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Poor miles per gallon, or MPG, can have many possible causes. In this article, I’ll touch on some of the most common causes of poor MPG and their possible fixes. Most mechanical issues with your vehicle have simple causes. The tendency is to overcomplicate things that we don’t understand, especially when it comes to our vehicles. Trust me, I’ve heard it all, or most of it anyway. If you take a common sense approach, there’s not much that’s beyond your reach, and poor fuel economy is one of those things. So if you have a vehicle that just isn’t getting the mileage it used to, read on and we’ll see if we can get you sorted out.

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Simple Checks: Tires

I like to start with simple checks when dealing with any automotive issue. In the case of poor MPG, I like to start with the tire pressure. This is an often-overlooked item, and one that should be first on the list. Your tires are everything when it comes to your vehicle, and gas mileage is no exception. The rolling resistance of your vehicle has a direct effect on your fuel economy. If your tire pressures are low, your vehicle will have to work harder to get itself down the road. The harder your vehicle works, the more fuel it will use. It really is that simple.

The type of tire you use has a direct effect on fuel economy. Some tires are better than others; some have less rolling resistance. You might spend more for better tires, but in the end, you will likely come out on top based on your fuel savings alone.

As for what you should set the pressure at, check your owner’s manual or the tire sticker that should be located somewhere on your vehicle. There are a lot of places they put these things. Mostly I see them on door jambs, or sometimes on a sticker in the glove box. Occasionally I find them under the hood. The important thing is to find it and set your tires to that spec.

Many people make the mistake that the pressure indicated on the outside of the tire is where you want to set the pressure. This could not be further from the truth. That listing is the MAXIMUM tire pressure for that tire, NOT what you should set your tire pressures at. I also often see that people want to inflate their tires till they look round, with no bulge at the bottom. Once again, the wrong approach. That bulge at the bottom of the tire is important. It’s called the “foot” or “contact patch.” The name implies its purpose, and it is an important one. After all, you wouldn’t walk very well without your feet, or even feet that didn’t have the proper shape. Overinflated tires have the same effect. Plus, overinflated tires can be dangerous. Sure, you’ll get great fuel economy with overinflated tires, but the safety risk just isn’t worth it. Vehicles with overinflated tires are unstable and difficult to control. So please, don’t go by appearances when it comes to inflating your tires. Inflate them to the proper pressure and move on.

Here are some videos to reinforce what I’ve talked about here.





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Poor miles per gallon, or MPG, can have many possible causes. In this article, I’ll touch on some of the most common causes of poor MPG and their possible fixes. Most mechanical issues with your vehicle have simple causes. The tendency is to overcomplicate things that we don’t understand, especially when it comes to our vehicles. Trust me, I’ve heard it all, or most of it anyway. If you take a common sense approach, there’s not much that’s beyond your reach, and poor fuel economy is one of those things. So if you have a vehicle that just isn’t getting the mileage it used to, read on and we’ll see if we can get you sorted out.

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Simple Checks: Tires

I like to start with simple checks when dealing with any automotive issue. In the case of poor MPG, I like to start with the tire pressure. This is an often-overlooked item, and one that should be first on the list. Your tires are everything when it comes to your vehicle, and gas mileage is no exception. The rolling resistance of your vehicle has a direct effect on your fuel economy. If your tire pressures are low, your vehicle will have to work harder to get itself down the road. The harder your vehicle works, the more fuel it will use. It really is that simple.

The type of tire you use has a direct effect on fuel economy. Some tires are better than others; some have less rolling resistance. You might spend more for better tires, but in the end, you will likely come out on top based on your fuel savings alone.

As for what you should set the pressure at, check your owner’s manual or the tire sticker that should be located somewhere on your vehicle. There are a lot of places they put these things. Mostly I see them on door jambs, or sometimes on a sticker in the glove box. Occasionally I find them under the hood. The important thing is to find it and set your tires to that spec.

Many people make the mistake that the pressure indicated on the outside of the tire is where you want to set the pressure. This could not be further from the truth. That listing is the MAXIMUM tire pressure for that tire, NOT what you should set your tire pressures at. I also often see that people want to inflate their tires till they look round, with no bulge at the bottom. Once again, the wrong approach. That bulge at the bottom of the tire is important. It’s called the “foot” or “contact patch.” The name implies its purpose, and it is an important one. After all, you wouldn’t walk very well without your feet, or even feet that didn’t have the proper shape. Overinflated tires have the same effect. Plus, overinflated tires can be dangerous. Sure, you’ll get great fuel economy with overinflated tires, but the safety risk just isn’t worth it. Vehicles with overinflated tires are unstable and difficult to control. So please, don’t go by appearances when it comes to inflating your tires. Inflate them to the proper pressure and move on.

Here are some videos to reinforce what I’ve talked about here.





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Simple Checks: Check the Oil

Here’s another simple check that can yield a big result. Engine oil is the lifeblood of your engine. If it’s low, your engine isn’t feeling very well, and as a result, it’s less efficient. A loss of efficiency equates to a decrease in fuel economy. I know your engine doesn’t really have “feelings,” but you get the point. The harder your engine has to work, the more fuel it will use. It really is that simple. Checking the engine oil is a great check if you are having issues with poor fuel economy. If you find that it’s low, top it off to the proper level and recheck your MPG.

Another thing to consider is changing your oil. Fresh oil lubricates better than old oil. As a result, your engine won’t have to work as hard to do its job. So if you’re in doubt as to when your oil was last changed, you might consider changing it if you’re having issues with poor fuel economy.

Something else to consider is using synthetic oil. Synthetic oil has better flow characteristics and lubrication ability than conventional oil, and as a result, your engine will run with less resistance when using synthetic oil. A word of caution on synthetic oil though. If your engine is old and leaking, you might forgo synthetic oil. If you switch to synthetic on an older, worn-out engine, you might see a negative result. Synthetic oil is very good at what it does, but it can’t repair a worn-out engine. In fact, it can make a worn-out engine worse. If you have an older engine, when switching to synthetic oil you might see more leaks and perhaps even have more engine noise than you did with conventional oil. However, if your engine is in good shape and doesn’t leak, you might consider switching to synthetic for better fuel economy. I often get asked if you can switch back and forth between synthetic and conventional oil. The answer to that question is, yes you can. Synthetic oil is designed to mix with regular oil, in fact many synthetic oils are really blends of synthetic and conventional oil. There are actually few fully synthetic oils. So if it’s already mixed in the bottle, you shouldn’t have any issues switching from synthetic to regular oil should you so choose.

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Simple Checks: Check the Transmission Fluid

Are you seeing the pattern yet? Yep, anything that makes your engine work harder costs you fuel economy. Your transmission is no exception. Primarily I’m speaking of automatic transmissions, but you could also apply this to the manual transmission as well. If you have a manual transmission, this might not be your first check, but if you’re running out of options with a manual transmission vehicle, it’s definitely worth a look. This section mainly refers to the automatic transmission, however.

If you have poor fuel economy and an automatic transmission, checking the fluid level is one of those quick checks that can yield big results. If the fluid is low, discolored, or full of air bubbles, your transmission won’t work as efficiently. It might even start to slip, which can cost you MPG. If the fluid appears dark or old, you might consider changing the fluid. Just like changing your engine oil, this can help your fuel economy a great deal. While you’re at it, you might also consider changing the transmission filter if applicable. This will ensure that your automatic transmission is in top form and is as efficient as it can be. You can find more information about automatic transmissions in the Solving Transmission Problems article.

Simple Checks: Coolant Level and Condition

In addition to checking the oil and transmission fluid it’s a good idea to also check the coolant level and condition. If the coolant is low, or the cooling system is malfunctioning and causing an overheat, your MPG will suffer.  So be sure to also check the coolant level and condition as well as the operation of the cooling fans if you have electric fans. More information on cooling system issues can be found in the Diagnosing an Overheat article.

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Beyond the Simple Checks

In the next sections of this article, I’m going to get a little deeper into the diagnosis of a fuel economy issue. Once you’re done with the simple checks, you might want to head here if you haven’t solved the problem yet.

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Tune-up Items

To be honest, I don’t really feel that “tune up”’ is a term that applies to modern vehicles. A better term for modern vehicles is “service.” Back in the day, engine management systems were mostly mechanical. Therefore, you would do a periodic tune-up of the engine and its systems to get it to run efficiently. This might involve some adjustments as well as replacing some of the serviceable parts and filters.

These days, those adjustments are a thing of the past. In some ways, this makes things easier, as you don’t have to have the experience and tools necessary to perform a proper tune-up or service. Theoretically, the only thing you need to worry about on modern vehicles is purchasing quality parts and installing them correctly. That is sometimes easier said than done, but you get the point.

So, if you have an MPG problem and you’ve gone through the simple checks, it’s time to turn your attention to the serviceable parts on your vehicle. Most modern vehicles have a service interval, usually consisting of a list of checks accompanied by fluid and filter changes within a given mileage or time frame. See your owner’s manual or service manual for the service items and service interval for your vehicle. You can find more information on diagnosing and repairing performance problems in the Diagnosing Performance Issues article.

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Air Filters

It seems simple enough. In fact, if you’ve gone to an oil change place lately, I’m sure they tried to sell you one, even if you just put one in last week. That issue is for another article. The reason service shops push air filters is because they know that there are a lot of vehicles out there with dirty air filters. Yes, they’re also looking to increase their profit margin. As we keep emphasizing, anything that makes your engine work harder compromises fuel economy. If your engine can’t breathe, it’s not going to be very efficient. A car with a dirty air filter would be the equivalent of you putting a shop rag in your mouth and running around the block. I’d wager you’d be close to passing out by the time you got halfway. Don’t do the same thing to your engine if you want good fuel economy. Keep a clean air filter. It’s a simple, inexpensive item that can yield big results.

One thing I’ll mention here regarding changing the filter. Be sure to check for debris or obstructions when changing it. It’s not uncommon for small animals to take up residence in your vehicle, especially if you live in the country or if you have pets. I’ll get to why this is significant in a minute. I have sometimes found that the air box was filled with “stuff.” I say “stuff” because it could be any number of things. I’ve seen straw, insulation, and even pet food clogging up an air intake system. This is why I mentioned pets. Other animals than yours might have an interest in your pet’s food. Some of these animals like to store this booty in your air intake system. To them it’s a great hiding place; to you, it’s poor fuel economy or worse, depending on the level of infestation. So when changing your air filter, just put your eyes down into the air box and make sure there isn’t anything in there that could restrict airflow.

Check the Intake Ducts

As long as we’re on the topic of air filters, it seems logical to talk about the air duct system. This is especially critical on vehicles equipped with a Mass Airflow Sensor, or MAF. This is a sensor in the intake stream that actually measures the air going into the engine. The computer uses this measurement to calculate a fuel mixture. One thing to watch out for is air leaks in the tube that runs from the air filter box to the throttle body of the engine. Any leaks here allow unmeasured air to enter the engine. This unmeasured air, or “pirate air,” has not been accounted for by the computer. This means that you have air with not enough fuel mixing in the combustion chamber. When this happens, your engine can perform poorly and get poor gas mileage. So take a moment to look over the air intake system for any leaks. Also, make sure all the clamps and connections are secure and not allowing any extra air to enter the engine.

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Spark Plugs

Another simple replacement item: spark plugs. Well, simple on most vehicles. Replacing your spark plugs can have a dramatic effect on fuel economy, especially if they’re old and worn out. Spark plugs have come a long way in the past few years. It used to be that you changed them out about every 10K to 30K. These days, most plugs go 100K or more. Part of the reason for this is that vehicle manufacturers are trying to keep their advertised maintenance costs down. The funny thing is, although they have increased the replacement interval for spark plugs, the cost of these longer-life plugs has gone up quite a bit.

The best place for answers about what plugs are in your vehicle and when you should replace them is your vehicle’s service manual. Some vehicles list the type of plug they want used on the under-hood emissions sticker. I would  strongly suggest you stick with this recommendation. I’m not an advocate of using different spark plug brands or types different from what the manufacturer calls for. It seems the engineers designed the ignition system around a particular spark plug, and I’d wager they did that for a reason. If you use a different type of spark plug in your engine, you run the risk of going outside the ignition system design, which could affect your fuel economy. I know there are spark plugs that claim to give you better mileage and more power, but don’t you think that if that were really possible they’d already be in your engine from the factory?

So if your plugs are old, replace them. When you do replace them, use the plugs recommended by the manufacturer for best results. Aside from using different spark plugs, old spark plugs can affect your engine’s performance. As plugs get old they can misfire and cause incomplete combustion. A misfiring engine is not an efficient engine, and efficiency is what we want for the best fuel economy.

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Brakes

You might be asking yourself why I’m mentioning brakes in an article about fuel efficiency. The answer is simple: Brake problems can increase the rolling resistance of your vehicle. Increased rolling resistance equates to a harder-working engine. A harder-working engine is not as efficient. Yes, we’re back to that again. It’s true, though. If you have a brake problem that’s causing the brakes to apply, or partially apply, it can make your engine work harder. In fact, I recommend you periodically check your brakes just to be sure that everything is properly lubricated and working as it should. Brakes are important — not just for stopping, but also for the overall efficiency of the vehicle. More information about brakes can be found in the Solving Brake Problems article.

Check Engine Lights

If you have a check engine light (CEL) or MIL (malfunction indicator light) on and you have poor fuel economy, you might already have the answer to your problem. Any time the computer detects a fault with the engine management system, it sets a code. Many of these engine management problems will affect fuel economy and emissions. This is one of the main reasons they started using this system in the first place. It’s there to alert the driver that there is an issue that can affect the engine’s operation and emissions. So if you have a check engine light, pull the code to see what you’re up against.

I’ll touch on some of the engine management systems that can effect fuel economy later in this article. For now, if you have a CEL, pull the code and do what you can to repair it. In the meantime you might consider reading the Solving Automotive Performance Problems article for more information on these issues.

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Clutch Problems

If you have a vehicle with a manual transmission and poor fuel economy, you might want to check your clutch operation. As a clutch gets old, it can start to slip in the higher gears. What this will equate to is a loss of motion and efficiency from the engine. In other words, less power is transferred to the transmission when you have a slipping clutch. So even if your engine is running perfectly, a slipping clutch can cost you MPG. Here’s a simple test to see if your clutch is working correctly.



Automatic Transmission Problems

One of the main ways the automatic transmission can affect MPG is if you have a problem with your TCC, or torque converter clutch. This is a clutch that your automatic transmission uses to eliminate power loss within the torque converter. It works very much the same as a manual transmission clutch and is controlled by the transmission. If this clutch does not engage or it slips when engaged, it can cause a loss of fuel economy. If you have this problem, your engine might run perfectly fine and you’ll have no idea why you’re losing MPG. The one thing you might notice if your vehicle is equipped with a tachometer is that as you’re cruising on the highway, your RPMs are higher than they used to be. This might be because the TCC is not working correctly. The principle is the same as with a slipping clutch on a vehicle with a manual transmission. Here’s a video explaining the operation of the torque converter and the TCC.


Besides the TCC, other automatic transmission issues can compromise MPG. One of those is a slipping clutch or band inside the transmission itself. Most automatic transmissions use clutches and bands to operate the different gears inside of it. If one of these bands or clutches fails, it can cause the transmission to slip in a particular gear. When this happens, it’s a loss of efficiency, and we all know what happens when an automotive system loses efficiency: poor MPG and a loss of power. More information about automatic transmissions and their operation, including some cool videos, can be found in the Solving Transmission Problems article.

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Cooling System Issues

One of the most important contributors to the fuel management system on a fuel injected vehicle is the coolant temperature sensor. You could almost look at this sensor like the vehicle’s “choke” that a carbureted engine uses. Engine temperature dictates when many things happen with your engine management system. If there’s a problem with the cooling system, it can have a direct effect on fuel economy and engine performance.

Now before you get all happy and replace your coolant temp sensor, think basic. Remember I said not to make things complicated. Try two things. First, make sure your cooling system doesn’t have any air in it. Here’s a video on how to do that. More info can be found in the What to Do If Your Engine Overheats article.


If you’re sure there’s no air in your cooling system and you still suspect a problem, you might check the thermostat. If your vehicle is equipped with a temperature gauge and you notice that it doesn’t come up to temperature like it should, you might consider replacing the thermostat. Your engine is designed to get up to operating temperature quickly so that it runs as efficiently as possible. If it doesn’t get up to temperature quickly and efficiently, your MPG will suffer. Aside from that, thermostats are wear items; they are mechanical devices that will eventually fail. So if you start to notice an issue with one, replace it sooner rather that later to ensure efficient engine operation.

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Overheating Engine

If your engine is overheating or running hot, it will affect fuel economy. The increased heat load of your engine will equate to a loss of efficiency. I cover how to deal with an overheating engine extensively in the What to Do If Your Engine Overheats article. Check that out if you have an overheating engine.

Misfires

I touched a little on misfires in the spark plug part of this article, but I’d like to cover them in more detail here. Misfires come in all shapes and sizes. They have an equal amount of causes. In this section, and some of the sections following it, I’ll talk about the possible causes of engine misfires. The bottom line is, misfires cause performance problems as well as poor fuel economy. A misfire occurs when a cylinder or cylinders don’t put out the power that they should. Misfires make the engine work harder, making it less efficient.

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Power Balance Testing

Power balance testing is one of the best and easiest ways to find the source of a misfire. The idea is to isolate each cylinder to see if it’s producing the same amount of power. If you find a cylinder that’s down on power, it’s likely you’ve found your misfire. Once you’ve located the problem cylinder, you need to find the cause of the misfire. This can be tricky, but as I said at the beginning of this article, if you take a logical approach, you’ll be successful in your diagnosis.

More information about what to do when you find a misfire, and how to diagnose engine performance issues, can be found in the Solving Automotive Performance Issues article. For now, here’s a video on power balance testing.



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Vacuum Leaks

I talk about vacuum leaks extensively in the Solving Automotive Performance Issues article. That’s because they are a very common cause of engine performance issues. They are also a common cause of poor fuel economy. If you haven’t figured it out by now, anything that causes your engine to run poorly can cause poor MPG.

Vacuum leaks can have many sources, but basically a vacuum leak is an air leak anywhere between the back of the throttle plate and the back of the intake valve on your engine. Vacuum leaks can be caused by a hose that came off or rotted, a leaking intake gasket, or a failed vacuum-controlled component.

A tip: Some HVAC systems use engine vacuum to operate their components. This means that you can possibly have a vacuum leak under your dash. If you have one of these systems, be sure to check it for vacuum leaks if you have a performance issue or poor MPG. Vacuum leaks are at the top of my list of things to check when I have a performance problem. Checking for them is simple enough.

This video shows how you can check for vacuum leaks with a can of carburetor cleaner. You can also use a spray bottle with a little water in it to do the same test to help eliminate the possibility of fire when doing the test. You can also use smoke to help you find vacuum leaks. More information on vacuum leaks and how to find them can be found in the Solving Automotive Performance Issues article.



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Voltage Leaks

Voltage leaks are the result of your ignition system breaking down. Voltage leaks can occur anywhere in the secondary ignition system. The secondary ignition system is everything after the ignition coil output. Any leaks here can cause performance issues and poor MPG. Here’s a video on how to find voltage leaks. It’s kind of a fun test.



Exhaust Restrictions

Exhaust restrictions can compromise engine performance. Anything that compromises engine performance affects fuel economy, as we all know by now. Many people want to condemn the catalytic converter when they have an exhaust restriction. To some degree, they are correct in naming this the prime suspect, but it’s not the ONLY suspect. You could also have a bad pipe or muffler that’s causing the restriction, so don’t rule those out, especially if you’ve already replaced your catalytic converter and still have an issue.

Exhaust restrictions are actually not that difficult to diagnose. All you need is a vacuum gauge and perhaps a buddy to work the gas pedal to do the test. Here’s a video on the process.



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Exhaust Leaks

You might be asking yourself why I’m talking about exhaust leaks as a possible cause of poor MPG. The reason has to do with how your engine calculates a fuel mixture after it warms up. After your engine warms up, it goes into what is referred to as “closed loop.” This means that your computer is taking information from your O2 sensors located in the exhaust stream to calculate the fuel mixture. Any exhaust leaks near an O2 sensor can throw off its readings. When this happens, it can upset your fuel mixture, thus compromising fuel economy. So if you have poor MPG and exhaust leaks, you might want to address those as part of your effort to get better gas mileage. Here’s a video you might find useful if you’re looking for exhaust leaks on your vehicle.



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O2 Sensors

An O2 sensor is a sensor placed in the exhaust stream that measures the oxygen content of the exhaust. This measurement has a direct relationship with how your engine is burning its air/fuel mixture. After the O2 sensor heats up to operating temperature, the computer begins to use the O2 sensor reading as its primary source to calculate the fuel mixture for your engine. It’s sort of a self-monitoring system to maximize power and efficiency. Modern vehicles don’t use O2 sensors; they use what’s called an AF, or air fuel sensor. These sensors are WAY more accurate than their ancestors. So if your vehicle is equipped with an AF sensor, you can ignore much of this section.

If an AF sensor has a problem, it will likely turn on the check engine light. Trust me, on modern systems, AF sensors are very closely monitored for any faults. The slightest hiccup with an AF sensor calls in the check engine light police. Older vehicles with single-wire O2 sensors are not as good about this. In fact, many of these older systems don’t have any form of O2 sensor monitoring. For this reason, it’s a good idea to check the operation of the O2 sensor if you have poor MPG on one of these older vehicles. It’s simple enough. Just grab a DVOM (digital volt ohm meter) and set it to a low voltage scale (less than 20v if you can). The O2 sensor’s operating range is in the 0 to 1v range. After you’ve warmed up the vehicle, check the voltage output of the O2 sensor. It should be switching rapidly across the .5v mark if it’s healthy. If it’s not healthy, you’ll notice it hanging out at a particular voltage, or it won’t switch very fast at all. This indicates a “lazy” O2 sensor. If you find one of these, replace it and then recheck your MPG. I’d wager you have marked improvements. In fact, we used to recommend O2 sensor replacement as part of a service on some of these older vehicles, usually at the 60K mark. After replacing one of these old O2 sensors, you might be surprised at how well your engine runs. Newer O2 sensors aren’t like this and they’re more expensive. For that reason, I don’t recommend replacing them as part of a service.

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O2 Sensor Heaters

Before AF sensors came along, they started using heaters in O2 sensors. They did this to help get the sensors up to operating temperature quickly. The quicker the sensor comes up to temperature, the more efficient the engine will be. The O2 sensor heater helps accomplish this. When an O2 sensor heater fails, it usually sets off the MIL or check engine light. There’s usually a separate code for an O2 sensor heater failure. When this happens, your O2 sensor will still operate; however, it will take a lot longer to heat up to operating temperature. This means that your engine is not running as efficiently as it could. A loss of efficiency means what, kids? That’s right, a decline in fuel economy. Technically, a failed O2 heater is not the end of the world, but if you want the best fuel economy, fix it sooner rather than later. Here’s a video about diagnosing and repairing an O2 sensor heater failure.



Fuel System Issues

If we’re going to have a discussion about fuel mileage, I think we need to work the fuel system into the conversation. Fuel-injected engines need a given fuel pressure to run properly. If that fuel pressure isn’t correct, it will affect the overall operation of the engine and the MPG. So if you suspect a problem with your fuel system, a good start is a fuel pressure test.

If your fuel pressure isn’t correct, don’t condemn the fuel pump just yet. It’s also a good idea to check the operation of the fuel pressure regulator and the fuel filter if equipped. A clogged fuel filter can cause a loss of fuel pressure. A faulty fuel pressure regulator can not only mess up the fuel pressure, but it can also leak. If you have a leaking fuel pressure regulator, you will have poor MPG. There’s a lot more to this testing, and I cover it in more detail in the Solving Automotive Performance Issues article.

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Fuel Leaks

It hardly seems worth mentioning, but I will anyway. If you have any fuel leaks in your vehicle, it will affect your MPG. If the fuel never makes it to the engine, it can’t be burned. If you have a fuel leak, address it sooner rather than later — not just because it’s affecting your gas mileage, but because it’s a safety issue as well. More information on finding and fixing leaks with the evaporative emissions system can be found in the article on Finding and Fixing Leaks on Your Vehicle.

Evaporative Emission Issues

You might not think that running your vehicle without a gas cap will cause a drop in fuel economy, but it can. You know gasoline as a liquid, but in truth it does its work best as a vapor. In fact, gasoline or petrol evaporates very quickly. If you don’t believe me, set out a small container of it and leave it there for a day. Come back the next day and it’s probably gone. The same thing can happen inside your fuel tank. The evaporative emission system on your vehicle is designed to help eliminate fuel evaporation into the atmosphere. If this system has an issue, it can effect your fuel mileage. Even though your vehicle seems to run fine with that check engine light for an EVAP problem, it could still be costing you fuel economy. So if you have poor MPG and a check engine light for an evap problem, you should address it sooner rather than later. More information about finding and fixing leaks with the evap system can be found in the article on Finding and Fixing Leaks on Your Vehicle.

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Octane and Your Engine

It’s a common misconception that using a higher-octane fuel in your vehicle will give you better gas mileage. This isn’t necessarily true. The long and short of it is that the RM2 method of octane rating is a rating of resistance to volatility. The higher the octane, the less the fuel wants to burn. They do this to accommodate the high combustion chamber temperatures you see in high performance engines. High performance engines often use high compression or forced induction to create more power. This increased pressure in the combustion chamber equates to heat. This heat can pre-ignite the air fuel mixture in the combustion chamber. This is bad, as it equates to a loss of power and possibly engine damage. On these engines, you use a higher octane fuel to prevent this pre-ignition from happening.

If you use a higher octane fuel in an engine that doesn’t call for it, you’re wasting your money in my opinion. Yes, some premium fuels come with a better additive package, but I don’t think you’ll see much benefit from them in your ordinary non-performance engine. In other words, you likely won’t go any faster and you won’t get better fuel economy when using premium fuel if your vehicle doesn’t call for it.

Conversely, if you run regular low-octane fuel in a vehicle that calls for premium fuel, you’re making a mistake. As stated, you run high-octane fuel to help prevent pre-ignition and misfires. Running low-octane fuel in an engine that calls for high octane will not only compromise power and cause a drop in MPG, but it can also damage your engine in some cases. The takeaway is to put in the fuel the manufacturer calls for and you should be just fine. I hope this video helps clear up any questions you might have about octane.



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Oil and Fuel Additives

My knee-jerk reaction here is to call all fuel and oil additives snake oil, meaning they really aren’t worth it. I think fuel and oil additives are a case of perceived value. You go to the auto parts store, pick up a bottle of “make your engine happy,” dump it in, and you suddenly start getting more power and better MPG out of your vehicle. This is why I say “perceived.” I’m sure there are people out there who swear by product X. I’m not saying product X isn’t a good product. I’m saying that product X is different things to different people. It’s money in the pockets of the people selling product X more than anything else. Am I opposed to this? No, I’m a capitalist. But as a capitalist I like to save money. For me, I’d rather save my money than put “make your engine happy” in my tank or crankcase. I prefer old-fashioned service and maintenance to keep my engine efficient. Dumping stuff in my tank just seems like a shortcut that will get me lost rather than to my destination. You, however, can do what you like. One thing I will say is that when used properly, fuel and oil additives don’t do any harm. So if they make you feel good, use them. If you want to save money, don’t spend it on fuel and oil additives.

Forced Induction Problems

Forced induction is anything that adds pressure to the intake on your engine. The most common forms are superchargers and turbo chargers. Both of these do pretty much the same thing, but in different ways. A supercharger is a compressor driven by a drive belt connected to the crankshaft of the engine. A turbo charger is also a compressor that forces air into the engine, but instead of being belt-driven like a supercharger, it’s driven by the exhaust gases coming out of the engine. Both of these systems help increase the power output and efficiency of the engine.

Superchargers produce max output at lower RPMs, which makes getting off the line quicker. Turbochargers, on the other hand, have a delay in power delivery. They need to “spool up” before they start increasing the engine’s power. A problem with either of these systems can cause poor fuel economy. Since they create pressure in the intake, you need to employ different testing methods when diagnosing these systems. For instance, instead of spraying carburetor cleaner or water on the intake when looking for leaks, you might try a mix of soap and water. When the soapy water reaches a leak, it will start to bubble when the intake is under pressure during boost, indicating the leak’s location. Other problems, such as bearing failures or control issues, can hamper the ability of these systems to operate properly. When these systems fail, you will see a decrease in power and fuel economy. Consult your vehicle’s service manual for testing and evaluation of the system on your vehicle.

Engine Mechanical

I’ve spent a lot of time talking about the systems that manage the fuel delivery to your engine. We haven’t said anything about the mechanical integrity of your engine yet, and we should. All of the aforementioned systems don’t mean diddly if your engine has a mechanical problem. It’s been my experience that engine mechanical is often overlooked in diagnosis. It shouldn’t be. If your engine has a mechanical problem, no amount of parts or sensors you throw at it will cure it. It is the heart of the system, after all.

One of the most basic tests you can perform to assess an engine’s mechanical integrity is a compression test. This test measurers the engine’s ability to draw in and compress air. In truth, your engine is nothing more than an air compressor. If it can’t do that efficiently, it won’t run right, and fuel economy will suffer. Here’s a video on compression testing that can walk you through the process.


Compression testing gets you in the ballpark. To find out the source of a compression problem, you would then perform a leak down test. The leak down test will tell you almost exactly where the compression loss is occurring should you have compression loss. Yes, in fact I do have a video on leak down testing.



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Driving Habits

I saved one of the easiest and best for last. Many times poor fuel economy is your fault. Yep, that’s what I said. The way you drive has a direct effect on fuel economy. If you have a lead foot, make sure you have a high credit card limit, because you’ll be spending a lot more at the pump. If you want an easy way to save fuel, slow down, don’t race at stop lights, and take it easy on your vehicle. Do it for one tank of gas and see if you go farther. I bet you do.



Summary

More in-depth information on engine performance, and the testing you can use to help you find a performance issue, can be found in the Solving Automotive Performance Issues article. As I stated in the beginning, there are many things that can affect your vehicle’s fuel economy. The main takeaway is anything that makes your engine work harder will compromise fuel economy. Therefore, in your search for a cause of poor MPG, be sure to look for things that will drag your engine down and make it not perform as well as it should. Take a common-sense approach. Listen for new noises, keep your eyes peeled for leaks or anything out of the ordinary, and generally feel how your vehicle is running. Be aware of its operation so that when something does go wrong, you’ll feel it before it gets to a critical point. If it feels weird or unsteady, look into it. Don’t be afraid to be intimate with your vehicle. Think of it as a modern-day horse. Treat it well and it will take care of you for years to come. Good luck in your search for better MPG, and thanks for reading.

Stay Dirty

ETCG

Written By EricTheCarGuy
Edited By Julie Hucke

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