Finding and Fixing the Causes of Poor MPG

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.

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.

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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