Finding and Fixing the Causes of Poor MPG
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.
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.
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.
