Finding and Fixing the Causes of Poor MPG
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.
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.
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.
