Finding and Fixing the Causes of Poor MPG
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.
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.
