Finding and Fixing the Causes of Poor MPG
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.
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.
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.
