Poor Gas Mileage

Poor Average Fuel EconomyAverage Fuel Economy on GM 3.6L

This article will dig up the details of some of the common mechanical malfunctions that can cause sudden and large decreases in fuel economy.

Before we climb into the engine compartment looking for problems there are some misconceptions we need to clear up first.

Sometimes when people feel their cars are getting poor gas mileage, they fail to run the numbers to verify exactly what kind of mileage they're pulling down.

I believe it’s possible for drivers to get a feeling that something has changed and they're putting more money into the tank than they use to.

With that said, we still need concrete evidence to back up this suspicion before searching for problems. This is important because if the average mpg is off by only two or 3 points this can be attributed to driving conditions and habits more than mechanical malfunction.

Finally before we start pulling up fuel trim data on our scan tool in search for fuel system problems, we need to know how many miles per gallon this car should be getting.

I'm referencing the Environmental Protection Agency rating that was assigned to the vehicle. It's important to know the highway and city driving specifications so we can compare our actual mileage to this range.

Motorists should never expect to exceed these ratings as these are often lofty goals set by carmakers. EPA ratings on new vehicles have changed a little as they now include three numbers and some fine print.

Highway is always higher than city and now manufacturers include a real world average number and sometimes a disclaimer in fine print saying your results may vary to prevent court cases.

Mechanical Problems Causing Low MPG

Vacuum Test GaugeVacuum Test Gauge

One of the most common issues found on older vehicles would be poor fuel economy due to a vacuum leak.

When an automobile has reached the 10 year point, rubber vacuum lines can start to become porous allowing for small vacuum leaks that may or may not turn on the check engine light.

The reason this affects fuel economy is because additional air is added to the combustion chamber and the PCM (power train control module) will add additional fuel to compensate for this imbalance.

Remember that an automobile emissions and fuel systems primary function is to provide a mixture that is environmentally friendly as its main goal.

In other words it's more important to your PCM that tailpipe emissions meet or exceed standards than any other function, including fuel economy.

In the situation of a vacuum leak the condition can be verified using the data stream on a scan tool. In fact, there are several readings in the data stream that can give you a clue that the computer is compensating by enriching the mixture.

The first sign is fuel trim will be in the negative numbers meaning that the injector on time has been increased and the injector off time has been decreased.

Another area on the data stream that can be monitored is the oxygen sensor voltage. The O2 voltage is measured on the millivolt scale. In normal operation the sensor will rise above 500 mV and below in equal amounts.

How many times the voltage rises and falls below the 500 mV threshold is known as cross counts. Results that are consistently above or below this halfway point will be an indication of an o2 problem.

Low MPG Caused by Air Induction Problems

Although poor fuel economy can be caused by fuel system components such as clogged injectors or high fuel pressure from a malfunctioning regulator these are not as common as air problems.

Another issue that is experienced on older automobiles would be leaks in the rubber intake hose between the mass airflow sensor and the throttle body. If air enters after the airflow sensor it has not been calculated by the computer.

This causes a lean condition requiring additional injector on time to compensate and return balance to the mixture. These rubber intake hoses are usually well-made but after ten or fifteen years in a hot engine compartment they can become brittle and dry rotted. Add to this, the situation when an air filter is being replaced on many models it’s required to pull up and down on this weakening hose.

Anytime air enters the engine that is uncalibrated by the PCM is an opportunity for poor fuel economy. This would include intake manifold or Plenum air leaks from deteriorated gaskets or loosening bolts.

In conclusion the most important thing to verify is what the actual miles per gallon of the automobile is. When a low reading has been verified, scan tools can often clue you in on whether or not it’s an air leak problem or a fuel system issue by monitoring the compensation adjustments of injector on time and the reaction of the O2 sensor.

Generally speaking when the problem becomes bad enough, check engine light codes should be set by the PCM. The newer the vehicle the more likely a code will be triggered. When the check engine light comes on this is actually good news because it will point us to the system experiencing the malfunction.