The first thing anyone should do when trying to improve their vehicle’s fuel mileage is bring the vehicle back to its normal operating specifications. This is a very important, often overlooked fact that is especially true of older high mileage vehicles with a lot of wear and tear on them. This is the first place we looked when trying to improve the mileage on our 3.3 liter 6 cylinder Chrysler with 186,000 miles on it. We knew something was fishy when we were going through a full tank of gas every 3 days on just our daily commute to work. We then started logging mpg’s and realized it was getting 17.4 miles per gallon from 90% highway driving! The EPA rated highway MPG on this 1993 vehicle should be 24 mpg. We were losing almost 30% of our original mpg rating because the Chrysler was not running up to factory spec!
As a result we then began to diagnose issues with the car and replace necessary parts to restore its performance. The first thing we realized was something I had been hearing for months but never made the connection: the popping noise coming from the catalytic converter underneath the car. What was happening was raw fuel was being dumped into the catalytic converter, detonating in the exhaust and causing the popping noise. Now this excess fuel was caused by an incorrect air fuel mixture due to vacuum leaks in the motor. This is a common problem with vehicles having over 100,000 miles on them. We proceeded over the course of a week to replace many of the top level gaskets in the motor to solve the air vacuum leak hoping this would help the computer readjust the air fuel ratio correctly. We proceeded to replace the valve cover gaskets, intake manifold gasket, and air intake plenum gasket.
While we were working on the car we thought about what other possible improvements we could make to improve our fuel economy. So we removed the fuel injectors and mailed them out to be cleaned and serviced. We also replaced the EGR valve and gasket. Read more about clogged fuel injectors and how they affect gas mileage in the fuel injector section. After we put the engine components back together, we continued logging fuel economy over the next few weeks. To our surprise we noticed the mpg rating increase from 17.4 the first week to 20.6 the next week and a half! It looked like our vehicle’s ECU was continuing to adjust the air fuel ratio back to normal as the third week fuel economy had risen to 24.5 miles per gallon! We had successfully improved gas mileage by 37% just by fixing all the problems with our high mileage car.
With the skyrocketing cost of gas of course we didn’t stop there in our quest. But suffice it to say, that the biggest gain we were able to make in improving our fuel economy was in restoring the performance of our vehicle and fixing the various issues with the motor. There are many ways to monitor your fuel economy such as using your vehicle's built in mpg calculator if it is a relatively new vehicle. Otherwise there are many products on the market such as ScanGaugeII that plug into your car's OBDII port to report gas mileage and help you save gas.