Contour Enthusiasts Group Archives Forums General Troubleshooting Bad bad bad gas mileage

Please note that you cannot tell if an air filter needs to be replaced by visually inspecting it. It can look OK, & still be full of dirt - if it looks filthy, then you're really in trouble! LOL If you don't know how many miles are on the filter, I would change it. It's so cheap & easy - it's not worth stressing your engine to save a few bucks (forcing an engine to run rich all the time can lead to major problems, poor mpg being the least of them!)

As for O2 sensors, we sell a ton of them at the store, & most customers claim that replacing them improved gas mileage, & they did not have a CEL come on - they were just changing them to try to solve a problem, or as regular maintanence, which is a good idea. They can deteriorate gradually, which may not trigger a CEL until they fail completely. Here's a good article:

http://www.autohausaz.com/html/emissions-oxygen_sensors.html

I'm guessing maybe the problem w/the vehicle in question here may be a combination of things.

How about clogged fuel injectors? Has fuel system cleaner been run regularly thru the gas tank? I use Chevron every 6 months - many agree it is one of the best.

Is your car idling high? After driving the car for 30 minutes, pull off the road and pull the dipstick out and smell the oil. If the oil smells like gas, you have an enrichment problem. It could be the aforementioned thermostat or an coolant temp sensor. If the coolant temp sensor is sending a crap reading to the computer, it is doing the same thing as the bad thermostat. If the computer always thinks it's cold, then it's going 5o keep the idle air control valve open and run rich, which wold give you bad gas mileage.

Once again, easier to replace the temp sensor than to try to figure out whats worng with it. Usually pretty cheap.

Now, I am not one to advocate throwing parts at a car, so if you want to do it the right way, find the coolant temp sensor and an ohmeter. Wait until the temp sensor is cold and take a resistance reading. I don't know what the spec is of the top of my head, but they should either be in the Haynes Shop manual or the factory shop manual. Drive the car until the car reaches operating temp and take another reading immediately (don't burn yourself!!!) then see if the resistance changes. If it doesn't meet specs, you know for a fact you have a bad sensor.

Let us know how you do!