Better gas mileage with the check engine light on

A few months ago, my check engine light came on and I started getting excellent gas mileage. A couple weeks later, the engine started idling a bit rough. Since it began, the rough idle has exacerbated, although it fluctuates: the engine idles smoothly within the first few miles of the day, and then, after those first few miles, the idling engine will waver in between 700 and 100 rpm. The improved mileage, too, fluctuates: a tank of gas sometimes will give me 35 mpg and others will give me 28 mpg without any drastic change in my driving habits.

I took it to my local mechanic and he agreed with me that the engine was running lean. For $450 he installed two new oxygen sensors. For a week after the repair, my check engine light was off, but it came back on and my car continues to have the same issues. I confronted him about his ineffectual repair job, and he said he needed to replace the oxygen sensors “to make sure they weren’t lying to us,” and now he would need to do further diagnosis to fix the problem.

1) What is the problem with my car?

2) Does it need to be fixed?

3) Could my mechanic have tested my old oxygen sensors without replacing the outright?

You may have a fuel related problem. When the injectors spray out less fuel then you get better gas economy at the expense of overall performance. This will also play havoc with the 02 sensors. I once had a 57 Belair with a 327 installed. I got unbelievable gas mileage but power was down. It was overpowered anyway so I drove it that way for several months. I finally removed the carburator and found remnants of a wasp nest inside after tearing it down. The jets were partially plugged. I installed new jets and my power came back. My gas mileage took a nosedive. Have the mechanic check your fuel pressure and your fuel filter. If the vehicle is driveable and you like the fuel economy then just leave it alone. If you want perfect driveability and less money in your bank account then have it repaired.

The “trimming” action done in the fuel trim section is just that a trim. It is done to keep the catalytic converter supplied with the proper gas mixture for it to fuction correctly. When your “check engine” light comes on with a "to lean’ message it is because action needs to be taken to get the catalytic converter functioning correctly again. We need a good Sunday brawl,so lets hear it.