Car Problem Question 2... Engine Misfires, O2 errors, odd time of running rough. What's wrong?

Question

To all those who posted answers to my first question, thank you. I know it is hard to diagnose an engine problem because of all the possibilities but I'm hoping that if I explain enough, I may get some where with this and not get hosed at the repair shop.
I have a 99' Jetta and 5 days ago the check engine light went on. My MPG went from 36 to 18 mpg. My car starts just fine with maybe one or two starts that took a couple extra cranks. The day my light first appeared, the light came on, then off, then on again and stayed on. I have a manual and when the car idols it sounds normal and feels normal. When I'm over 2300rpms it sounds normal and runs normal. But, when I drop below 2300 I can occasionally feel a couple of jolts and I can feel the misfires. I went to autozone and they told me the first O2 sensor was reading errors and two of my cylinders were misfiring. Anybody have any ideas??? Please help..

Answer 1

try a TUNE UP!!! it can't hurt

Answer 2

I would first change the O2 sensors (about 100 bucks for both...can be done in your drive way with simple hand tools), there should be two of them one before the cat and one after the cat, when an O2 sensor fails it can cause misfires. IF after you do this and it still running poorly, I would take it to a shop and have them look at the cat-converter...they will listen to it and also use a heat sensor on it to see if it is heating up and also run it on a machine (like a emissions tester) to see if what the mixture (hydrocarbons and such) It is possible that your cat is bad/clogged or cold.

Answer 3

Your "check engine" is telling you that a sensor has failed.

You need to hook it up to a code reader and get trouble codes. Since your gas mileage sunk, it is fuel related (O2, Manifold Air Pressure, coolant temp, Mass Air Flow, Throttle Position or other sensors may have gone bad).

If your car is running rich (it is) you can melt your catalytic converter. The excess HC will overheat it. Don't drive it until you fix the fuel issue.