I just finished replacing the clutch in my 99 4runner V6 and after gettting it all back together, fired it up and it ran terrible! Would barely idle, backfired and just generally ran rough. I drove it for 3-4 miles (new clutch is sweet) and the check engine light came on. Went home and pulled a P0171 code, cleared the code and went for another drive. As the miles accumulated the truck started running better and better and after about 20 miles is running great again. The only gues I have as to what happened was that I removed the catalytic converter and took the front O2 sensor out. Is the O2 sensor sensitive to getting "dirty"? It just dangled from the wiring harness for a couple of days. Any thoughts on what went wrong, should I have covered the O2 sensor while I had it out? Thanks

 

How many gaskets did you remove during the clutch R&R? One for the front cat flange and one for the 02 sensor itself, correct?

That is what I am leaning towards anyway...

 

what was the code??? P0171?? is that an 02 sensore code, or what is it?, and do you can't really get an 02 dirty, unless you poor oil or something all over it.

 

did you disconnect the battery when you did the clutch, it may have been just relearning

 

Check the oxygen sensor connections. Maybe one started to come loose after it had been dangling for a few days. I am not too familiar with the 99 4Runner, but is there anyway you could have switched the oxygen sensors?

 

Usually the P0171 code means the MAF sensor needs cleaning, but since this happened right after you did work on the vehicle it may be something else causing the code to appear. If it only came on temporarily then went off again then I wouldn't worry about it.