I went to change my downpipe today and after changing it the car fails to idle properly (when warmed up). Revs drop to 500rpm, then raise above 1000rpm and oscillate between the two values. The car may stall after a few seconds.

The car was OK before changing the downpipe. They also did an ECU reset at the service. They told me that it is the MAF sensor that is gone but no error codes were reported (neither CHECK ENGINE nor on the SELECT MONITOR).

1. Is it the MAF that is in bad shape but not completely knackered thus not giving an error code? Could an ECU reset reveal a MAF problem?

2. Could it be the Lambda sensor contaminated with the special lubricant, used to remove screws, which was used to remove the Lamda from the old downpipe?

3. Could it be the idle control solenoid? But it wouldn't explain why the car was OK before the downpipe change?

All members please help on possible causes of the problem.

 

What MY is your car.
I`ve only had early cars but all three have had this problem, I must admit I never fully cured it on my first car MY94but have succesfully cured it on later cars by cleaning the hot wire in the MAF and flushing out the idle speed control valve then reset the ECU. p.s the MY 94 car did not reveal any faults when hooked up to the select monitor.

 

MY99 with 45000 miles. No problems so far.
I have only removed the resonator box (standard filter).