Help! Version IV, V limited WRX STi (1998). Intermittent erratic idle. Yup, seen lots of examples posted but can anyone help with this one. After thrash (not red lining but serious acceleration), come to stop, idle drops then up, repeats number of times and then settles. On quick reverse known to do the same on occasion. Even pulling into drive, which is a hard left, steep incline up and then down into drive, same can occur.

Self diagnostic shows no fault. Changed MAF unit (this model whole thing) for one known to be OK and no change, self diagnostic still showing no fault. Anyone out there with any ideas as to what to try next or just now wait till something fails.

 

I've got a version 3 STI and mine has done it for at least 3 years, plus the check engine light comes on sometimes on deceleration! I put it in neutral for 10 seconds and the light goes off. Neither of these have caused a problem for a long time, but the rev thing is annoying I know. Mine goes from 800rpm to 1000rpm roughly, but it does settle after a while.

 

lambda?