Just fitted a 3" Mongoose de-cat downpipe to my 98 STI RA, this is mated to a HKS Hiper. After fitting I noticed idle was bouncing around from 400 - 800 rpm. I re-set ECU and all was fine for around 15 mins then it starting doing it again.

Question I suppose is, downpipe causing problem ?

More likely a coincidence and MAF is on it's way out ? (disconnecting MAF and engine dies like it should though)

Lambda issue ? (though this is onlt 4 weeks old)

Any thoughts ?

Clive

 

Well thought is must be Lambda but wrong, still doing it after changing
Will try a new MAF