Engine missfire most noticably at low rpm
 

Hi guys, the car seems to have developed a slight missfire, most noticeably between 1000-2000rpm. I've changed and gapped the plugs to 0.6mm and swapped over the MAF but to no avail. The Power FC's knock reading is jumping around too, mostly when i'm cruising or when the engine is cold. The car is also black smoking so obviously over fuelling. Its had new coil packs and injectors a year ago when the engine was rebuilt at API. Any help would be appreciated, thanks guys. The car is a 93 wrx by the way.

Could well be your O2 sensor, if it's not working correctly then this would result in over fueling. This does not always throw up a CEL warning. You can get a genuine part or many use a universal type with no real problems.

Had similar problems with the missfire under 3000rpm recently changed the Maf Sensor didn't help but i've just had new O2 Sensor fitted not sure if it's helped yet will let you no when i get my car back next week :thumb:

Cheers guys, had a new lambda sensor about a year back. It is a possible on my problem hit list though. Someone mentioned something about a "Cold Start Valve" to me, any ideas if this could be an option too?

Hiya,

could be an air leak. might be worthwhile checking that all the hoses between the maf and intake manifold are ok. If you can't see anything obvious, then try spraying some easy-start/WD40/etc onto the hoses and connections, with the engine up to temperature and idling. If the engine revs change just after you've sprayed, then you'll know you've got a leak from that area.