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hey guys, i have a 99 wrx.

Started out that i had a hesitation/miss on acceleration,checked for vacuum leaks, theyre ok.
I cleaned out the AFM and now the hesitation is gone but its now blowing black smoke out of the exhaust and the engine is hunting at idle.

Any ideas on wats going on would be muchly appreciated, cheers

Maybe a blown head gasket?

Smoke is a bad sign

Black smoke is to much fuel. Hunting at idle I normally associated with an air leak which is generally a faulty BOV. However with you cleaning the maf it may have left a residue on the sensor. Is it a hotwire or pad model air flow in yours?

what OB said, i use contact cleaner to clean my afm, leave no residue. also it maybe worth running a can of subaru upper engine cleaner through the IACV (idle Air Control Valve) system, as the IACV could be dirty and causing the hunting?..

bryan

ps the IACV is on the pass side of the car on the back of the manifold and is a creamy color.

it is the O2 sensor that trims fuel. If it is no longer working or connected ECU just goes max rich making black exhaust smoke.

Thanks for the ideas guys, will get back to u when i try a few things

Or oil leak burning off in turbo area? or back pressure from the engine into turbo?
Cheers
AP