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Join Date: Jan 2009
Location: S.F. East Bay
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Compression/leakdown test would tell if you have internal engine damage such as cracked rings or bent valves which would cause misfire code.
You can also move ignition-coils, coil wires, injectors around one at time to see if code changes to different cylinder. This would narrow down list of possible suspects.
Vacuum leak at intake-manifold gasket for that one runner may cause lean mixtures in that one cylinder only. Would not throw off combined AFR enough to trigger P0171.
Also canít pinpoint leaks by spraying stuff externally. Only real way is to plug exhaust with banana, potatoe, cork. Then pressurise intake to 2-3psi and squirt soapy water at every single seam, joint, hose-clamp all 360 around from AFM to intake valves. Have to crawl under car with flashlight and mirror-on stick to get to under side of each intake-manifold runner. This test has found leaks where all others have missed it. I then fire mechanic that wasted time and money trying and failing.