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I am looking for some ideas as to the cause of a misfire
Car: 2002 Subaru Legacy, 2.5L 4 cylinder, non-turbo
History: Recent engine out overhaul. Heads rebuilt becase timing belt had failed and bent valves. New valves put into it but they were not machine-ground, the valves were just replaced. Heads were milled flat at machine shop. Pressure tested with the new valves at machine shop and passed.”
Problem: consistent cylinder 2 misfire, A pulsing air sound also audible coming from exhaust-to-head connection on the opposite engine bank so I suspect that there is an air leak here but it is not on the side of cylinder 2.
Diagnosis so far: Compression test seems equal on all 4 cylinders about 130ish psi. Leak down: One time with cylinder 2 at TDC I show a 100% leak but when I turned the sprocket a couple times and reset it at TDC I got <15% leak. I can't hear any hissing coming from exhaust or throttle or oil fill tube so I am not sure where the pressure leaked out to. I tested fuel pressure and it is good. I tested my fuel pressure regulator and it is good. The injector seems to be firing normally. Spark is present and I also moved the plug to cylinder 1, and then the plug wire, and the misfire stayed on cylinder 2.
Hypothesis: I favor a valve leak since I did not have the new valves reground into place. I am not sure if it is possible to get an intermitent valve leak though. Is it possible to have the head just slightly out of seal with the valve so that sometimes the valve makes a seal and sometimes doesn't? As mentioned I think there is an exhaust leak too just by the sound. I was going to put some smoke into te exhaust and see. I would think that a misfire from an exhaust leak would not stick to a specific cylinder though.
My plan: I was going to pull the engine and have all 24 valves ground in then replace the engine and see what happens. Any other ideas?
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Replies
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Couple things that can cause a misfire fuel, spark and compression. see if this link helps further.
http://www.ericthecarguy.com/faq/solving-automotive-performance-issues
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