Ive gone back to basic as regards to doing my own diagnostics for misfire on idle. Obviously the first thing was to carry out a compression test. The result are as follows : both cylinders drivers side measured at 180 psi & both cylinders passenger side measured 165/170 psi. My concern is that there is a tolerance of 10% or 28psi i think ? difference between all cylinders. Im am sure a previous thread stated the diff should be 7psi (Bob). How concerned should i be. As then i could continue with the remainder diagnostics. Timing,coils, maf etc. Your help would be grateful
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Not being funny... but:D the pass side spark plug holes are much harder to get to than the drivers side, are you sure the tester is inserted tightly on the passenger side??
Rob |
what car and engine, with those pressures its indicating a 9:1 comp ratio, Sti4 wagons had that, and early STi 2 wagons.
does sound as though there was some issue with the gauge on the lower side, Subaru spec is no more than 7psi difference. bob |
The car is a 95 wrx ra, but the engine had a red manifold. The gauge has a flexible rubber hose, one end has the dial and the other has a brass type threaded connector (just longer than the spark plug) I tried this test 3 times on each cylinder just in case. Might have the other side wrong though. i was dreading to here about the 7psi difference. With the actual results ive got what would it translate to my engine condition besides the obvious
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Ticky,
I wouldn`t be to bothered with your results ;) When my last engine gave up i was seeing a difference of 70 psi i.e the 3 good were at 165 / 150 and the bad one was at 80 :eek: BTW the engine still ran it was just a bit off balance. Still 2 open her up but we recon its probably just the land gone or the ring cracked ;) Pete |
Thanks Pete . After trying to get so much info on possible causes for misfire. I just looked at what could be the worst thing, firstly low/uneven compression. i believe all of the cylinders have good compression but do not fall within the 7psi max difference. I was seeing a dif of 10psi & worst 15 psi. Could this cause my engine to misfire. I was going to by new Maf & Lamba sensor but then thought about this 7psi. Should i carry on with the purchase.
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If it were me I`d take a step back m8.
Are your plugs ok ? No cracks, gaps correct for boost your running? Are the coils ok ? Check for hairline cracks (if u have coils). Do you have oil in the inlet manifold etc. Loads of things to check before lookin at the electronics bud ;) Pete |
Ticky
Is the engine tending to shake at idle, but runs OK if you raise the revs a bit? If so I suggest you look for a leak on the intake side. Either the manifold or one of the many pipes, possibly even the brake servo. I got my MY97 back after a short engine change under warranty. The first time I stopped at traffic lights the engine was trying to shake itself out of the car. Dealer thought it was OK. I had to insist they checked the intake manifold thoroughly. Eventually they fixed it by refitting the manifold. Good Luck. |
Good point there STi ;)
When I last took my manifold off I noticed it had been blowing (ever so slightly) around number 3 at the gasket. Probably not torqued down properly :rolleyes: Pete |
Re-misfire
Have you changed the coil packs and plugs?? coil packs seem to last about 6years and 100k miles at most in my experience.. I resisted to start with but after changing them it resolved the lumpy idle and running on 3 cylinders from cold and a miss in first on boost.. also the car felt like a rocket again.. I then changed the plugs later and it was back again.. checked plugs and gaps and fitted NGK rather than sh1te Champion plugs and it was back to normal again.. I know nothing about the compression though.. Simon |
Good point about the intake manifold & relevant hoses, Ive been waiting for the gaskets from Heritage/Subaru dealer to send them as well as other bits & bobs three weeks so far. What would cause oil in the inlet manifold, i dont think i got oil in mine but will check when gasket arrive. Got little traces of oil in the turbo to intercooler pipe ? As for the coils I visually inspected & tested them using instructions from a thread, got reading of 1.2 on all four even though it should have been 0.63 - 0.83 or something like that but i just got this feeling its not the coils. Thanks all anyhow, will be rejecting
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Yeah I didn't think it was the coil packs either..
but if they are the originals on the car then I would put money on them needing changing anyway.. Buy one at £50 from a dealer.. fit it.. bet it runs better... Which is what I did.. intension was to move it about until I found the one causing the miss.. it still missed but drove some much better with just one replaced that I bought all four.. JGM :) |
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