ticky

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

tweenierob

Not being funny... but 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

Bob Rawle

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

ticky

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

teknopete

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
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

ticky

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.

teknopete

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

STi8 Mate

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.

teknopete

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

Pete

Jolly Green Monster

iTrader: (4)

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

ticky

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

Jolly Green Monster

iTrader: (4)

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