Porsche Enthusiasts Club Forum

On my 944S I am on a quest to sort out a slight idle misfire. Ignition side all new other than the coil and ignition module.

Decided to do a compression test a few minutes after being out for a decent run.

#1 : 195psi
#2 : 155psi
#3 : 190psi
#4 : 190psi

Now cylinder #2 is down slightly on the others. I've read that no cylinder should be less than 10% different and also that the lowest cylinder shouldn't be less than 70% of the highest. Which is correct as my results could be good or bad?

The car pulls hard all the revs, loses no oil or water, and all 4 plugs are a good colour and the same. It really does drive well and doesn't feel down on power at all.

I am clutching at straws thinking there could be a mechanical issue causing the slight misfire at idle?

Any advice always appreciated!

Rich

I had a poor idle on my 968 after a 'bodged' chain and slipper change, turns out the cams were missed timed slightly, car was fine driving, but idling in traffic was a tad embarrassing, slight hesitation, missing, hunting etc...had the cams timing rechecked at a specialist, and alls well now.

Well that's another avenue to look down, thanks.

Have you done a wet test as well on the bores, or possibly even a leakdown test?

Without trying to sound too pessimistic I would try to get a boroscope down No.2 bore....

No unfortunately I didn't have time to do a wet test and won't get chance to till Friday.

Don't have access to a borescope unless I hire one or find a garage with one.

I'm wondering if I could have a broken valve spring as symptoms are similar to what RS2000CUSTOM had on his S2. There is a light ticky sound from #2 area but it isn't desperately noticeable.

Compression readings on mine didnt indicate very much

I had an intermittent rattle which was due to the hydraulic tappet "taking up the slack" and then it would go quiet

Also a cylinder leak test was no good as the 2nd valve spring was able to hold the valve closed

I identified which cylinder was misfiring by removing plug lead (when misfire was happening) and removed the rocker cover then with a torch I could see the snapped spring

The reason for why I said to check it out with oil and an boroscope is because I have seen something like this before a couple of times, including on my S2.

For your sake I do hope that maybe a spring is broken (but as said it wouldn't throw readings off massively unless the spring was very badly damaged but that would be going down another avenue or something even simpler and cheaper to fix .

Unfortunately next two days are out but Friday I'll do a wet test on #2 and post the result.

Thanks for the help

Get one of those little USB cameras off ebay snake eyes think it was called 10 quid

Plugs in your laptop very handy thing to have

I redone the dry test on #2 this afternoon and it was 185psi so must've left the gauge hose slightly loose the other day when it read lower.

Took cam cover off to check for any obvious broken springs and none apparent. Cam tensioner pads all good too.

So back to something on the ignition side or a vacuum leak for the cause of the misfire I'm assuming.

Fire it up in the dark with the bonnet open, look for some runaway sparks

Cruddy injector?

I sent them away for testing and cleaning a few weeks back. They came back fine.

Going to borrow a smoke machine I think and see if there is a vacuum leak somewhere. There were none obvious when I had inlet manifold off to clean the ISV but that doesn't mean there isn't one somewhere!

Try carefully spraying carb cleaner round joints tiny bit at a time and if the revs rise, there's your leak.
Warning kids dont try this at home

I will try that later. It has new Beru leads, Bosch dizzy cap and rotor arm, plus new plugs. Only ingition parts not changed are the coil and the ignition module, which looks like an unbranded aftermarket one.

I've also done a voltage drop test and thats all within tolerance so the earths seem ok.