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| My dry compression test results https://forums.tipec.net/viewtopic.php?f=25&t=19488 |
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| Author: | pilch [ Mon Mar 16, 2015 4:17 pm ] |
| Post subject: | My dry compression test results |
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 |
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| Author: | sstman [ Mon Mar 16, 2015 5:56 pm ] |
| Post subject: | Re: My dry compression test results |
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. |
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| Author: | pilch [ Mon Mar 16, 2015 6:17 pm ] |
| Post subject: | Re: My dry compression test results |
Well that's another avenue to look down, thanks. |
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| Author: | AlpineTurbo [ Tue Mar 17, 2015 12:42 am ] |
| Post subject: | Re: My dry compression test results |
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.... |
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| Author: | pilch [ Tue Mar 17, 2015 1:17 am ] |
| Post subject: | Re: My dry compression test results |
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. |
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| Author: | RS2000CUSTOM [ Tue Mar 17, 2015 9:23 am ] |
| Post subject: | Re: My dry compression test results |
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 
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| Author: | AlpineTurbo [ Tue Mar 17, 2015 6:17 pm ] |
| Post subject: | Re: My dry compression test results |
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  .
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| Author: | pilch [ Tue Mar 17, 2015 7:43 pm ] |
| Post subject: | Re: My dry compression test results |
Unfortunately next two days are out but Friday I'll do a wet test on #2 and post the result. Thanks for the help |
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| Author: | Waylander [ Thu Mar 19, 2015 10:46 am ] |
| Post subject: | Re: My dry compression test results |
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 |
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| Author: | pilch [ Fri Mar 20, 2015 7:00 pm ] |
| Post subject: | Re: My dry compression test results |
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. |
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| Author: | Waylander [ Fri Mar 20, 2015 8:06 pm ] |
| Post subject: | Re: My dry compression test results |
Fire it up in the dark with the bonnet open, look for some runaway sparks |
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| Author: | white944 [ Fri Mar 20, 2015 9:38 pm ] |
| Post subject: | Re: My dry compression test results |
Cruddy injector? |
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| Author: | pilch [ Fri Mar 20, 2015 10:21 pm ] |
| Post subject: | Re: My dry compression test results |
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! |
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| Author: | Pink piggy [ Sat Mar 21, 2015 12:02 am ] |
| Post subject: | Re: My dry compression test results |
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 
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| Author: | pilch [ Sat Mar 21, 2015 9:58 am ] |
| Post subject: | Re: My dry compression test results |
Waylander wrote: Fire it up in the dark with the bonnet open, look for some runaway sparks 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. |
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