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porschehopeful · Joined: 08 Dec 2007 Posts: 5 Location: Philadelphia Suburbs

I just recently purchased a 79 931 and am in the process of checking the engine out and finding the parts I need to make it run. I did a compression test of the car yesterday and these were the results (I don't know the numbering scheme of the cylinders so I just did 1 through 4 from front to back):

1- average 119
2- average 93
3- average 121.5
4- average 123

I also re-tested 2 and then got an average of 85

After reading on here that the spark plugs need to be removed and WOT I did the test again:

1- average 121.5
2- average 86.5
3- average 122.5
4- average 123.5

also retested 2 again and the average was 96

So, what does this all mean? I can't do a warm test because the car does not run. I'm missing CIS and some other things so I can't start it just yet. Are there major engine issues?

The car is actually in the registry under the PO's name shenry1307
(although its not in very good shape any more)
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79 931 with lots of problems

endwrench · Joined: 07 Dec 2002 Posts: 1631 Location: Victor, Montana

You definetly have some sort of problem on #2 but I would go ahead and do a wet test. Squirt a couple tablespoons of oil in the sparkplug holes and try the compression test again. The cyliders may be washed down with fuel.

Todd
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'79 924NA. Rebuilt 9.5:1, MSDS header, Mega Squirt Injection, MJLJ-EDIS Ignition, 1.6L Whipple Charger and Intercooler, 10lbs Boost, 944 Trans, Custom HD Clutch.
"simsport" said....superchargers are better than turbos its official!....

porschehopeful · Joined: 08 Dec 2007 Posts: 5 Location: Philadelphia Suburbs

Unfortunately I already took the compression tester back to the mechanic. I'll have to see about borrowing it again sometime next week to do the wet test.

There wasn't any fuel wash because the car hasn't been running in a long time (half a year atleast) and the CIS is missing/fuel pumps don't work.

What do you think the problems could be? Is this going to require engine disassembly/rebuild?
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79 931 with lots of problems

sandgroper · Joined: 07 Dec 2002 Posts: 108 Location: in limbo

broken rings,re ring it

Smoothie · Posted: Thu Dec 20, 2007 11:21 am Post subject:

Fuel wash during its' previous life could have caused the problem with #2.

Some other possibilities are broken ring/s, stuck ring/s, blown head gasket, or sticking, burnt, or out of adjustment valve/s. Get some Marvel Oil or other top cylinder cleaner-lube and squirt about 10 cc into each cyl., replace the spark plugs, crank the engine a few times to distribute the lube and let it sit until you can re-do the compression test (or repeat a time or two during the week as time and motivation allow). If this fixes the problem, then it was just stuck rings.

Another thing to try is to pump air into the cyls, one at a time, with each at TDC where both valves are closed - and listen through the exhaust, intake, and oil filler. Wherever the air sound is loudest, tells you where the leak is. Loudest through the exhaust means leaky exhaust valve, loud at intake means leaky intake valve, and loud through the oil filler means it's mostly leaking past the piston rings.
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"..it's made in Germany. You know the Germans always make good stuff."

'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox

tuurbo · Posted: Thu Dec 20, 2007 2:29 pm Post subject:

I don't know the intricacies of these cars. I 'think' I understand engines a little. A cold test doesn't sound right. The test condition is cold, but the running condition is hot. Everything expands and is unstuck when hot. So I don't know.

Relative compression, ok. The low cylinder might be lower than the others on a warm test. I'll grant you that much.

If it were a warm engine you were testing here with these results I'd say "yes, likely" time to push the investigation further.

Cold test? Dunno. Rings and/or bores, valves and head gasket. With these cars the latter wouldn't surprise me. It also wouldn't surprise me if a warm test yielded a higher compression for #2 cylinder, maybe enough to get by - but this isn't possible, I realize.

Try blowing putting the cylinder under compression and listening really carefully - try to find out where the sound is going. Smoothie has a method for that.

If you want certainty, remove the cylinder head. It's going to bug you until you narrow it down anyway. And if your cold test result is valid yer not gonna get out of this without cracking the head off the block.
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1980 924 turbo, MSD, Meth. Inj, otherwise stock.

tuurbo · Posted: Thu Dec 20, 2007 4:04 pm Post subject:

Ok just did some reading and I see why Smoothie and the others take the cold test so seriously.

I guess I've never had to do a compression test cold and have always assumed it's supposed to be hot. So how much difference could a test on a hot engine make anyway?

sandgroper · Joined: 07 Dec 2002 Posts: 108 Location: in limbo

broken rings,youve got the early 79 lowcompression detonator to start with,done smoothies test and once found burnt exhaust valve so its a good idea as well.

Paul · Posted: Fri Dec 21, 2007 9:24 am Post subject:

I hope you squirted some oil into the cylinders before running these compression tests
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Owned and repaired 924s since 1977
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