[Kim]

I am having a leakdown test done in my white 92 coupe tomorrow at the local Porsche dealer. The car is using about 1 litre of oil in 2000 km. What should this test tell me and how should I interpret the "numbers" - assuming that I will get some numbers. Compression was 188/185/185/155. I am also having the cam belt taken off and put back on and am hoping (boy, am I hoping) that although it is tight the Nissan place did not install it correctly. If so, and my pinging gets fixed, it might let me work on the oil leak at a more relaxed (read <<<$$$) pace.

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[flash]

i'm afraid you're probably going to be in for bad news - that 155 hole does not look good - hopefully the leakdown will show a blown gasket (can you believe i said that?) - otherwise, you could be looking at either a chewed valve or a worn out ring - because of the oil consumption, it is very possible it's a ring - are you smoking at all?



just a double check - when you did the compression test, were all 4 plugs out, throttle body open, and you gave it at least 6 revolutions for each hole? gauge calibrated?

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[bob blackwell]

Kim, the leakdown test is done to determine why some cylinders have less compression than the others. Basically, air is pumped in and then you listen for where the air is escaping to determine if the compression leak is caused by valves or piston rings. If the air is escaping into the exhaust it indicates a bad exhaust valve. Air escaping around the base of the cylinder head indicates a blown head gasket. If the air is escaping into the crankcase then it indicates bad piston rings, etc.. good luck, Bob Blackwell.

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[Kim]

"just a double check - when you did the compression test, were all 4 plugs out, throttle body open, and you gave it at least 6 revolutions for each hole? gauge calibrated?"



No idea - had it done at an independent shop recommended by one of the local PCA members.



Also, I have never seen any smoking nor have I ever had anyone say the car is smoking. I do note that sometimes the car can seem to be "sooty" at the back fenders and under the wing. I have always assumed this to be "road grease" and tire particles. There is NO coolant loss.

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[flash]

no coolant loss does not mean no blown head gasket - common misconception - while it is very common, it isn't always the case



i'd call the guy who did the test, or run down and buy a gauge from kragen and do one yourself and verify that the test was done correctly before yanking stuff apart



sooty stuff is usually carbon from running rich - this would be consistant with what is going on in hole #4 if it was a bad valve - if the sooty stuff is slimy though, it's likely oil, pointing to possibly a ring



again, though, speculating is easy - verify the compression test

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[Kim]

Here is the poop from the car, hot off the presses. Cylinders listed from 1-4.



Compression 180, 205, 180, 175

Last test before adding ring conditioner (650 km ago) 185, 185, 185, 155

% leakage 30%, 15%, 40%, 40%, all into block



Camshaft belt was off by one tooth in advance direction

Balance shaft belt off by one tooth



Both belts taken off and reinstalled. Car does not ping (raining hard today, didn't want to step into it hard, however) and runs a lot smoother. Idles better also. Data on oil consumption will follow when I check it next. If the knocking stops I will delay the engine work a bit and see if the oil treatment does anything else.



I might go out looking for a Corvette to scare tonight!!

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[orphanowner]

[quote name='Kim' date='Apr 17 2006, 02:26 PM']Here is the poop from the car, hot off the presses.  Cylinders listed from 1-4.



Compression  180, 205, 180, 175 

Last test before adding ring conditioner (650 km ago) 185, 185, 185, 155

% leakage  30%, 15%, 40%, 40%, all into block



Camshaft belt was off by one tooth in advance direction

Balance shaft belt off by one tooth



Both belts taken off and reinstalled.  Car does not ping  (raining hard today, didn't want to step into it hard, however) and runs a lot smoother. Idles better also. Data on oil consumption will follow when I check it next.  If the knocking stops I will delay the engine work a bit and see if the oil treatment does anything else. 



I might go out looking for a Corvette to scare tonight!!

[right][post="19378"]<{POST_SNAPBACK}>[/post][/right][/quote]



Those leak down numbers can't be right. Anything over 15% indicates a serious engine malfunction. Even 15% is pretty bad. If these were correct, you'd have a trail of oil following you or the car would barely run from a bent or burned valve. I'd be looking for the cause of the low compression on number 4. The shop should pull the cam cover and inspect the cam lobes for number 4 to rule out a worn cam and give the timing chain etc an eyeball. You may want to use a shop that has 944/968 experience. A car with those leadown numbers wouldn't seem normal in any way. Who did the last cam and balance belts? Hopefully not this shop....

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[Kim]

The test was done by continuously pressurising the cylinder with 80 psi air from the garage compressor and measuring the staic pressure that could be maintained. 40% loss would then be 80 psi in, 48 psi maintained. Don't know if this is standard procedure or not. The Porsche shop was not really concerned that it was "bad" given the car's age and km.

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[orphanowner]

[quote name='Kim' date='Apr 18 2006, 05:56 AM']The test was done by continuously pressurising the cylinder with 80 psi air from the garage compressor and measuring the staic pressure that could be maintained.  40% loss would then be 80 psi in, 48 psi maintained.  Don't know if this is standard procedure or not.  The Porsche shop was not really concerned that it was "bad" given the car's age and km.

[right][post="19412"]<{POST_SNAPBACK}>[/post][/right][/quote]



I usually use 100 psi as it makes it easier to figure the percentage of leak down. Obviously they did the test wrong and the fact that your driving the car and pretty happy about how it is running is the proof. I do leakdown tests on racing engines often enough to know that at 15 percent, the owner's are ready to spend some money to fix them or get the lost power back. I've done high mileage 944's and only seen 10%. These engines used moderate amounts of oil and didn't have any oil deposits on the spark plugs or visible oil smoke in the exhaust. They ran good enough so that the owner's didn't worry about some minor engine wear.

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[Eric_K]

Was the leakdown test done before or after the timing belt was set right? If the timing is off, the valves might not be fully closed.

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[Kim]

It would seem as though the leakdown test was done first followed by inspection and reinstallation of the camshaft belt. Your comment makes sense to me but note that the compression test was done before the leakdown test. Should it not, too, be affected?

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[Eric_K]

It could possibly effect the compression test but it wouldn't be as obvious as the leak down test where the motor is not turning over.

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[gbarrier]

forgive me for hogging the subject but would like a little more info on the leakdown test. When we do this type of differential pressure test on an aircraft engine we do it at TDC. Somewhere I read to do it on these engines at the bottom of the stroke. Is this common?

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