Oddjob

Ive have three sets of leak down test numbers on the same motor that do not come close to matching. Two tests were done by two different reputable shops and the third was a test I did with my own equipment.

Shop 1
1: 0%
2: 0%
3: 5%
4: 5%

Shop 2
1: 39%
2: 5%
3: 36%
4: 15%

Test 3
1: 12%
2: 3%
3: 5%
4: 7%

I do not know the procedures or the equipment that either shop used, but the differences seem extreme. I can understand some inherent error with different gages, but that should be proportional, and these results are not even close. How repeatable is leak down testing? Outside of just plain mistakes, can any variables account for the difference in test numbers?

Perry 951

Temperature will play a pretty large part in the results you have above. A cold motor is going to be much worse off than a hot one, so if the car was at the same temp when each test was done, something is screwy.

Also, if the rings are shot, and the car was recently ran, the oil film on the bores and rings can help hold back leaks.

I would test the car again with it at operating temp and see what you get.

Oddjob

I did my test cold, assuming that it would be worst case results. I will retest the motor hot to compare.

I would not expect this, but any possibility that at operating temps, there could be a higher leak down through the rings than when cold?

Brian Fugok

I had a high number on a cylinder so I releived the pressure and checked TDC again, I was off, it must have moved when pressurizing. Maybe the same happened here.

Warm up the engine and try again and make sure you are at TDC.

Good luck.

Does any oil run out of your crankcase vent tube?

Jeremy Himsel

I'd be willing to say that shop 2 did'nt have the cylinders at TDC. Otherwise you car is in pretty sad shape.

When comparing tests 1&3, the only that was a major variation was cylinder # 1 @ 12%. I'd make sure the car is warmed up and the #1 is @ absolute TDC and re-check (I've had it off just a few degrees and it blew 12%). I'd trust my own reading before I'd trust anyone elses.

Oddjob

What I noticed for my testing, is that if you dont find the dwell or toggle position right at TDC, the pressure will push the piston down and turn the engine over. This seems to be true even if the car is in gear, because I did have it move a couple inches during the testing. I tested the motor twice (cold both times), on two consecutive days and the numbers matched within 2% of each cylinder. And my numbers showing each cylinder holding at least 88psi, dont indicate I missed TDC.


But what is also unusual and why I dont just dismiss Shop 2's results (Shop 2 also is the one with the best reputation and specializes in Porsches, specifically 944s), is that this car has been down on power for about a year, and has been blowing a lot of oil through the intake. The oil is also jet black, even at well under 1000 miles on it. Both symptoms can be an indicator of blow by, so the sad shape numbers did not surprise me (The turbo was just rebuilt and the symptoms persist). But obviously the results from the first shop and my own test numbers, dont indicate a significant problem.

kasturbo

Jim, sorry to pull up an old thread, but what did you ever figure out with your motor?

Oddjob

I pulled it out and put in a spare motor that I rebuilt. I pulled the head and oil pan on the old motor to look for any obvious problems, but everything looked ok. Never had time to look into it any further. Probably wont do any more diagnosis until I need another motor, and have to rebuild it.

After that experience, I am somewhat skeptical of leakdown test results. Its certainly more telltale than a basic compression test, but it seems that there can be a lot of variation and potential for erroneous results.