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jwasbury jwasbury is offline
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Join Date: Jan 2008
Location: Weehawken, NJ
Posts: 3,384
I can offer up a few opinions about leakdown/compression tests and well known/respected p-car shops with years of experience based on my personal experiences with both of them.

I think the shop's statement that the "numbers will be all over the place" is BS. This shop simply doesn't want to do the job. This shop sounds a bit lazy to me, maybe they don't need the billable labor hours. These tests are not a cakewalk on a turbo porsche, but they aren't very difficult either. I've performed both compression and leakdown tests myself on my 930. The leakdown probably took me 2-3 hours in total, and that was the first time I've ever done a leakdown test on any vehicle. A shop with 40 years experience should be able to do it in half the time.

These tests are certainly one of the best ways to assess the health of the engine, but I think they will only tell you so much. Good numbers are certainly no guarantee that the engine does not have some real issues. Perhaps the shop's BS answer is really a way to dodge this issue: if they give you a test with "decent" leakdown numbers and you buy the car but later find a problem, you may come after them.

Bad numbers, or more importantly significant variances between cylinders are probably a sign of something very serious and obvious. You should have them done to rule out the obvious, but realize that they are not a guarantee. I think that many people have elevated "the car passes a PPI with leakdown with flying colors" to be the equivalent of a certified pre-owned vehicle.

My personal experience with leakdown/compression testing on a 3.3 liter turbo:

44% leakdown on #6. That's a number that would have most people run away screaming Look closely and you can see how large the top ring land is...this piston got very hot and melted a bit, deforming the crown and enlarging the top ring land. The 2nd ring land was compressed and the ring would not rotate properly.

10% leakdown on #5. This was the 2nd worst leakdown number of the bunch, but its perhaps not a number that everyone would say "run away"

%6 leakdown on #2...3rd worst of the bunch but the most obviously damaged ring. %6 is not a such bad number and was actually very close to the "good cylinders"

All other cylinders had intact rings and had 2%-4% leakdown numbers which I believe would be "flying colors" territory.

Incidentally, a compression test I performed about 1 year prior to the leakdown showed 135-140 psi on cylinders 1-5 while 6 showed 120psi. So I knew something was up with #6, but the problems with #2 and #5 were not obvious to me with either of these tests.

I should also mention that this car had no visible smoking and ran very strong prior to the teardown. I can't imagine that anyone who test drove it would conclude there was anything wrong. Oil consumption was maybe the only telltale, but wasn't too bad for long trips on the highway (when mostly off boost) and well within "normal" limits.

Good luck with the search
Current: 1983 911 GT4 Race Car / 1999 Spec Miata / 2000 MB SL500 / 1998 MB E300TD / 1998 BMW R1100RT / 2016 KTM Duke 690
Past: 2009 997 Turbo Cab / 1979 930
Old 08-09-2010, 08:35 PM
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