911 SC Cylinder leak down test questions

mike sampsel · mike sampsel's Garage

At 110,033 miles in March of 2016, my mechanic performed a cylinder leak down test on my 78, 911SC.

The results were:

#1=6% #2=7% #3=6%
#4=9%R #5=6% #6 = 6%

I'll assume these numbers indicate the engine is likely okay for another 20K-30K miles
or more without a top end rebuild? What does the R after #4=9% mean?

Have added 3 K miles in 2 years and all seems good. Looking for advice on how this information might support the sale of the car.

wgwollet · 07-02-2018, 01:22 PM

Well
Leak down tests must be done correctly on Porsche engines and I find a big variation on the numbers. One mechanic tried to tell me my engine needed a top end and it had 8,000 miles on a rebuild. I latter told him that is was just rebuilt ...but he tried to say it was leaking from a bad piston ring in 2 cylinders...I tested it and got good numbers

The car runs great, go figure, I am told the % of the leak down per cylinder should all be close together and yours seem to be wearing the same...

Trackrash · Trackrash's Garage

A leak down test can be an indication of where a problem is, if one is suspected.

I am guessing that the R might mean rings. The mechanic should indicate where the leakage was occurring, if the number was high.

If the motor is not using oil or fouling plugs I would not worry too much. A leakdown will not show worn valve guides.

Have it done again by another mechanic after a while and see how it compares if you are concerned.

If a compression test was done and one cylinder was low, a leakdown could indicate where the problem lies. IE, rings or valves.

FWIW, I did a leakdown on a motor I just sold. I got 6-7% on all but #5 which was about 10% with leakage through the intake valve. Later the buyer did his own test and he was getting 2-4 % on all cylinders except #2 which was 12% through the exhaust valve. I pulled the valve cover and banged on the rocker arm and it went up to 6%. Probably both issues due to carbon flakes on the valves.

If I was you, I would wait until you are selling and have the buyer do a test if he wants.