Leak Down needed if Good Compression Test?
12-11-2006, 06:26 PM
#1
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Leak Down needed if Good Compression Test?
The title says it--if the compression test is good (equal high pressures) does a leak down test add anything and if so, what? By the same token, if I'm having a PPI done and the compression test is not good is there any sense in doing the leakdown if I'm not willing to fix the engine? Finally, I've read on this forum that a compression test is not reliable on the 911--is that true and why?
Thanks for any input--I wasn't able to find straightforward answers doing a search--lots of posts but not too many whys.
Russell
Thanks for any input--I wasn't able to find straightforward answers doing a search--lots of posts but not too many whys.
Russell
12-11-2006, 07:30 PM
#2
Rennlist Member
Russell: I've posted on this in the past but can't access that file right now. Baically, if a certain procedure is followed, a leakdown test is failsafe. On the other hand, a compression test requires that all six spark plugs are removed prior to doing the test. When those plugs are loosened the chance exists that carbon pieces will come loose and fall down, landing on the valve seat of an open exhaust valve. When the engine is cranked that exhaust valve can close against carbon chunks, which are strong enough to prevent the valve from sealing properly. Result? Flawed test. Always do a leakdown at PPI time.
Pete
Pete
11-04-2011, 10:45 AM
#3
Parts Specialist
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Rennlist Member
REVIVE:
I know its an old thread...but Pete is SPOT ON!!
I am chasing a slight hesitation on mid range...checked all the injectors, and then ran a compression test...the #'s looked OK ...then ran a leak down....I had one cylinder at 40% !!!
exhaust valve suspect
pulled the valve cover and with a block of aluminum on the adjuster and pressure in the cylinder tapped (repeatedly) on the valve....well, it got better 25%, then worse!! 80%, then cleared!!! - 6% ! repeated tests, rotated motor several times, banged without pressure, with, without....seems good!
new plugs as the old ones (1.5 years old) looked worn...I may talk to my master tech regarding that one.
so my scare was fixed....but I would not have found that without the leak down test - just saying.....
I know its an old thread...but Pete is SPOT ON!!
I am chasing a slight hesitation on mid range...checked all the injectors, and then ran a compression test...the #'s looked OK ...then ran a leak down....I had one cylinder at 40% !!!
exhaust valve suspect
pulled the valve cover and with a block of aluminum on the adjuster and pressure in the cylinder tapped (repeatedly) on the valve....well, it got better 25%, then worse!! 80%, then cleared!!! - 6% ! repeated tests, rotated motor several times, banged without pressure, with, without....seems good!
new plugs as the old ones (1.5 years old) looked worn...I may talk to my master tech regarding that one.
so my scare was fixed....but I would not have found that without the leak down test - just saying.....
11-04-2011, 01:30 PM
#4
RL Technical Advisor
That's exactly why we've not done compression testing in 20+ years. 
Leakdown tests are the only way to go.
(I'd kindly suggest not using anything metal to stake the valves; use wood instead)
Leakdown tests are the only way to go.
(I'd kindly suggest not using anything metal to stake the valves; use wood instead)
11-04-2011, 05:41 PM
#5
So a leakdown test covers both? In other words, a good leakdown result = good compression?
