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Compression test results - who's right?
Compression test results - who's right?
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Old 05-02-19, 06:38 PM   #1
Pault1
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Default Compression test results - who's right?

Just want to get people's experience on this. Here's my situation:
'06 STI, was supposedly given an ECUTek tune with Perrin CAI and a de-catted downpipe back in 2007. But when I bought it and moved it to Cali, had to replace 100% of OEM hardware to get it smogged. I never changed the supposed tune. FWIW no dealer or smog tester has ever mentioned finding a tune....

It's at ~105K miles, and is used as a daily driver with multiple autocrosses / year. So brief hard use for 1 minute at a time, maybe 120 runs/year, generally NOT beat on at any other times.

At around 90-95K started having issues (coolant level changes) finally diagnosed as head gaskets. At that time, heads done, valves done, new OEM turbo due to a visible crack.


Last few months, it has seemed to be down on power, has a tip-in hesitation, and occasional blue smoke on start-up. I asked three different garages to diagnose; one was clueless as to cause; two have done compression tests.
First one:
COMPRESSION TEST RESULTS (COLD ENGINE):
- Cylinder 1: 140 psi
- Cylinder 2: 105 psi
- Cylinder 3: 140 psi
- Cylinder 4: 80 psi
CYLINDER LEAKAGE RESULTS (COLD ENGINE):
- Cylinder 1: 5%
- Cylinder 2: 40%, intake and exhaust valves leaking
- Cylinder 3: 5%
- Cylinder 4: 60%, intake valves leaking
Diagnosis of valves leaking was determined by the sound. Valve clearances not checked at that time. AVCS confirmed operating normally. Garage is a Subie specialist and tuner.



Second one, I don't have the results in a file yet, but basically 2 and 4 found to have low cold compression, consistent with first one's test. But valves checked and only #4 exhaust valve was found to be very slightly tight (0.7 instead of 0.8, I think I heard) and on leak-down both 2 and 4 were blowing by. They stated that when pressurized, air could be heard coming out of the oil filler. Stated diagnosis is probable cracked ring(s). Recommended new short block. Garage owned by a friend but the mechanics are not specifically Subie specialists.



Now, I could be paranoid, but second garage is who was responsible for the head gaskets and valves last year. They are OK with standing behind any problem that their subbed out head and valve machining resulted in but don't think that it is the valves.



So comment away. Please try to give meaningful comparison to the way a leak down test may be interpreted. And would start-up blue smoke be more likely to be a cracked ring or some problem with guides/seals? Thanks.
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Old 06-14-19, 12:06 PM   #2
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Default Re: Compression test results - who's right?

Hmm, mighty quiet here!

OK, final was "new short block" plus a finding that the valve job during the previous head gasket job was defective so new work on the valves. There was definitely wear on #2 and #4 cylinder walls, deep enough to detect with a thumbnail. When they have some free time they are going to disassemble the bottom end so we can look at the piston & rings.
Based on the wear and compression loss found, the mechanic said that the engine would have failed in a few thousand miles. Will try to attach pic.
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Old 06-15-19, 02:54 AM   #3
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Default Re: Compression test results - who's right?

Same cylinders involved No real difference except clueless. Needs to be pistons and rings at a minimum.

Its a mistake to drive any turbo Subaru at with any boost in a state of unknown tune.
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