Compression and leakdown test (not my car)

[moribundman]

These figures are from an Audi 2.8 V6 12 valve engine. The engine consumes a little less than 0.5 qts per 1k miles at this time.

Compression of new engine: 9-14 bar (130-203 psi)
Minimum compression: 7.5 bar (108 psi)


At 130,000 miles, the results seem very typical for this particular engine:

Compression (psi):
Cyl 1: 200
Cyl 2: 210
Cyl 3: 205
Cyl 4: 200
Cyl 5: 195
Cyl 6: 200

0% Leakdown on all cylinders

How would you read these figures and the compression variances? Do they indicate (some) sticky rings?

Considering the compression numbers and leakdown test result, how do you explain the oil consumption?

 

[Chris142]

quote:

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Originally posted by moribundman:


0% Leakdown on all cylinders


Considering the compression numbers and leakdown test result, how do you explain the oil consumption?

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0% leakdown? Impossible! Every engine will have some leakdown. I'd have that retested.

I'd look to the valve stem seals for the oil consumption.

 

[moribundman]

I guess when they said "0%" leakdown they meant that they could not confirm any considerable leakage. Knowing who had the leakdown test performed, I would say he picked a place with a competent mechanic.


What I notice is that the #2 cylinder exceeds the new engine compression specs, which leads me to believe that this engine has some carbon buildup in the combustion chamber. Maybe the variances between the cylinders indicate more or less sticky rings.

 

[Chris142]

I'd say the carbon buildup is from the oil getting past the valve seals.

 

[moribundman]

Would you think the leakdown test was botched and the leaky valve stem seals were missed?

 

[1 FMF]

sources of oil consumption can be valve stem seals like was mentioned, also pcv valve or however the crankcase vent is setup, feeding into the throttle body. Also highly dependent on how the car is driven, if it's a manual and you do a lot of engine braking. May also be normal if the car is run hard and the oil used is light and is getting past the rings, not necessarily a bad thing.

The reported compression #'s to me don't say anything other than the engine is in very good health.
Before drawing any other conclusions aside from leakdown, I would inquire about the gauge used, how old it is, and how the test was performed. 10-15 psi difference can easily be attributed to gauge accuracy and resolution.
I imagine the 0% leakdown report is the simplification of a very low # which means there is no leakdown problem.

Higher than normal/nominal compression #'s indicate carbon buildup in the cylinder, on the piston and cylinder head. Very common with high mileage and oil consumption. Lower than normal numbers, like 50% lower, may be caused by a sticking piston ring if not a worn out or broken ring, which is saying it isn't sealing against the bore. In some cases a sticking ring can be fixed by solvents which will break up the carbon that is causing them to stick and not seal, but that's not the case here with your #'s.

 

[1 FMF]

quote:

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Originally posted by moribundman:
Would you think the leakdown test was botched and the leaky valve stem seals were missed?

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you wouldn't see the affect of a leaking valve stem seal from a leakdown test. The leakdown will only pinpoint the valve to head seal of the intake/exhaust valve, not the seal on the valvestem. The valves are both closed on a leakdown test.
A vacuum gauge hooked up at idle will possibly diagose valve stem seals, by showing unsteady vacuum and/or less than optimal vacuum, or by an unsmooth idle. But that might only tell you they don't leak at idle. At higher rpm is a different story.
Parent's lincoln, early 90's I think with 200k miles, had shot valve stem seals. It was common for that engine with high miles if I remember hearing correctly, and the tell tale sign was blue smoke out the exhaust only on acceleration. If the car was driving city in a lot of stop and go it would eat a quart in a day or in 100 miles. If you did a long steady highway trip and never accelerated hard, there was no oil consumption.

 

[moribundman]

Thanks, 1 FMF. So you're saying sticky rings would always cause lower compression?

You confirm what I thought about the "0%" leakdown result. It was a simplified diagnosis.


That car is most likely driven in a fairly sporty manner. If he drives anything like me, then he is also doing his fair share of engine brakinng, and yes, I'm aware of oil getting sucked right through the valve stem seals under those conditions.


What COA would be advised?

 

[mechtech]

The zero leakdown is obviously incorrect, but most likely is at a very good rate, and I would be happy.
Maybe a visit to the websites of those who have your particular vehicle would help pinpoint common oil consumption issues, and the viscosity that seems to work best.

 

[moribundman]

quote:

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Maybe a visit to the websites of those who have your particular vehicle would help pinpoint common oil consumption issues, and the viscosity that seems to work best.

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That's funny. We (12v engine Audi guys) come from Audiworld, where we have our own (performance) forum. Oil is not our main topic there, performance mods are.

From what I gather, the above result is typical for an 12 valve motor with this mileage, or mileage up to 200k miles. Oil consumption on our cars varies, but 0.5-1 qt per 1k miles is rather common. In my opinion, 5W-40 oils offer generally the best compromise in our engines.

 

[1 FMF]

quote:

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Originally posted by moribundman:
So you're saying sticky rings would always cause lower compression?

What COA would be advised?

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yes, sticking ring = not sealing against bore = loss of compression.

try valvoline maxlife or other hi-mile oil in 10w-40 that advertises to help valve stem seals. Can't hurt. If you're in to doing your own tuneups, some seafoam or other top end cleaner to dissolve and burn out any carbon in the cylinders that might be causing the hi compression. Do this before the oil change.