Purpose of doing a wet compression test?


Bgunner

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I have finally collected every thing I need to do the compression tests on my 94 3.0L w/ 147,xxx and I have a few questions I'm hoping someone can answer for me. The first being: What is the purpose of doing a wet compression test?

I know I am supposed to look for a large difference between cylinders but if there is a difference on the wet test between cylinders what does this tell me?

I purchased a pump oil can to squirt oil in cylinders but roughly how much should I squirt in the cylinders to do the test properly? And what type of oil should I use, regular motor oil?


I have never had a motor with weak compression so this is new to me. The reason for the testing is I am trying to track down a shake while under heavy loads. No CEL, Newer ignition system including distributor and coil but acts like an intermittent skip. Fuel pressure is fine so I am thinking injectors but want to be sure the cylinders are fine before dropping the cash for new injectors. The last time I used my compression tester was about 13 years ago when I lost an injector in a Saturn to make sure it was an injector issue.

Any tips and tricks you may have are welcomed also. Thanks for your time and advise.
 
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Dirtman

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The point is to determine if the compression issue is due to worn rings or if the valves are leaking. Adding oil in the cylinder will seal the rings so if you add oil and the compression is still low its a good indication that the problem is a bad valve. If the compression goes up after adding oil then you have bad rings.
 

kimcrwbr1

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A leakdown is more accurate IMO the wet test is a waste of time. You should at least lap the valves and replace stem seals any time you pull the heads.
 

Bgunner

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A leakdown is more accurate IMO the wet test is a waste of time. You should at least lap the valves and replace stem seals any time you pull the heads.
I don't have a leak down tester this is why that test wont be done.

Compression test results:

Cylinder 1: Dry 165 psi Wet 205 psi
Cylinder 2: Dry 175 psi Wet 200 psi
Cylinder 3: Dry 155 psi Wet 197 psi
Cylinder 4: Dry 160 psi Wet 200 psi
Cylinder 5: Dry 160 psi Wet 220 psi <- issues? Roughly 10% difference I think this is within tolerances, yes/no?
Cylinder 6: Dry 170 psi Wet 200 psi

Wet test performed with motor oil.

Fuel pressures:

KOEO: 37.5 psi
KOER: 31.5 psi
KOER blip the throttle: drops to 28 psi then jumps to 40 psi then levels back out at 32 psi. Holds steady at 32 psi @ 2000 rpm no load on engine. Is dropping to 28 psi an issue since the specs call for 30-40 psi?

I'm thinking I need to do a vacuum test to check the cat next. Opinions on this since I do have a rattle in the exhaust?
 

RonD

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Your average compression is 163
You don't use the highest or lowest result, add up the remaining 4 and divide by 4 = 163
So top is 179, 163 +10% = 179
Lower is 147, 163 - 10%

Dry Compression is OK

WET test always increases compression test results unless there is a leaky valve, then it will barely go up, so you are good.

Yes, vacuum gauge is one of the better "low tech" tools that can tell you ALOT about an engine.
Including partially blocked exhaust
 


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