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Odd Compression Test Results

 
Old 10-07-2018, 02:43 PM
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Do you have a lot of experience doing compression tests? Its possible the results were not accurate. Seems like you would have some sort of symptoms with numbers like that.
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Old 10-07-2018, 03:08 PM
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Originally Posted by rpg51 View Post
Do you have a lot of experience doing compression tests? Its possible the results were not accurate. Seems like you would have some sort of symptoms with numbers like that.
I have plenty of experience with compression and leak down testing. It's pretty tough to screw up a compression test, simple stuff here. I owned DSMs for years, and those cars needed repairs constantly. I ran the compression test twice because the numbers looked so far off.
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Old 10-07-2018, 03:21 PM
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Yea. I agree if you have done these tests before your numbers are probably good. I guess something must be going on in there. Hope it turns out to be something easily dealt with.
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Old 10-08-2018, 10:38 AM
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Originally Posted by treimche View Post
Engine should be warm. I crank until the gauge stops increasing.
Last time I replaced my truck's plug when engine was warm, the threads were extra tight, almost stripped them, was scared to do it on my S
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Old 02-19-2019, 05:23 PM
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Finally revisiting this. I forgot to update the thread with my compression test results after the valve adjustment.

Compression test 9/20/18 before valve adjustment:
Cyl 1 - 190
Cyl 2 - 218
Cyl 3 - 220
Cyl 4 - 265

Compression test 11/16/18 after valve adjustment:
Cyl 1 - 216
Cyl 2 - 212
Cyl 3 - 230
Cyl 4 - 262

Also performed a leak down test a couple days ago:
Cyl 1 - 40% leak down
Cyl 2 - 37% leak down
Cyl 3 - 28% leak down
Cyl 4 - 3% leak down

When performing the leak down test, air was leaking up and out the spark plug holes, except for cylinder 4. The numbers all align between compression and leak down, and hearing air coming up out of the spark plug holes leads me to believe the rings in cylinders 1, 2, and 3 are all worn. It's surprising how much the compression in cylinder 1 changed after the valve adjustment, but it makes sense.

So, I'm not sure what to do from here. I have a PTuning turbo kit, AEM Infinity, fuel system, etc, all sitting here ready to be installed. But I also don't want to blow the motor up and wreck a brand new turbo. What would you do? From what I'm reading, you can't just bore the cylinders and put in new pistons/rings. Might just order a borescope and see if I can see what the cylinder walls look like before worrying about it. Maybe I will install the turbo kit and run low boost, but start stock piling parts and whatever is all needed for a rebuild while taking it easy on the tired stock motor.
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Old 02-19-2019, 09:55 PM
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Why did it leak through the spark plug holes? That's where you put the compressed air adapter correct, did it not seat correctly?
If there is a leak there then all measured values would be useless.
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Old 02-19-2019, 11:25 PM
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If it was the piston rings, you'd hear the air coming from the PCV valve, oil filler hole, dipstick tube.
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Old 02-20-2019, 02:43 AM
  #18  
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The classic #4 cylinder liner failure will result in high compression in number 4.

Check the cylinder with a scope, with piston at BDC. Liner failure will look like a wide black stripe.

Do not pull the full pump fuse for a compression test. Fuel will still spray into the cylinders and skew the readings. It is also unhealthy for the engine.

Disconnect the injectors instead.

Last edited by Billman250; 02-20-2019 at 02:51 AM.
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Old 02-20-2019, 04:11 AM
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Originally Posted by Billman250 View Post
The classic #4 cylinder liner failure will result in high compression in number 4.

Check the cylinder with a scope, with piston at BDC. Liner failure will look like a wide black stripe.

Do not pull the full pump fuse for a compression test. Fuel will still spray into the cylinders and skew the readings. It is also unhealthy for the engine.

Disconnect the injectors instead.
What's the mechanical/scientific reason behind having high compression with damage to the liner? interesting.

Yea, I'd say 1st step is to establish the condition of the bores before anything else.
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Old 02-20-2019, 04:33 AM
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Originally Posted by flanders View Post
Why did it leak through the spark plug holes? That's where you put the compressed air adapter correct, did it not seat correctly?
If there is a leak there then all measured values would be useless.
Let me explain a little better. With the leak down tester in cylinder 1, I can hear air escaping through cylinder 2 and 3 spark plug holes. It wasn't leaking through the spark plug hole where the tester was.
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