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Old 02-13-2018, 03:19 PM   #1
ljgsxr750
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Default Compression questions

Hello, I have been searching and reading a lot of information on the forums. Just a few questions, I just did a compression test on my 06 STI and the results came back 125, 123, 125, 125. The engine was not at operating temperature, the car has been sitting since it last overheated. Short story, I drove the badass home from work as usual with no issues, after being home for an hour or so I started her up to go pick up my son. On startup she struggled, sounded like she was missing so i immediately shut her down. Started her back up with no issues, drove to the 76 less than a mile away and filled up. On startup at the gas station, it stumbled so i turned it off and then back on. This time a slight puff of whiteish smoke came out then stopped. On the way to get my son it overheated. So I took her home and made sure my coolant levels were topped off etc, started it back up, no issues overheating at idle for 5 mins or so, as soon as i got down the street instantly overheated. popped the hood to find coolant boiling out of the overflow. Now at this point i assumed it was a Headgasket, however i dont have milky oil, nor do i have crud on the top radiator cap. The new fluid doesnt look contaminated either. I have just ordered a radiator pressurizer kit and a leak down kit. My question.... Are those compression numbers good and would that compression test have told me if I had a headgasket leak? I am by no means no mechanic so please dont flame me for lack of knowledge/experience. horrible shop experiences have led to my DIY chronicles.

And there is no power loss when driving, just idle issues at startup and while idling sometimes.

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Old 02-13-2018, 03:36 PM   #2
STi320
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I am not a mechanic so I don't have any concrete info for you but what I am confident in saying is this:
Your compression numbers all being within 2 psi of each other is good.
Your compression numbers all being low is not so good. When all four cylinders are nearing the 120 level its usually time for a refresh.
A compression test wont necessarily tell you that you have bad head gaskets.
You most likely wont feel a loss of power if the head gaskets are leaking unless its really bad.
When my stock block started to go, lost cylinder #4, it stumbled on start-up and while at idle, I could feel the car shaking, but my comp numbers were 130, 130, 135 and 60.
Did you get a strong wiff of coolant the last time you drained the oil?
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Old 02-13-2018, 04:13 PM   #3
Valarius Starchaser
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Default Compression questions

Do a leakdown test in combination with a compression

Edit how many miles?

Last edited by Valarius Starchaser; 02-13-2018 at 05:08 PM.
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Old 02-13-2018, 05:04 PM   #4
ljgsxr750
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Quote:
Originally Posted by STi320 View Post
I am not a mechanic so I don't have any concrete info for you but what I am confident in saying is this:
Your compression numbers all being within 2 psi of each other is good.
Your compression numbers all being low is not so good. When all four cylinders are nearing the 120 level its usually time for a refresh.
A compression test wont necessarily tell you that you have bad head gaskets.
You most likely wont feel a loss of power if the head gaskets are leaking unless its really bad.
When my stock block started to go, lost cylinder #4, it stumbled on start-up and while at idle, I could feel the car shaking, but my comp numbers were 130, 130, 135 and 60.
Did you get a strong wiff of coolant the last time you drained the oil?
Ah were those numbers at operating temp or on a cold test? I didnt get a strong wiff of coolant when I drained the oil. Also had the car idling with the oil cap off to see if it puffed smoke and it didnt.

Quote:
Originally Posted by Valarius Starchaser View Post
Do a leakdown test in combination with a leak down

Edit how many miles?
The motor has about 22k on it since the build.
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Old 02-13-2018, 05:09 PM   #5
Valarius Starchaser
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Rings maybe didn’t get beded properly how’s oil consumption?
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Old 02-13-2018, 05:17 PM   #6
ljgsxr750
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Quote:
Originally Posted by Valarius Starchaser View Post
Rings maybe didn’t get beded properly how’s oil consumption?
The oil consumption isn't too bad, usually only about 1/4 of a quart low after 3k running 15w50 Redline Synthetic.
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Old 02-13-2018, 09:49 PM   #7
Charlie-III
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Quote:
Originally Posted by STi320 View Post
I am not a mechanic so I don't have any concrete info for you but what I am confident in saying is this:
Your compression numbers all being within 2 psi of each other is good.
Your compression numbers all being low is not so good. When all four cylinders are nearing the 120 level its usually time for a refresh.
A compression test wont necessarily tell you that you have bad head gaskets.
You most likely wont feel a loss of power if the head gaskets are leaking unless its really bad.
When my stock block started to go, lost cylinder #4, it stumbled on start-up and while at idle, I could feel the car shaking, but my comp numbers were 130, 130, 135 and 60.
Did you get a strong wiff of coolant the last time you drained the oil?
Good basic info, I have no real issue with it.
Cold engine and/or lowish battery can yield low numbers.
I have yet to see a failed Subaru HG that mixed oil and coolant. Pushing into overflow tank?, yep.

Quote:
Originally Posted by Valarius Starchaser View Post
Do a leakdown test in combination with a compression

Edit how many miles?
Leakdown may help, worth a shot.

Last edited by Charlie-III; 02-14-2018 at 08:40 AM.
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Old 02-14-2018, 02:01 AM   #8
PDXREALTOR
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I had compressions numbers that low on cold motor, but bigger spread, yet still within 10% and leak down numbers in the 5-6 percent range. Mechanic said because leakdown test was good he didnt bother trying things like oil in the low cylinder to see if compression went up.
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Old 02-14-2018, 08:34 AM   #9
STi320
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Quote:
Originally Posted by ljgsxr750 View Post
Ah were those numbers at operating temp or on a cold test?
That was at or near operating temp.
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Old 02-14-2018, 04:39 PM   #10
ljgsxr750
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Quote:
Originally Posted by Charlie-III View Post
Good basic info, I have no real issue with it.
Cold engine and/or lowish battery can yield low numbers.
I have yet to see a failed Subaru HG that mixed oil and coolant. Pushing into overflow tank?, yep.



Leakdown may help, worth a shot.
Yeah I ordered a leak down tester and radiator pressurizer that should be arriving today and tomorrow. I am well prepared for it to be a headgasket, because it was the initial thought, however the compression numbers made me question myself. Is it possible that it could be a clogged radiator? Will the pressure test confirm the radiator is working correctly or is that something that would need to have the car running to diagnose?
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Old 02-14-2018, 04:41 PM   #11
ljgsxr750
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Quote:
Originally Posted by PDXREALTOR View Post
I had compressions numbers that low on cold motor, but bigger spread, yet still within 10% and leak down numbers in the 5-6 percent range. Mechanic said because leakdown test was good he didnt bother trying things like oil in the low cylinder to see if compression went up.
If you dont mind me asking, what issues were you having that made you do the leakdown/compression test?
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Old 02-14-2018, 05:10 PM   #12
Charlie-III
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Quote:
Originally Posted by ljgsxr750 View Post
Yeah I ordered a leak down tester and radiator pressurizer that should be arriving today and tomorrow. I am well prepared for it to be a headgasket, because it was the initial thought, however the compression numbers made me question myself. Is it possible that it could be a clogged radiator? Will the pressure test confirm the radiator is working correctly or is that something that would need to have the car running to diagnose?
Neither an engine compression test, or leakdown, or cooling system pressure test will say squat on the cooling thermostat or clog/unclog of the radiator.

For the most part, all they will say is......does that "system" hold pressure or not, compression test (wet or dry) only determines if the cylinders can make correct compression.

Period.

Sorry.
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Old 02-14-2018, 05:39 PM   #13
ljgsxr750
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Quote:
Originally Posted by Charlie-III View Post
Neither an engine compression test, or leakdown, or cooling system pressure test will say squat on the cooling thermostat or clog/unclog of the radiator.

For the most part, all they will say is......does that "system" hold pressure or not, compression test (wet or dry) only determines if the cylinders can make correct compression.

Period.

Sorry.
Got it.
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Old 02-14-2018, 05:53 PM   #14
PDXREALTOR
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Quote:
Originally Posted by ljgsxr750 View Post
If you dont mind me asking, what issues were you having that made you do the leakdown/compression test?
I blew a half Quart of oil out of the head ports / home made AOS on a 120mph long sweeping corner.

Before I knew/understood what happened I panicked and went straight for compression and leak down tests.
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Old 02-14-2018, 06:21 PM   #15
Charlie-III
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Quote:
Originally Posted by ljgsxr750 View Post
Got it.
NP......

Sorry.....

I will say, a cooling system pressure tester is what I use to test for most blown HG's (combustion chamber to cooling system).

I have posted this before, if you want, I can post here. I'm just lazy and don't want to write again......
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