i just did a compression test this morning and was wondering - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


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Old 09-08-2004, 11:30 AM   #1
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Default i just did a compression test this morning and was wondering

cyl # 1 142 cyl # 2 140
cyl # 3 145 cyl # 4 132
cyl # 5 140 cyl # 6 145

Anybody else ever do a compression test? Does this sound OK, I am wondering why #4 is lower than the rest and is 140ish the right ballpark.
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Old 09-08-2004, 12:10 PM   #2
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They seem fine, #4 is a bit low though hard to get to?
Try a shot of engine oil in the cylinder and try again, the oil will create a better seal, if the compression does not come up it could be a valve or head gasket?
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Old 09-08-2004, 12:13 PM   #3
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Quote:
Originally Posted by PontiacDad
They seem fine, #4 is a bit low though hard to get to?
Try a shot of engine oil in the cylinder and try again, the oil will create a better seal, if the compression does not come up it could be a valve or head gasket?
and if it does, you just have some slightly worn rings.


not too shabby though. I put a junk yard head on my Shadow in an emergency, and the car didn't run to great... in the higher RPM'*, it would scream, but you could barely keep it running at idle. I did a compression check, and wouldn't you know it - 35 psi! yes, that'* right, I didn't forget the 1 there... it was amazing the little beasite even ran!
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Old 09-08-2004, 02:38 PM   #4
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Most importantly is that all cylinders are within 10% of each other. You can try the oil as mentioned above, and continue to monitor the situation, but in the meantime you have nothing to worry about.

Cheers,
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Old 09-08-2004, 07:53 PM   #5
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your compression is fine, yea one of your cylenders has lower compression, but its not that bad... i wouldnt worry about it
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Old 09-09-2004, 12:25 AM   #6
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Aren't our cars (Series I anyhow) supposed to have around 175-180 when new? I had a compression test done on Ol Blue last summer and she showed 165-170 across the board. When I get my plugs exchanged for the correct plugs I will get the tester out and check Goldmember'* compression.
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Old 09-10-2004, 09:21 AM   #7
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So is it supposed to be around 180 ? Im at 140 does that mean my engine is worn out?
Anybody have actual compression readings from your own cars?
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Old 09-10-2004, 09:36 AM   #8
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Quote:
Originally Posted by marksssei
So is it supposed to be around 180 ? Im at 140 does that mean my engine is worn out?
Anybody have actual compression readings from your own cars?
any car will wear out a little bit over time. i'd be happy with the readings you have, because as it'* been pointed out - they're all w/in 10% of each other, so any wear that has occured has been pretty uniform. just a broken in engine, tis all
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Old 09-10-2004, 12:35 PM   #9
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The online GM Service Manual doesn't give a specific psi number as optimal, but it does say;
  • The lowest reading should not be less than 70% of the highest reading.
    No cylinder reading should be less than 689 kPa (100 psi).

I would guess 180 psi to be quite high.

Cheers,
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Old 09-10-2004, 03:07 PM   #10
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Here'* a silly question:

Are all of the plugs supposed to be out while testing compression??
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