Compression check

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Compression check

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Compression check - 4/12/2008 10:48:22 AM

1986c10

Posts: 15
Joined: 11/26/2007
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I did a compression check on a 305 in a '86 C10, and i couldnt find the PSI that it should be making. 5 out of 8 cyl. were at 150psi while 2 others were down at 120psi and the last was the worst 95psi.

Any help on this would be great, Thanks
Mike

Post #: 1

RE: Compression check - 4/12/2008 3:05:53 PM

NoBrakes

Posts: 20
Joined: 4/9/2008
Status: offline

I am no expert but try this. And good luck

Interpreting Compression Test Results If all cylinders are performing equally and engine performance is acceptable, the engine passes the test.

Variations in compression between cylinders should be no more than 20%.

When two cylinders next to each other have low compression, a blown head gasket is usually indicated.

A blown head gasket.

One or several cylinders with low compression and no apparent pattern of loss often indicates burned exhaust valves.

Rough idling is a symptom.

At higher rpm, the rough running from the burned valves disappears.

NOTE

On an engine at idle, a burned exhaust valve will cause a dollar bill to be sucked against the end of the exhaust pipe every time the bad cylinder's piston has an intake stroke.
Wet Compression Test If any cylinders show poor results, perform a wet compression test.

Squirt about a tablespoon of oil into each low cylinder.

The oil makes a seal around worn rings, boosting the compression reading.

When cylinder readings that were low increase to normal during a wet test, a piston ring problem is indicated.

NOTE

Adding too much oil takes up volume and raises compression. When the compression test is completed, install the spark plugs. Be especially careful not to strip the threads on an aluminum cylinder head and use anti-seize compound on the spark plug threads. To more accurately diagnose engine problems, use a cylinder leakage tester to perform a cylinder leakage test on the engine.

NOTE

A compression test will not tell the condition of oil control rings, only compression rings.

(in reply to 1986c10)

Post #: 2

RE: Compression check - 4/12/2008 3:11:36 PM

NoBrakes

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Joined: 4/9/2008
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Make sure you remove all spark plugs, have carb open, & distributor unplugged.

(in reply to NoBrakes)

Post #: 3

RE: Compression check - 4/12/2008 5:57:59 PM

1986c10

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Joined: 11/26/2007
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All that was done, we figured just by the rough running that a head gasket was toast, so we did the test just to verify... is there anything else that can be damaged as a result of the destroyed head gasket other than the burnt ex. valve?

(in reply to NoBrakes)

Post #: 4

RE: Compression check - 4/12/2008 7:14:42 PM

shawnvw

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Joined: 1/18/2008
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If enough antifreeze gets into the oil you could spin a bearing from lack of lubrication

(in reply to 1986c10)

Post #: 5

RE: Compression check - 4/13/2008 8:18:43 AM

1986c10

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Joined: 11/26/2007
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quote:

ORIGINAL: shawnvw

If enough antifreeze gets into the oil you could spin a bearing from lack of lubrication

What would be an indication of a spun bearing?

(in reply to shawnvw)

Post #: 6

RE: Compression check - 4/13/2008 6:28:28 PM

NoBrakes

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Joined: 4/9/2008
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possibly a knocking sound that becomes more rapid when you accelerate

(in reply to 1986c10)

Post #: 7

RE: Compression check - 4/14/2008 8:21:55 AM

1986c10

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Joined: 11/26/2007
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Ok...well guess I'll just play it safe and pull the engine and check it all out. right now its running really rough and dont want to take my chances and have something get really damaged.

(in reply to NoBrakes)

Post #: 8

RE: Compression check - 4/14/2008 7:30:32 PM

NoBrakes

Posts: 20
Joined: 4/9/2008
Status: offline

Just an idea --- but instead of pulling motor how about you pull the heads --- have them shaved and valves pressure tested at
a local parts store. I dont think it is to expensive -- install all new gaskets and tune up --- Can you even notice water in your oil right now? If not then this could be the route to go. I could be wrong with your bottom end but you need to have your head or heads done anyway. Good luck and I would like to hear on what you did and found out......

(in reply to 1986c10)

Post #: 9

RE: Compression check - 4/15/2008 5:37:43 PM

1986c10

Posts: 15
Joined: 11/26/2007
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I havent checked the oil yet, ill do that next time im able to. Ill be sure to let ya know how things go, might not be for awhile as i have exams coming up, but soon as things happen ill let ya know.

(in reply to NoBrakes)

Post #: 10