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Old 10-02-2011, 10:29 PM   #1
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Join Date: Mar 2008
Location: Waterloo IA
Posts: 101
#1 #2 #3 low compression in L6 250

In my 66 GMC I did a compression test today because it would stall out many times when starting in 1st gear. The results:
#1: 90psi
#2: 85 psi
#3: 75 psi
#4: 120psi
#5: 120psi
#6: 120psi

I was thinking the rings are leaking in the first 3 cylinders but after doing some searching ive found that it could possibly be from stuck valves or possibly even flattening of the camshaft.

Any help would be greatly appreciated.
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Old 10-02-2011, 10:39 PM   #2
Fred T
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Join Date: Aug 2002
Location: Bel Aire, KS
Posts: 6,265
Re: #1 #2 #3 low compression in L6 250

Best way to tell is to set the cylinder with both valves closed and connect compressed air to the spark plug hole. The listen for the air out the exhaust or crankcase.

Another method is to repeat the test with a small squirt of gear oil in the cylinder. That will usually seal the rings enough to raise the pressure if rings are the problem.

There is no such thing as too much cam...just not enough engine.
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250, 292, compression, cylinder, inline

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