360 Compressiont test ....
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360 Compressiont test ....
Today I tested the cylinder compression on my 1969 360 engine. This truck sat for a few years. I only sprayed a little carb cleaner in and on the 2 barrel carb... drained the old gas from the tanks and put new gas in .... primed the carb and shortly it started. Missing bad at first, ran it for a minute or two, rev'd it us and it really started running pretty decently. Good idle and slow take off were good... but it seemed to have a cylinder missing with aggressive acceleration. I replaced all spark plugs, ( everyone was a nice light coco color at the tip)
BUT ... the compression test has the #5 cylinder at just 25 pounds compression ?? All the rest are 100 psi to 115 psi. I am wondering if this maybe a bad or weak valve. ?? The truck has unkown miles because the speedo cable was disconnected by the previous owner, since the truck was brand new (previous owner has pasted away). The speedo mileage shows 23,000 miles. Actually, from the condition of the truck it might not be too far off. The truck sat for years, the owner only used it when he needed to 'go get some wood' . . . .
Bad Valve?? Bad Cylinder?? Any educated guesses ??? THANKS IN ADVANCE. Jim.
BUT ... the compression test has the #5 cylinder at just 25 pounds compression ?? All the rest are 100 psi to 115 psi. I am wondering if this maybe a bad or weak valve. ?? The truck has unkown miles because the speedo cable was disconnected by the previous owner, since the truck was brand new (previous owner has pasted away). The speedo mileage shows 23,000 miles. Actually, from the condition of the truck it might not be too far off. The truck sat for years, the owner only used it when he needed to 'go get some wood' . . . .
Bad Valve?? Bad Cylinder?? Any educated guesses ??? THANKS IN ADVANCE. Jim.
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Joined: Apr 2002
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From: Coal country
Watch how the pressure goes up while cranking. You may also want to remove all the spark plugs to improve cranking speed if the battery or starter are weak. Don't lose your plug wire sequnce though.
On a cold engine, check the cylinder's compression, and see if it goes to the maximum value it will reach immediately or gradually over several compression strokes.
If it goes up to the max over 5 or more strokes, it is probably a valve.
If it goes up quickly to the maximum it will go within 2 or 3 strokes, it is probably a ring problem.
Next, squirt some motor oil into the cylinder, and re-test. If the values change significantly, it is a ring problem.
One exception to this, is if a ring has broken or an object of some sort has caused a deep score in the cylinder wall. Usually, the oil test won't make much if any difference.
Once the engine is fully warmed up, and been driven at least a little bit, redo the checks.
Post back so we can see what you find.
On a cold engine, check the cylinder's compression, and see if it goes to the maximum value it will reach immediately or gradually over several compression strokes.
If it goes up to the max over 5 or more strokes, it is probably a valve.
If it goes up quickly to the maximum it will go within 2 or 3 strokes, it is probably a ring problem.
Next, squirt some motor oil into the cylinder, and re-test. If the values change significantly, it is a ring problem.
One exception to this, is if a ring has broken or an object of some sort has caused a deep score in the cylinder wall. Usually, the oil test won't make much if any difference.
Once the engine is fully warmed up, and been driven at least a little bit, redo the checks.
Post back so we can see what you find.
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Thanks BanjoPicker66 !! That is just the good advice I was hoping to get for someone on this site. Thanks for taking your time to help me. Sounds like Very Good advice. I will do these checks and let you know how it goes. It may take me a couple days to get the time. My weekends are busy work days for me. Thanks Again Mr. BanjoPicker66. JIM
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All good advice gentlemen. Thanks I will be checking it out with your help again.. Thanks Loads!
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From: South Central Utah
Doesn't matter how you do the test, one cylinder is lower than the others. Being a 360 I would bet a bad valve.
In 1973 I worked at a Ford dealership and saw a brand new F-250 4x4 with 7 miles on it and a burnt valve. I have seen very few bad pistons and/or rings in just one cylinder in a 360.
In 1973 I worked at a Ford dealership and saw a brand new F-250 4x4 with 7 miles on it and a burnt valve. I have seen very few bad pistons and/or rings in just one cylinder in a 360.
