How bad is this compression test?

[Arl37]

72 C10 307 engine. 7 cylinders between 108 and 120. 5 are close to 120. One, number 3, is 90. What do you think. Really dont want to do a crate at this time but dont want to get it painted and then have engine crap out.
This is the cleanest engine I have ever seen with the valve cover and intake off. Not a drop of sludge or gunk anywhere.

[webfoot]

Did you do it with the engine warm? I'd try that one again and see if you get the same reading. If so, something is wrong.

[earlthegoat2]

Though the 90 is a pretty good deviation, I would still consider that acceptable unless you are a perfectionist. True, something may be wrong, but the cylinder is not necessarily going to crap out in the next month just because of this reading.

[terry b]

shoot a little oil in number three and retest. Let us know what happens.

[trac209]

If the engine isn't burning any oil and runs ok I would think you'll get many miles out of it. You could try adjusting the valves you might have one a bit tight and losing some psi.

[Arl37]

Shot oil in it and it popped right up to 120. By way of background I cant check it when warn at this time. Exhaust off awaiting new intake. It has been siting cold for a week. The first time I check # 3 it registered 110 and then all later attempts were at 90. Waht is the reccomended range for this motor.

[1972RedNeck]

When you do get it running, I'd get some seafoam and slowly dump it down the carburetor at idle until it dies. Then empty the rest of the can in the carburetor and roll the motor over a couple of times. Wait 30 minutes. Start the truck outside and take it for a drive right down main street.

This should loosen up any stuck rings and carbon buildup and help get the pressures up.

[terry b]

If cyclinder pressure went up after you shot oil in the cyclinder the problem is with the cyclinder wall or the piston rings. How long has the motor sat? Could be a stuck ring. I dont think the valves are the problem.

[68gmsee]

If it's not burning oil and otherwise running okay and you don't want to spend time and money, then keep going until you have to do something.

The reason I say this is because once you tear into the engine, you may as well go all out and do a complete rebuild otherwise you will not be happy with it.

[WAAF]

stuck ring.. maybe.. leave the seafoam on the shelf.. grab the marvals mystery oil..
add a qt to the oil.. 2 qt's if you really want to clean it out..
drive around in low (after it's warm)and let the engine do the braking..
then wot and engine braking.. then drive around some more normal like and then do it again.. do 4 sets of 5 and then do a normal oil change.. if it's a stuck ring.. it should be free now..
good luck.
the seafoam works also.. but is not an oil and I really don't like stuff like that getting in the crankcase(oil) and then running through the engine..
most times it causes more problems than it solves.. opening a few can of worms on it's way to fixing one..

[d4xycrq]

Art,

If I were you, I'd have left the engine alone and run it for weeks, or even months before deciding to do mods to it. Only then can you know if there are other issues that render your mods a waste of money.

The 90 PSI on one cylinder is not enough to hold you up. I bet there are a stuck rings that resolve themselves once run for awhile.

All that said, your are not me. It's your truck. It's your motor. I presume this is not your daily driver - and in that case, no matter which way you go is just as good as the other.

To answer your question about compression - textbook answer is 140 PSI would be nominal, and down to 125 on a given cylinder would be the limit.

Ray