Thread: 350 engine compression testing

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I looked under FAQ but could not find posts about compression testing. I have a 1974 Chevy Stepside 4x4 with a 350ci. About 10 years ago it was running fine then it felt like a cylinder was missing. I basically parked it and it's been sitting up ever since.

I know I'll have to get the old gas out of the tank and clean out the line but what I need to do if try to figure out if there is one bad cylinder. I have a compression tester but I've never used it or done a compression test. I think I am supposed to disconnect the main coil wire, take out all the plugs then put the tester in each cylinder, turning the motor over for a short time, then comparing the compression numbers.

Can I disconnect the fuel line so the old gas won't get into the carb (letting the pumped fuel run into a container) or do I leave it hooked up? Since the motor's been sitting do I need to put some oil or Marvel Mystery Oil or similar into each cylinder before I test? How much for each cylinder? I do know the motor is not seized.

Also, what would be considered a "normal" compression number for a good cylinder?

Thank you in advance for any help you can give!

RP

If you don't give it any gas you won't get any into the cylinders, but if you get it running it would be wise to clean out the tank, fuel lines, new fuel filter and possibly a new fuel pump.
I would look for all cylinders to be 130 and above after five or six rotations.

Standard operating procedure:
Ideally, start the motor and bring it up to operating temperature. A cold motor will show less pressure.
Remove air filter assembly
Wire the primary throttle blades wide open so the motor can breathe
Disconnect the coil hot wire to prevent a fire. Tie the wire back so it cannot arc against any metal in the engine bay.
Remove all spark plugs to make the starter's job easier to turn the crank.
Turn each cylinder through at least 5 compression cycles to max out the gauge. Each time you go through a compression cycle, the needle on the gauge will jump. ...It will stop moving after about 5 cycles.

As Ken said, most run of the mill motors will show around 130, although I have seen daily driver motors still haulin' the mail with 80 or 90 psi. On the other end of the scale, I have seen motors with well over 200 psi on pump gas with everything polished in the combustion chamber, a very tight squish/quench (think 0.030") and a "spot-on" tune. Crane cams says that somewhere around 165 is the limit for using pump gas with a run-of-the-mill street motor if you want to stay out of detonation.

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The advice given by both Ken and Tech is spot on for a true compression test to determine the overall health of the engine, but you're looking for a dead cylinder. If it were mine I think I'd pull the plugs and squirt about a tablespoon or so of Marvel Mystery Oil in each hole and let it sit for a day or two, just to get some lube on those dry cylinder walls. Since you know the gas is bad, and the tank & lines need to be cleaned out unhook the line at the inlet to the pump and shove a golf tee into the line, and while you're at it take off the line from pump to carb, too since it'll be coming off to clean, anyway. Then wire the throttle open and crank it like Tech explained. Don't worry to much with the value of the compression number you get, but look for consistency between holes, all pretty close to the same. If you have one outlier you know where to look for your problem.
Once you have your problem identified clean out your fuel system, fire it up and follow Tech's process to the letter to know where you are for engine health. Just my $0.02 on chasing the unknown.

Thanks VERY much, my 3 amigos!

I am printing your instructions out so I'll have them handy when I test.

Best,

RP

keep the spark plugs in order as you remove them because there will be evidence of what cylinder is missing----- the compression could be fine on all cylinders but if you mixed up the plugs you'd have missed which one--------