Pull all the sparkplugs out before you do the test---it is more accurate that way, and there is zero chance of the engine starting and cutting of your head (just be sure and label each plug wire with a peice of masking tape so you can get them back onto the correct sparkplugs. It is a good idea to have a friend hold the throttle wide open while the test is being done---you can tie it open, but its easy to forget and grenade your engine when you are finished the compression check. Be sure that the engine turns over at least 3 to 5 times for each cylinder being tested. Do use the starter---turning it over by hand 5 times will not give a true reading because of "leak-down" and it would take 80 complete turns of your engine to check 8 cylinders (a piston only comes up on the compression stroke every other revolution of the crankshaft)---a full days work.---if you are really into it, after you have completed the compression test and recorded all the numbers, give about 3 good squirts of 10w30 oil down each sparkplug hole, and do the test again. If there is a signifigant rise in each cylinders compression, this will indicate that it needs piston rings---the oil temporarily helps seal against compresssion leak-down. If the numbers stay the same, but overall compression is lower than factory spec. calls for, then this indicates that you need a valve job. The oil into the cylinder trick will not do anything to seal burned or worn valves.
A couple other tips: Run the engine first, get it up to operating temp. Disconnect power to the coil or distributor, especially if you have electronic ignition and are using the key to crank the engine--it can blow out the module cranking it with no plug wires connected.
Hey Squirrel---I never knew that, about blowing the module by cranking an engine without the plugs hooked up. Of course, my technology kinda stopped about the same time they quit making carburetors and HEI ignitions. Thanks for the tip----and your right---warm the damn car up first---I missed that.---Brian
Great thread! Two tools that always come out first...sparkplug socket with ratchet, and compression gauge...
it can blow the module and start a fire. i've seen the aftermath from someone leaving the plugs in the wires, must have looked like an 8 hole flamethrower. to be really safe pull the efi fuse..........but of course thats a new car.
What if you can't get the engine to fire? How will not warming the engine up affect the test? I have a small nailhead in my 55 buick that I'm trying to get started. I've replaced all ignition parts, and verified the timing and firing order are good. I'm squirting a little ether down the intake, so I know I have fuel. Still, won't even catch. My next step is to test the compression. Can't exactly warm the engine up if it won't fire.
BostonCat---It really won't make all that much of a difference. The main reason to bring up your engine to operating temperature is the thermal expansion of the metal engine. The difference in ring gap, piston fit, etcetera from ambient (room temperature) to operating temp (about 180 defrees F) will have a slight effect on compression readings, but not as much as some people would think. If you are getting spark at your plugs, and have tried a little gasoline down the carb, and it still won't fire, you are probably out 180 degrees on your timing.
Remember, #1 cylinder on a nailhead is the front RIGHT (p***enger) side cylinder bank plug, not the left side like SBC's. I agree, it sounds like you're 180° off on the dizzy.
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