I have a 302-cu.-in. V8 engine that's run 106,000 miles. The left cylinder bank shows 142-psi compression on all four cylinders. The right bank is as follows: 142, 127, 124 and zero to 90 psi. The last cylinder swings from zero to 90 psi with every turn of the motor. Is this a valve, valve spring or head gasket thing? It seems that everyone I ask gives a different answer. There's no water in the oil, and no oil in the water. Is there any way to check without tearing off the cylinder head? What is the normal compression pressure for this engine?
TERRY HALL
VIA INTERNET
I suspect you're going to wind up removing one cylinder head, Terry. At least.
In theory, the correct pressure is variable. The exact pressure reading you see on your compression gauge should be equal to atmospheric pressure multiplied by the engine's compression ratio. You'll rarely see this, for reasons I'll detail later. More important is the consistency of the readings between cylinders. I'm worried that your battery got discharged or your engine cooled off before you completed your readings.
Here's how to do a compression check correctly. Start with a cold engine. Crack all the spark plugs loose, turn them out five to six turns, and then tighten them to 5 to 10 ft.-lb. Start and warm up the engine to normal operating temperature. Be sure the battery is in good condition and fully charged, as you want the engine to spin over rapidly until the end of the test.
Disable the ignition in a fashion that won't damage the electronic-ignition amplifiers. (You'll have to consult the appropriate service manual for this.) Then, remove all the spark plugs.
Prop the throttle open with a wooden stick or the plastic handle of a screwdriver. This is important, because you want the cylinder to get all the air it needs to completely fill.
Install the compression tester in one cylinder, and turn the motor over for six to eight compression strokes, or until the gauge stops climbing. Record the gauge reading and move to the next cylinder promptly to make sure you finish before the engine cools significantly.
This will give you consistent readings. Because of variances in the barometric pressure, speed of cranking, altitude above sea level and what brand of gum you chew, the pressure will be lower than the theoretical maximum. No matter. What's important is the variation between cylinders, and 20% is enough to constitute a problem.
In your case, you have a problem with one cylinder. You could turn the engine to Top Dead Center on the compression stroke for that cylinder and put compressed air back into the cylinder to see where it's leaking, such as back into the intake, out the exhaust or into the crankcase. Nevertheless, this will likely be an exercise in futility. If you've taken your compression test correctly, that head's probably going to have to come off.
I googled and got this if it will help you to determine how to do it correctly. But from what is there it seems 100 is low but most important is that they are all around the same. If it is low the engine is worn. Good way to tell is if you pull the pcv out and pop the oil cap off. If you got alot of smoke coming out where you put the oil in the rings are going bad and the engine is worn out.
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