Compression Test BPT

[nolig2278]

I did the compression test last night:
As my wife cranked it would hit about 140 psi for each plug then immediately drop back down and back to 140. Is the dropping normal?

She did not hold the gas pedal down, just the clutch and held the key at the crank position.

Thanks

[JWMotorsports]

You should hold the throttle wide open during cranking...also recommend unplugging disty or cam sensor to stop fuel injectors from pumping fuel into the cylinders during testing. This will help get a more accurate reading as fuel will cause the rings not to seal as well by disolving the oil film on the cyl. walls.

You should only have to turn the motor over about 4-6 times. Count how many times you hear the motor turning over as you watch the needle rappidly rise on the compression tester. Once the needle stops rappidly rising that is where you need to stop turning it over and read the gauge...it should also hold that pressure for a while until you bleed it.

I belive 130psi is the rebuild level and the cylinders should not very by more than a couple psi (more than 5psi variance between cylinders can be an indicator of a problem which will need to be diagnosed via a leak down test).

A strong motor should really post more than 150+psi. If I remember correctly my drag car was somewhere in the 180-185psi range with the cylinders showing virtally no variance between them on the fresh race motor. Every engine is different as compression ratio has a decent effect on scores. You should consult a Haynes or Chilton manual for your motor's specs.

P.S.

Your engine should be at operating temperature before testing as well.

[nolig2278]

what does it mean when the compression tester does not hold the pressure?

i haven't done the test with the engine hot so maybe that will make a difference

i was able to see into the cylinders and 1 3 4 are mostly a silver color while 2 appears to have black on it

the top of thread (or bottom of spark plug well) on 3 and 4 has what appears to be oil.

the car does run fast as hell and with about 12 psi but i do have smoke

[JWMotorsports]

Means you could have a defective gauge. The gauge is supposed to hold pressure until you bleed it off.

Testing while hot should yeild different (better) results because the pistons have swelled up from heat creating a better ring seal.

If it looks like you have oil on the spark plugs this could mean many different things. I'd suggest doing a leak down test on it to further diagnose the problem. It could be the rings or a valve stem seal ...I'd guess a valve stem seal may be leaking if it is only one or two plugs with minimal oil residue...allthough 2 adjacent plugs could point to a possible head gasket problem. This is why a leak down test is needed. If oil is on all the plugs it would likely be excessive crankcase pressure.

It likely smokes because of the oil your seeing in cyl. 3 & 4. I'm assuming the smoke your seeing has a bluish tint to it? If it is white you could have either a head gasket problem from a warped head, a cracked head, or a cracked block. Again a leak down test will help identify the problems. I'm guessing maybe valve stem seals are leaking if your compression test shows all 4 cylinders are pretty close across the board. How much oil residue are you seeing? Any pics of the spark plugs I can see?

[nolig2278]

i stupidly didnt take any pics
but i did replace the recently installed cheap pcv with a custom home depot one i had and
now there is no smoke.

am i to assume a cheap pcv valve allows boost to push oil through the valves or does it prevent oil from retruning to the pan by causing extreme pressure in the entire oil system? is it jamming, cause there is pressure from cumbustion chambers that is not escaping past the pcv?

thanks

i will test my car out more on monday so i will then know for sure