Compression numbers

[TrueBlue95GT]

I did a compression test and this is what I got:

5 65psi 1 130psi
6 142psi 2 130psi
7 130psi 3 110psi
8 130psi 4 135psi

Is this bad? If so, what does it mean?

 

[BlueOvalStangGT]

That is not looking good to me. How did you do it? Was the car warmed up/ were you at WOT for all cylinders? Did you have all the plugs out? How does your car run?

 

[TrueBlue95GT]

The car was still warm, I might have messed up by leaving the throttle closed. For cylinders 1-4, I still had the plugs in cylinders 5-8, but for the rest of the cylinders, all of the spark plugs were out. My car runs like absolute sh@t when it warms up.

 

[ARVtheHPjunki]

I had numbers similar to yours on my blazer last year and it turned out to be a burned exhaust valve seat. Pull the valve cover off and look for obvious stuff first, then work your way to more severe probs.

 

[Venom351R]

shouldent see anything below 130 . Mine were 130 when it was cold so I know Im all set.

 

[HISSIN50]

i personally dont think absolute numbers matter so much; as you are seeing, there are enough variances in methodology to skew the numbers. i look at relative relationships.

did you do a wet and dry test? the wet aspect might help you figure out what is wrong in cylinder 5, as well as shedding some insight into the condition of the other cylinders. a leakdown test would also help.

good luck to you.

 

[TrueBlue95GT]

What are wet and dry tests? I just put the adapter in and cranked for 3 seconds and wrote down the sustained pressure.

 

[helix007]

there should be no more than 10% variance between cylinders

 

[BlueOvalStangGT]

For 3 seconds? I thought you had to keep cranking till it stopped goign up. Oh and did you have a battery charger when you did it?

 

[VibrantRedGT]

Not good man. Might need head gaskets. Remove radiator cap, start the car and if you see lots of bubbles then it's head gasket time. Hopefully it's just head gaskets and not something fatal.

 

[HISSIN50]

What are wet and dry tests? I just put the adapter in and cranked for 3 seconds and wrote down the sustained pressure.

dry test is done first. then add a TINY amount of oil into the spark plug hole (if you add too much you run the risk of hydrolock). the recheck compression. if the number goes up a good bit, that suggests worn rings.

i also crank it till the reading maxes out (within reason).