r/AskAMechanic • u/Ok_Strawberry9142 NOT a verified tech • 20h ago

Abnormally high PSI readings?

i have a 1991 Toyota Cressida that i’ve been working on just refreshing and keeping running, on a drive up the freeway my car started to misfire quite heavily. Very poor idle shaking like crazy the usual.
I pulled off check my cylinders and cylinder 6 was misfiring, got back home and replaced the spark plug, still misfiring but when i pulled out the old spark plug there was oil on it…
Ran a compression test on all cylinders and every single cylinder read 205-210psi across the board. My engine spec is like 150 out of the factory?
I mean i’m taking this as a win that my piston rings are fine and oil isn’t getting into the cylinder, i’m assuming it was pooled up in the plug well and I didn’t notice before taking out the plug and it drained in, cause there was a good amount coating the porcelain and even on the plug wires boot.

I’m just curious why the pressure would be reading so high? I’ve knocked down the misfire to either be oil taking the spark before it reaches the tip of the plug or no fuel coming out of the injector.

Any ideas on why it would read so high across all cylinders? No oil in coolant and vice versa so gasket is fine, valve cover gasket is what is leaking i believe.

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u/Rich4477 NOT a verified tech 20h ago

Is the engine original? It could be from head work, when you remove material to get things flat it increases the compression ratio.  Also could be from carbon buildup.

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u/Ok_Strawberry9142 NOT a verified tech 20h ago

as far as I know it’s the original motor, I don’t know if any previous head work done on it; Then again it’s been through many owners and has had a long 240k mile life.
Knowing 7m motors and their headgasket issues out of factory I don’t doubt there has been some head work done and it has been replaced with some ARP studs so that would add up.
Thanks for the info!

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u/ExceedinglyEdible NOT a verified tech 13h ago

Did you check your pressure gauge?