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Q: Why am I getting low compression?

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i did get the head surfaced and "only the head surfaced" I am getting 85psi in cylinder 1 125psi in cylinder 2 120psi in cylinder 3 125psi in cylinder 4. why would I get low compression all of a sudden? should I try the test with air tool oil? I have not changed the oil will that make a dif? thank you so much for your time.

My car has 207000 miles.
My car has an automatic transmission.

The low compression in cylinder 1 can be that your rings are not sealing or the head has a crack that did not get seen during the resurface. You can try the oil in the cylinder and see if the pressure increases to what the other cylinders have. This would be rings on the piston if it does. You can pressurize cylinder #1 with about 120 psi air pressure and see where the air leaks out of. If you hear air coming out of the crankcase through the oil fill cap hole then you have rings on the piston are not sealing. If the air comes out of the exhaust or intake then a valve is not sealing. If the air comes out of the radiator when cap is off then the leak is a cracked head. You should change the oil and filter to get any water that may have got in it out of the engine. If you would are still having a hard time diagnosing this issue by yourself, you can seek help by hiring a professional technician, such as one from YourMechanic. They will be able to accurately diagnose your rough running concern.

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