low compression

[bobtail]

I just ran a compression check. cylinders 1 thru 7 have 120psi and number 8 only 3psi. what is the next step?

 

[bmc69]

With a number that low, and the rest "normal", I'd be suspecting a bad/burnt valve. I'd pull the valve cover first thought, and watch the valve motion on number 8 to see if it looks normal when turning the engine over.

 

[pcf_mark]

Check that the valves are working - maybe you get lucky and a nut has backed off the rocker or a cam lobe went flat. Then I would repeat your test but squirt some oil in the cylinder. If psi goes up appreciably then you may have a ring problem and you need to look deeper. If it does not you have a burnt valve(s) or head gasket.

 

[Rustytruck]

Take cylinder # 8 and put it on top dead center with both valves closed take a spark plug thread to air fitting adapter. feed compressed air into the cylinder. The air will either leak out the intake, the exhaust or the crank case. if there is no leak then one of the valves is not opening suspect cam and valve train. if it leaks out the intake suspect intake valve or spring. if it leaks out exhaust suspect exhaust valve or spring and if it leaks into the crank case suspect piston or rings. there will always be some leakage on a used engine but where is it leaking the most.