valve problem quick question
 

hey guys well im sure i hav a valve problem on cylinder 4
compression on crank reads 140 but a compression retest while the cars running reads 50

i dout its a piston ring so im fixing up to buy the stuff i need

i no for a fact a head gasket but

wat im asking is is it a bad jus to change valve springs n stem seals on jus cylinder 4 cuz im really low on cash but if i hav to ill put a wait but get back to me ne one plz cuz i was thinkn jus
to buy a
head gasket
4 valve springs n stem seals jus for piston 4
n is there ne thing else i am missing
appreciate ne help thanks

You don't test compression with the engine running. Ever. It is best to test it with the engine warmed up. But to do that you run the engine for a while, then turn it off and take all the spark plugs out. Then test the compression in each cylinder one at a time while cranking.

If cylinder 4 is dead due to a faulty valve, that's usually a crack or bend in the part on the cylinder end of the valve and replacing the springs and seals won't help. Your test getting 140 psi seems to rule that out. Another thing that can happen is if the valve adjustment is too tight, the valve won't close fully after the engine warms up. You should though consider other reasons why #4 is not working.