I just replaced head gasket in 2002 accord and after doing so I did a compression check and the no. 4 piston is only at 90 psi. while the others are about 160. There is a lot of blow back coming from oil cap when I take it off. I put a lid full of oil and compression came up to 240 and I let a little oil blow out and checked again and it was about 150. The car does not smoke it just runs rough when idling. What I'm wondering is it possible that the compression ring cracked or wore down much more than the other cylinders. I don't really see how but could it be the head gasket and not the rings? The car is not using water or oil, and they are not mixing. If it is the rings how hard are they to replace without taking engine out?