I have a " 91" Nissan Stanza with a 2.4 liter, A/t. It has a vibration at idle like a miss. Its not a tune up problem. I notice with the engine running and remove the oil fill cap, it seems to have compression in the valve cover and engine seems to want to stall. I figure timing belt/chain maybe slipping. Engine has 107k miles. Is there anything other than valves that would cause compression in the valve cover?