Re: Is it time to pull the head off ?
with a good compresion check not likely you have bad valves. Remove and look very closely at the rubber boots in the intake system. past posts have noted people finding cracks that were only visible when the boot was manipulated (ie bent). Is your throttle plate closing completely? if you reduce it to the basics when the piston goes down it is trying to pull air with it through air intake system. Vacuum is a measure of how well that system is sealed i.e. higher the vacuum less air leaking in through something. either the intake system, the exhaust valve, up past the rings from the crankcase or the head gasket. a valid compression test should eliminate the exhaust valve head gasket and rings. pretty much leaves the supply system. boots, and intake manifold. If throttle plate not closing completely then you are pulling air in through it and your vacuum test is useless.
86T, 350 Nismo 08 (#1415)