92 cav crankcase vacuum

trying to fix a car over the phone, problem one. the guy says his car will run with the oil fill cap off but when he puts it back on it dies. says there is a strong vacuum on the oil dip stick tube. is this where the PCV valve comes into play? any help would be appreciated thanks

It's probably the breather that is plugged allowing vacuum to build in the crankcase, but I'd guess the PCV valve is due for replacement anyway. If so, have him replace it. There should be almost no vacuum in the crankcase, I believe 1-3" Hg is where you want it. Any more than that and it can suck gaskets inside the surfaces they seal, especially the oil pan and valve cover gaskets. Also, see if the car will run with the oil cap on and with the throttle held open slightly. It may be an IAC problem that "goes away" when air is allowed to enter the engine through the PCV valve.