Have you got a one way valve on the vaccum pipe between the inlet manifold and the engine breather and if so is it working?
If the valve is not there or it’s sticking open when the manifold goes to positive pressure (boost pressure) it will pressurise the crankcase via the engine breather and can blow the cap off.
Originally Posted by B1G_RST
Originally Posted by mikedaf2
have a leak down test done I have found this to be more accurate in the past
whats this mate ?
A leak down test is where a metered amount of air is put into a cylinder via the spark plug hole and the amount "leaking" out is shown on a gauge. It is done with the piston on TDC and the end of the compression stroke when both the inlet and exhaust valves are in the closed position.
If the rings are not sealing properly or there is a problem with a piston the air will be herd escaping from the dipstick tube or the oil filler cap. All engines will have some degree of leakage past the rings, the greater the cc the more leakage there will be.
It can also be used to diagnose leaking valves and head gasket failure.