You have a pile of miles on your engine, so worn piston rings would be my first suspect. A good test for that is a cylinder leakage test. A compression test might show low compression, but the problem with that test is your compression might not actually show up as very low, and if it IS low, you don't know why.
With a cylinder leakage test, also called a "cylinder leak-down test", you turn the crankshaft by hand until the piston is at top dead center on the compression stroke, so the valves are closed, then you use a hose similar to the one for a compression tester, but without the check valve, to force in compressed air through the tester. The tester limits air pressure to around 30 pounds, and it has a gauge to show the percent of leakage. Less than ten percent is considered normal or acceptable.
There's four places to look for the results of leakage. If you hear hissing at the tail pipe, the exhaust valve is leaking. If you hear it at the inlet to the throttle body, it's a leaking intake valve. Bubbles in the radiator indicate a leaking cylinder head gasket, or much less-commonly, a cracked cylinder head. In your case, worn piston rings will show up as hissing heard at the oil fill cap or dip stick tube.
If testing does show excessive leakage past the piston rings, pressure will build up in the bottom of the engine. That blowby is supposed to be drawn out through the PCV valve, but when it gets to be too much for the system to handle, you will get that pressure buildup. That pressure pushes on the seals and gaskets, and if there is a tiny or insignificant leak already, any oil running against it will get pushed out. Valve covers and spark plug tube seals are the perfect place for that to happen. Oil under pressure is in passages far away from places it can leak out. Valve cover leaks don't normally result in a serious leak because that oil isn't under pressure, ... Until you have excessive blowby.
Monday, November 10th, 2014 AT 1:59 AM