Jeep was bought new in 2004 and replaced the gasket last year due to it leaking around 70K. Is that around the usual time? 1 of the good things with a straight 6 is what a easy fix. Gasket was $40+

 

Everything that I have read make me believe that the valve covers on the 4.0 tend to leak quite easily.

 

We replaced ours at 120k. By the time I inherited the thing, it had been leaking for many years. I replaced the OEM rubber gasket with a FelPro cork gasket and sealant. We just turned over 151k, and no leaks so far.

My theory on why OEM rubber gaskets leak prematurely is the acidity from using petroleum-based engine oil and a lack of proper maintenance. Since switching to synthetic oil and implimenting a strict maintenance schedule, I haven't had gasket problems (incl. the OEM pan gasket) issues since. Food for thought.

 

IMHO, valve cover gaskets only begin to leak when the oil drains get plugged up. When more oil is supplied to the valves than can drain back, it pools in the head, rising above the gasket surface. This is especially common on V8s.

People say that when you get that little puff of oil smoke on startup, your valves seals are bad. Not true. If the drains are crapped up, oil will remain in the head long enough to leak down the valve stems and burn off on startup.

Anytime you have the valve cover off, check/clean the drains.

Those "rubber" gaskets you talk about are neoprene and have been used in aircraft since 1940.