I did search for info on this but my question is about the pros and cons of replacing valve stem seals... My long time mechanic said my valve seals are leaking, not much, and not enough to notice any change in performance, etc. Just a puff of smoke when the engine's stone cold (not condensation). He recommends no repair at this time but to be aware that this problem can be a CAT killer. Based on what I've read here, just the seals can be replaced without pulling the head. He seems to think that's incorrect for the 4.1, but in any event said that just doing the seals may create other problems. I bet him he's wrong. I'll owe him a pint of Guinness if he's right. So, what do you folks think? Can it really be done on the car? Anyone done it and had unforeseen problems?