I've recently been looking into picking up an older Jeep YJ as a second vehicle. I found one that I really like, but it has a noisy lifter. I've done some research, and this seems to be very common problem with these vehicles. This has made me a little curious as to what a lifter actually does in the engine, and what makes one lifter noisy and another one not. I've got a basic understanding of how engines work. Ie: camshaft, crankshaft, timing belts, pistons, valves, etc. I don't know where the lifter fits in though. Anyone feel like dumbing this down for me? Now to remedy this, I've heard suggestions like: live with it; put some thicker oil in it; replace all the lifters; replace the lifters and the camshaft; and a couple others. Is replacing lifters a big job? Is it expensive to get done? Is it even necessary? Should I find another vehicle? I definitely want this vehicle to last for 4 or 5 years. Is this a sign of a badly maintained or abused vehicle? Thanks for any input you people may have. Sorry, it's a 91 2.5 liter if that helps.