90eliminator Posted April 4, 2009 Share Posted April 4, 2009 ok, just bought a 92 Cherokee of ebay, sight unseen but a good deal.. 92 limited, power everything, needs only minor stuff so I was told. It was raining when I picked it up so I didn't have a chance to put a battery in it, do a tune or whatnot, I'm not upset at all, I still got a great deal.... but the motor is knocking at startup and very little at idle with a increase while accelerating, sounds like it runs strong. Ok now for the dumb question.... how long should I expect this motor to hold out. Can I run synthetic oil in it, or any other tricks to keep her running for a little bit longer....??? Thanks Link to comment Share on other sites More sharing options...
jeepcoma Posted April 5, 2009 Share Posted April 5, 2009 Mine's pretty noisy at idle since I change the oil, once it's warmed up and the oil pressure is down a bit it seems to quiet down so I'm figuring just lifter noise, which I'll try to compare with next time I change it. I'm not worried about it, especially since it goes away above idle. If it were a real knock problem I wouldn't expect it to go away after warming up. Link to comment Share on other sites More sharing options...
Eagle Posted April 5, 2009 Share Posted April 5, 2009 The knock could be one of three things: rod bearings; wrist pins; auto tranny flex plate. The flex plate always quiets down when there's load on the tranny and comes back when idling in neutral, so that's probably not the problem with yours. Bad wrist pins are unusual in the 4.0L engine, so the likely culprit is the rod bearings. Those are not expensive and can be changed with the engine in the vehicle, just by dropping the oil pan. It's a good idea to do the main bearings and the rear main seal while the pan is off. Do that and you're probably good for another 100,000 miles. Link to comment Share on other sites More sharing options...
mvusse Posted April 5, 2009 Share Posted April 5, 2009 Could also be lifters. Link to comment Share on other sites More sharing options...
Eagle Posted April 5, 2009 Share Posted April 5, 2009 Lifters "tick." Rod bearings "knock." Link to comment Share on other sites More sharing options...
BLHTAZ Posted April 5, 2009 Share Posted April 5, 2009 If it's a significant knock, I would agree that it's likely rod bearings. One thing to remember is, the HO 4.0L engines were known for having a "slight" knock to them from new. Our '95 YJ has had it since we bought it in early 1997 and still does it now. I would go record the sound, but it is not running right now while I am awaiting a new PCM to get here for it :shake: . Link to comment Share on other sites More sharing options...
jeepcoma Posted April 5, 2009 Share Posted April 5, 2009 One person's knock is another's ping, so it makes it more interesting to figure out what's going on. Another thought I had, is that cracked exhaust manifolds are fairly common on these (I think, correct if wrong). Depending on where and how it's cracked, it could make some "puff", "tick", or "rapping" type sound, that would be louder when cold and lessen as the manifold heats up (but never goes away if the hole is large enough). Link to comment Share on other sites More sharing options...
90eliminator Posted April 6, 2009 Author Share Posted April 6, 2009 this is def. a lower end knock.... what is the repair cost on bearings and caps on a 4.0. Any ideas..... Link to comment Share on other sites More sharing options...
Eagle Posted April 6, 2009 Share Posted April 6, 2009 Main bearings +/- $85 Rod bearings +/- $45 Pan gasket $21 Rear seal $16 Link to comment Share on other sites More sharing options...
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