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Discussion Starter #1
I have a very slight knock only at very low RPM's on my 90 YJ 2.5L. I have tightened the exhaust manifold down and that helped a bit but, it isn't gone completely. I had assumed it was an exhaust rattling but when I climb underneath and shake things around I am unable to replicate the sound. I read on another thread about a timing chain knock. Could this be where my problem is? Does anyone know how I can identify it?
 

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Only at low idle? It goes away completely after that? Does it change when the engine warms up? How many miles on the engine?

It is also possible that it could be hydraulic-valve lifter tick, but you can troubleshoot a little further before you start replacing parts. You can take a piece of garden hose and while the engine is running hold it up against you ear and put the open end near the timing chain plate, you'll hear it if its bad.

You will also need to take off your fan to do this bc you may get some blowback sounds from the wind across the hose. The engine will be ok for a shortened amount of time with the fan being off, just make sure your coolant levels are ok. It won't take long but it'll point you in the right direction and get you started. A bad chain is a bad chain but you can treat sticky-lifters up to a certain point before replacing them.
 

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Discussion Starter #3
I only hear it with the clutch out or being let out in 1st or if I downshift too far say about 20 MPH in 4th gear. As soon as the RPM's come up a hair then it disappears. That's why I've always assumed it was exhaust related due to the idle being a little rougher at low RPM's. The knock doesn't increase with engine revs. It disappears. The engine has lots of miles but who knows how much work has been done in the past. There is absolutely no noise when the Jeep isn't rolling so it's hard to get my head up under the hood to diagnose. It's been doing this for years and seems to not be getting any worse but, not knowing what it is is driving me crazy.
 

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Discussion Starter #7
No. Put in a new transmission, clutch, pressure plate and throwout bearing about a year ago. No change at all. It's really more of a rattle than a knock and I only get it while rolling when the rpm's are way low. It could still be an exhaust issue I guess. I just can't find it.
 

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Is your fuel pump mechanical or electric? Not to familiar with your rig. If it's a mechanical fuel pump then that can cause low end knocking noise. As mentioned before use a piece of hose near the pump and listen with your ear to the other end of the hose.
 
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