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I have a 90 cherokee Laredo 4.0, 198,000 miles on it. it sounds like its knocking when it idles and then dissipates when I get to higher speeds. Is it just piston slap?
Its probaly wrist pin noise. Check compression to check for piston slap. It could be rod bearings but thats usually accompanied with lower than normal oil pressure. I have the same knocking and been told its pretty common.
If you have piston slap you should also have a noticeable loss of power. Correct me if i'm wrong, but does the noise seem like it's coming from the bottom of the engine and is louder to the side of the jeep than the front.
I replaced my rod bearings and my compression is in the mid to high 130s and i still have the knock. Im pretty sure its the wrist pins. I've been told not to worry about it by many jeep owners. I think this topic has been covered on some other threads. If you want to isolate the problem cylinder pull the spark plug wires one at a time(without shocking yourself) while the engine is running. The noise will most likely change.