Ive noticed that my 96 XJ with the 4.0L is making a knocking sound when I start it up when its cold and when I start it when its warm. It only does it for a couple minutes then goes away. When I give it gas the knock disappears. I crawled under and it sounds like its definitely coming from the lower end of the engine, but I cant pinpoint the exact spot or component that its coming from. Any ideas guys?

 

How's the oil pressure look?

 

Oil pressure is great! usually about 45-70psi all the time

 

Try Seafoaming if you never done it before.

 

yep, tried the seafoam too lol...

 

Tuff is it an automatic trans? I had this same symptom. I was going to change the engine but when I pulled it out I found the flexplate was cracked. Replaced it and re-installed the engine and the knock was gone. You may have the same thing.

 

Yea its an automatic. I read your other post about your hairline crack on the flex plate. Do you think that could be it though? Especially since it goes away when it warms up or after about 2 minutes on a warm start. Thanks for the input though guys!

 

Mine was always there but much worse while it was cold. But I think it varies due to how close that the cracks are to the crankshaft bolts.

 

1TUFFXJ,
Well I don't have an answer to your problem just want you to know your not the only one with the same issue. Mine does the exact same thing....I have a knock from lower part of engine for about 15 minutes when it's cold after it warms up the knock seems to go away. Tried seafoam in the crankcase and intake did not help. Only difference with mine is it's a 5 speed...if I figure out what the problem is I'll hit you up and let you know!

 

great thanks!

 

Wow good info. Never heard of this symptom on a 5 speed. Can't be flexplate then because a stick has a real flywheel. Only thing they share is flywheel bolts could be loose. It has been suggested that loose bolts are what cause the flexplate cracks.

 

Try replacing the harmonic balancer. If the Flexplate is bad,cracked or lose bolts it should rattle all the time or should anyways...A bad harmonic balancer from my experience can cause a low end rattle as well..Anyways something to consider.

 

sea foam ? its made for 2 strokes whats the deal ?

 

have a five speed also same deal except mine is about 10 seconds on start and then comes again in the higher rpm's i dont think it could be the motor it starts and runs to good for a crank or a cylinder to going bad