knock knock

96 XJ here, 180k 4.0. Recently I've had a repetitive knock sound show up. You don't hear it on a cold start but after it warms up there's a distinctive lower pitched knock with a frequency of about 10 times a second, at idle. Seems to be coming from the bottom of the car/engine as you can hear it better from the sides and rear. The normal engine sounds seem to drown it out when you listen from over or in front of the engine. Not sure if the knocks increase in frequency along with acceleration.

Any ideas?

I would suggest a piece of pipe to further sound it out. Try to be listening at the rate with what operates at that frequency.

Or use a long screwdriver. Place the tip against the block and your ear to the handle. Watch out for parts the spin or spark!! Move the tip to different places until the noise is the loudest.

Yep. Read my response on Cherokee forum.

My knock sounds exactly like whats in the video where the guy is fixing his loose flexplate bolt.


https://www.cherokeeforum.com/f51/how-fix-flexplate-knocking-125547/


Is there any danger in running it like this for a few weeks? I've got a more pressing problem with my other car I'm trying to take care of right now.

Yes. Danger.

RCP Phx said:

I would suggest a piece of pipe to further sound it out. Try to be listening at the rate with what operates at that frequency.

Click to expand...

I use a piece of plastic water pipe. If you listen systematically you can likely localize the noise. I have a stethoscope, the plastic water pipe works just as well and keeps you at a safe distance from moving parts and hot exhaust.

Or you can add a can of engine Moly and see if it reduces the knock, May be your connecting rod bearings are worn. I've replaced the connecting rod bearings on three 4.0 motors so far, seems like a common issue.

If it is the flex bolt(s) loose, that is a 10-20 minute fix, jack up pull 2-3 cover plate bolts, tighten the FP bolts, reverse and done.

It could just be the exhaust pipe rattling near the transmission plate thingy.