Knocking noise coming from bottom end

My 1991 XJ has a knocking noise, It only seems to do it in park and netural.... first thought was main bearing :rattle: I hope not. I popped the valve cover and checked the rocker arms there was no play in anyone. any advice?

Hummm nouthing?

knocking noise is a pretty vague symptom, can you be more specific? what kind of knocking, where does it seem to eminate from, is it consistent with engine rpm, by telling us it only does it in P and N, suggests that it only does it when there is no load on the engine, can you be sure it's the engine and not the tranny? stick or auto tranny? upload a sound file?

First place I'd start is with the flexplate and torque converter bolts. Check for bolt tightness and cracks on the flexplate, and a lil' more info on the noise would help.

Maybe not as bad as you think. Do a search on "Knocking noise."
.
A few have mentioned on this forum about the "piston slap" noise/knock and how some engines came from the factory with the noise.
Mine has it and, like yours, it seems most evident while idling (5 spd) and my ear is close to the pan.
I searched here, found some info, but then I Googled it and found more.
If yours sounds a bit like a diesel -what I describe as that low frequency heart-sinking rumble/thrashing of a gunny sack full of large smooth stones - you may be in luck.
Read on.
.
Best,
.
~~~~~~~~~~
The lowdown on the 4.0L engine noise:...
.
In their original complaint, plaintiffs alleged that Chrysler's model years 1991-95 Jeep vehicles with 4.0 liter (4.0L) and 2.5 liter (2.5L) engines were defective due to an idle knocking noise in the engine. ... aka "piston slap."
.
http://caselaw.lp.findlaw.com/scripts/getcase.pl?court=il&vol=app/2001/1994079&invol=3
During 1993 and 1994, Chrysler created an "Idle Knock Task Force" and a "Diesel Noise Task Force" to address customer complaints about the engine noises. Chrysler also had an ongoing task force known as the "noise, vibration, and harshness task force" (NVH) that studied and recommended improvements for overall engine sound quality. NVH recommended that, in order to reduce sound radiation in the 4.0L engine, a main bearing brace could be added to the bottom of the engine block. NVH indicated that by tying the main bearing caps together, bearing movement is kept at a minimum and sound is reduced. This change was incorporated in the 1996 models.
.
***
.
From some other source I don't recall and didn't copy/past:
.
As part of the Jeep powertrain program in 1996, the engines were extensively refined, receiving state-of-the-art aluminum pistons and revised camshaft profiles. The 4.0-liter block was stiffened with engine block ribbing. The valve covers also were isolated and a main bearing brace added. As a result, key sources of noise, vibration and harshness were eliminated on both engines, while the 4.0-liter engine's torque curve was broadened.

Check your idler pully for play and such.

Just saw one go out, the Jeep sounded like a diesel right before.

id also first check for a cracked flexplate or lose bolts, easy to get to by removing the inspection cover.

Next, go to your timing belt/chain, w/e...

On the XJ I sold, sounded like a rod knock, ended up being the timing component slapping the cover...

I wouldnt say it's internal in the motor simply because they motor is moving internally anytie its running, not just in park or nuetral (not that you dont know that, just being a smartass and stating the obvious)

Speed_racer said:

id also first check for a cracked flexplate or lose bolts, easy to get to by removing the inspection cover.

Next, go to your timing belt/chain, w/e...

On the XJ I sold, sounded like a rod knock, ended up being the timing component slapping the cover...

I wouldnt say it's internal in the motor simply because they motor is moving internally anytie its running, not just in park or nuetral (not that you dont know that, just being a smartass and stating the obvious)

Click to expand...

ditto on the flex plate. when in gear there is a static load on the assembly and it seems to quiet it down. my torque convertor bolts loosened up on me once and it was the same.