Lately I've had a knocking in the 4.0 in my `97 TJ. I cant tell for sure if its coming from the bottom or the top, and it sounds like a diesel at idle when cold. The noise lessens when its warmed up, but is still present. Granted most of my driving is around town, stop and go type stuff, but I've kept the oil changed every 3k and dont abuse it (often).

There has been no performance loss at all, no overheating, no loss of oil pressure. I went back to stock ECU programming last night, even though its never had a tank of anything less than 93octane since I bought it, but the noise persists. I checked for trouble codes when I did the programming, but there were none. I also changed the oil last night (about 800mi late <img src="/forums/images/graemlins/scared.gif" alt="" /> ), going to 10w40 instead of 10w30, hoping the thicker viscosity in the high temp range might quiet it down, but that did not help.

Am I being paranoid? Maybe its the "normal" valve train clatter from the 4.2/4.0 and its only bothering me because I've been listening to it so intently for the past couple weeks? I'm pretty sure one or more of the injectors is not performing optimally (leaky seals, and one or two are "ticking" louder than normal), could that contribute?

Your thoughts?

I'm going to go start it again and see if I can narrow down where its coming from.

I just ran it for several minutes, and the sound seems to be coming from both the top side, around the valves, and from the bottom side, right around the middle of the oil pan. It's more prounounced on the bottom side, but when I pulled the oil cap and watched the valves, the timing seemed to match them. Sticking lifters/rods maybe?

Possibly piston slap. Bearings would get noisier as the engine gets hot and the sound would be from the bottom end. Piston slap normally gets quieter as the engine heats up and the piston expands and yes, it can make your engine sound like a diesel. Doesn't sound right at only 84K miles, though.

Have you tried the old "screwdriver to the ear" trick? You can get real close to the origin of the noise thataway. If it is piston noise you can even tell which piston it is. A grateful friend saw me doing just that to his ride and when repaired he brought me an electronic ear with a metal rod on it. Man it is nice to put those earphones on and not have to lean into a running engine.
Big Jim <img src="/forums/images/graemlins/pfft.gif" alt="" />

You were running a performance chip and a K&N filter? You need to run a compression check. It only took 60K miles on my jeep to get almost .025" worth of wear running a K&N. Mixed with an aggressively timed over rich engine from a chip, the engine might be wearing quickly.

I've only had the K&N on for the past 20k or so, the exhaust was done about 12k ago, and its only been programmed for the last 4-6k, I cant really imagine it wearing out *that* quickly. I dont drive a lot (telecommute from home full time), and when I do, I try not to be abusive (cops around here are pretty strict), but I do on occasion stick my foot in it.

The main goal of the performance mods was to bring it back up to "normal" after sticking 31" tires on 3.07 diff gears. Yeah yeah, I know swap the gears, I just wasnt comfortable with the black art of setting up a ring and pinion myself, though doing the spider gears wasnt too bad.

Back to the knocking, it completely disappeared a couple times over the past few days, then came back, seemingly at random. For instance, it was doing it when I started it up this morning to take girl home, wasnt doing it at a stop light 4 blocks from my house on the way back, and was doing it when I pulled in the driveway 3 minutes later. Its not just the location (driveway), as I have heard it at several stop lights/signs with nothing close for sound to bounce off, the exhaust is too loud to hear much of anything at speed. Frustrating, very frustrating. <img src="/forums/images/graemlins/scared.gif" alt="" />