Help! Engine knock!

First off I'd like to say that I am new to the group. I look forward to sharing lots of XJ info with all of you!

Ok on to my problem.

I have a 90 XJ 4.0 w 167K miles on it.
It runs like a raped ape and has good oil pressure but it has what was thought to be a rod knock. I did a archive search and looked at the usual suspects. The flex plate and bolts were fine. I pulled one plug wire at a time and no change.

To find out what was going on, I went ahead and pulled the oil pan and checked the rod bearings. They were worn but I didn't really see one that looked bad enough to knock. While I had each rod apart I pushed each piston up and down to check for play in the wrist pins. They all seamed nice and tight. I went ahead and replaced the rod bearings while I was in there and used a plasti guage to check the clearance. I pulled one main bearing cap and put it back since the main looked very good. While in there I noticed the timing chain had quite a bit of slack in it. So I replaced that too. Since I had it apart I decided to go ahead and put in a new high flow oil pump as well. After all of that I wrestled the oil pan back on.

Once everything was back together we fired up the jeep... Knock knock knock! Darn!

I know piston slap is common on these jeeps, but if it is that it's only one cylinder.

After using a wooden dowl to listin for the sound it apears to be comming from the rear of the head and we now think it is in the valve train. I pulled the valve cover and checked each rocker with it running and I doubt it is a lifter.

I'm at the end of my rope here! any ideas or suggestions for what I have missed here? Any help would be greatly appreciated.

Bruce Smith

if it only happens at idle, i woudnt worry about it. if it increases with rpms, then you probably will have problems. mine only does it at idle and when low on oil or when the oil has broken down and needs an oil change. also try a heavier weight oil

I suppppose I should have said when the knock happens. The knock happens at all engine speeds. Speed the engine up and the knock speeds up. Engine temp doesn't seem to effect it either. I was thinking about trying the heavier oil. Right now I'm running 10-40. Maybe a couple of cans of motor honey might help LOL.

ok now the Knock as become more of a knock rattle sound. I've located it to be at the rear of the engine. It seams to be close to the the head but it is hard to tell. Could this be piston slap?

I have an 87 with a, "tick tick tick". It only happens occasionally, so we have diagnosed it as a floating lifter. I paid for an engine flush, which didn't help. Maybe I'll run 20W50 for a while..? Any opinions..?

Have you looked at your flesplate. I had a knock that sounded like the bottom end eas about to let go. Turned out the bolts holding the torque converter to the flexplate have loosened off. Tightent them and no more knock.

well, I have narrowed the noise down to the rear of the engine. IT sounds like it is near the head though. I did look at the flex plate bolts but I didn't put a wrench on them. I will soon though.

wingtime said:

well, I have narrowed the noise down to the rear of the engine. IT sounds like it is near the head though. I did look at the flex plate bolts but I didn't put a wrench on them. I will soon though.

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It could be the flex plate itself, not the bolts. They sometimes crack in a ring around the reinforcing plate where the flywheel bolts are. If it is cracked, idling in neutral you'll get a knock that sounds a lot like a bad rod bearing, but if you put it in gear and give it a bit of gas, the noise goes away because the flex plate is under load.