Rattling sound at low RPM's?

First let me say, this is great forum guys. Lot's of good info on here.

My rig: 94 XJ, 5 speed, with the I6. Abt 160K miles.

My problem: At low RPMs, no matter what gear I'm in, when my come up to a small hill and hit the gas to maintain speed, there is a rattling coming from the engine compartment. I think the key is when I'm in low RPMs and the engine is under a load, i.e. more gas equals louder rattling. If I down shift to raise the RPMs, the rattling is still there, but not as loud.

Fixes I've tried: New water pump, plugs and wires, fuel injector cleaner, fuel filter, Shell 93 octane gas, airfilter and cleaned the intake.

I don't see a knock sensor, so that rules that out. Timing is controlled by the computer so I can't really mess with that, that I know of.

Someone suggested the cat might be busted, but the sound is coming from the engine.

Any ideas would be greatly appreciated.

Thanks.

Could it be the timing chain? This has happened to me twice due to a tensioner failure. With mine it only made sounds when the engine was hot--maybe having to do with lowered oil pressure. When the rpms got high enough (about 1500) the rattle disappeared. Its something to look into. And by the way, if it is the chain, make sure you change the everything that is involved in the timing set. If not, you might have to do the job again soon. Ask me how I know!

Are you thinking this is pre-ignition? Does it sound like a dozen elves hitting little hammers under the hood? Usually running premium will correct that. Some years do have a knock sensor, but I don't know about yours. One common cause of pre-ignition can be carbon build up. Do you ever take it from a rolling start up to the speed limit (or above if so inclined - flame invitation, I know) to blow the carbon out. Others will chime in with the wonders of seafoam, which I have never used.

I would put a "stethoscope" of some sort on the timing chain cover. When my chain failed, originally I would notice the noise it only while moving but it got to the point where I could clearly hear it at the front of the motor at idle. If it is the chain, you might avoid some of the in-between time by listening now. Just a thought.

I was hoping to stay away from the timing chain. I thought of that as well, but from what I've read XJ's timing chains are know to last till the engine dies.

How many miles did you all have on your rigs before the chain gave?

I think I will purchase a stethoscope. (This is a good excuse for me to buy a new tool.)

I have used seafoam, but not through the tank. I ran it through the fuel rail on a old toyota I used to have with a 22RE. My brother, a mech, showed me how. Merchant's tire and auto charges you about $80 to do something similar. You can do it for the price of a can of seafoam. (Plugs did foul out during the process though) The vehicle will spudder and scream and smoke like it's going through an exercism. But it DOES work. Smoothed out my rough idling, acceleration increased and the engine just ran better.

Come to think of it, I never did it on my XJ. I'll give that a try too.

Thanks for the tips.

maybe bad motor mounts? mine rattles and knocks around cuz i gotta change those. maybe you do to

Ok, did the Seafoam thing. Didn't help. I was looking at the motor mounts and I think you might be right. I don't know if that's what causing it, but I do know my whole the engine and vehicle shits when I give it gas.

You could try the Mopar Combustion Chamber Cleaner, if it is carbon other have reported success with it.

For sure buy the mechanics stethoscope, handy tool, but you should also buy yourself some vinyl tubing, about 3 feet of it. The stethoscope's metal end will often pickup harmonic resonance making identifying the source of a noise difficult. Use the vinyl tubing to locate the general location, then the stethoscope to isolate it.

EchoSixGolf said:

Ok, did the Seafoam thing. Didn't help. I was looking at the motor mounts and I think you might be right. I don't know if that's what causing it, but I do know my whole the engine and vehicle shits when I give it gas.

Click to expand...

Note I meant to say "shifts" when I give it gas.

Job #1. Inspect motor mounts. Your motor should not shift much at all.....

To check the motor mounts, have someone you trust put it into gear with their foot firmly on the brake, and see how much the motor moves. With everything off, look for marks on the radiator from the fan. Don't you have an idea yet if it is pre-ignition knock or something hitting? Once you have heard pre-ignition, you can't confuse it. It happens on acceleration and sounds like elves with hundreds of hammers. And stops as soon as you back off the gas. Premium gas almost always stops this condition.

I took a quick vid of the engine and uploaded it to photo bucket. You can kind of hear it at the 1 min mark and right after I rev it.

I use 93 octane gas, always have, so I don't think it's pre-ignition. It has me stomped.

http://s720.photobucket.com/albums/ww209/EchoSixGolf/?action=view&current=Jeep.flv

A couple of years ago I had the same scenerio on my '88 (@115k) a short time after I brought it. With the suggestions of a lot of people trying to help me, I replaced the rod bearings, oil pump, balancer & seal, and rear seal, all in-car. My mistake was not checking the main bearings while I was in there...
Got it all back together and the same noise was still there, just no more oil leaks. Anyway, my next step was to keep driving it while I rebuilt a junkyard engine. Got the new engine in, and a couple months later got around to tearing down the old one. Sure enough, the front main bearing had spun and tore up the engine casting pretty good. Funny thing is, it always started every time without fail, oil pressure stayed pretty decent even when hot, and it always had plenty of power!
So, here's hoping this is not your problem, but, something to think about if you decide to tear into it for a look around.