Front engine noise

[Hobby]

Hi, I have a 96 cherokee 4.0. I know these motors are noisy any way but I cant seem to figure this one out. Lately I have been hearing a sort of rumbling noise from somewhere in the front of the engine. I don't know what rod bearing noise sounds like but I hope this is not it. It has 97,000 miles on it and has been taken very good care of all of its life. Noise starts about 10 to 15 seconds after start up and gets quiter when it warms up. Doesn't quite go completely away though. Has good oil pressure and has only started this about 3 or 4 weeks ago and is getting louder. Any thoughts or suggestions would be greatly appreciated.

Thanks,
Ryan

[juicexj24]

Could be idler pully, or power steering pump. Is it a whine or a rattle noise? Pop the hood, start it up and listen for the noise, check both of those. Also, could be the fan clutch as well. Juice

[Ralph]

Spend a couple of bucks on a mechanic's stethoscope. Whe you hear the noise pop the hood and start probing the areas mentioned above with the stethoscope. You should find it pretty quick. Could also be the water pump.

RR3

[Hobby]

I just came back in from trying to figure it out. I was able to use some earplugs and a long screwdriver to make a sort of stethescope. Still couldn't pin point it exactly though. It seems to a very low pitched rumbling sound. I found it loudest down where the belt is routed around the crankshaft. What is that? Is that a vibration damper? I also pulled the plug wires off one by one and it still made the noise. I guess that is a good thing.

Thanks,
Ryan

[Hobby]

Ok, stopped by the mechanic today because the noise is getting pretty annoying. 2 mechanics came out to listen. One said he did not know what it was. The other was pretty sure it was in the valve train. noise occured at 1/2 the rate of the engine speed. Made sense I thought. He specifically thought it was in the rocker arms. Told me to drive until it quits or spend about 1,500 or 2,000 on a upper end rebuild. Thats crazy I thought. These are hydraulic lifters right? Can they not be adjusted or is this the end of my jeeps life. Also this is a shop that works on a lot of jeeps. Any suggestions would be appreciated.

Thanks,
Ryan

[Matthew Currie]

I'd check again to make sure the noise isn't coming from some common source like the crankshaft pulley (harmonic balancer), or a bad water pump, and then if it's nothing identifiable, I'd ignore it. It seems way too young to be having problems with timing chain wear.

Piston slap is common with 4.0's, and one characteristic of this is that it will improve as the engine gets hotter.

Rod bearing noise will increase under load, and usually get worse as the engine heats up.

[Sir Fairnuff]

Onboard ship all I worry about on my systems an engines are hours. You've got 100k on your rig that equates to 1,700hrs at 60mph highway only, 7,000hrs city driving only. My bet is at 43 mi/day you've got 4-5k hrs. That engine series has problems with the harmonic balancer. If your sure the sound is 1/2 the RPMs take a serious look at the cam bearings & chain.
Craig
'90' 4.0 8k hrs. (106k mi) and yes I have a hour meter.

[Hobby]

Never really thought about it in hours. Being a pilot I probably should have. The small airplanes I fly have 2500 hour TBO's (time between overhaul) on them average. I took it to another shop this morning to get another opinion. Mechanic came out and said it sounded like piston slap, I personally have never heard piston slap that bad before. The are going to keep it for the day so we will see what happens.

Thanks,
Ryan

[Sir Fairnuff]

Thanks Matthew,
I have my Detroits on such a regimented schedule they never get to a piston slap state, 2k on fuel, 5k on heads & 10k on majors. Not since my SS 396 in '69'! Is this a regular issue with the non High Output '90' 4.0 and which cylinders?
Hobby, can you keep us up todate, I love local knowlege insted of generic charts.
Craig

PS: I can understand why those punny little engines in planes have a 2,500 TBO. I run 1100hp ea. in my VW bus size Detriots.

[pancake]

Originally posted by Hobby
Never really thought about it in hours. Being a pilot I probably should have. The small airplanes I fly have 2500 hour TBO's (time between overhaul) on them average. I took it to another shop this morning to get another opinion. Mechanic came out and said it sounded like piston slap, I personally have never heard piston slap that bad before. The are going to keep it for the day so we will see what happens.

Thanks,
Ryan

2500 hours? Damn...my Citabria only has ~1700 hours. :(

[Matthew Currie]

Originally posted by Sir Fairnuff
Thanks Matthew,
I have my Detroits on such a regimented schedule they never get to a piston slap state, 2k on fuel, 5k on heads & 10k on majors. Not since my SS 396 in '69'! Is this a regular issue with the non High Output '90' 4.0 and which cylinders?
Hobby, can you keep us up todate, I love local knowlege insted of generic charts.
Craig

PS: I can understand why those punny little engines in planes have a 2,500 TBO. I run 1100hp ea. in my VW bus size Detriots.

The piston slap seems to be quite variable, but less common in pre-Chrysler engines. My 87 never had it. My 95 has it for sure. My stepson's 93 has almost none. It doesn't seem to matter what the mileage is either. The 95 was noticeable at 110K miles when I bought it. The 93 has 190K, and the 87 235K. I've heard on this forum or its predecessor that some HO's were even replaced under warrantee for it. But if you have piston slap, other noises will seem to blend with it. I almost lost a water pump because its rattle was so similar.

[Eagle]

Originally posted by Hobby
Ok, stopped by the mechanic today because the noise is getting pretty annoying. 2 mechanics came out to listen. One said he did not know what it was. The other was pretty sure it was in the valve train. noise occured at 1/2 the rate of the engine speed. Made sense I thought. He specifically thought it was in the rocker arms. Told me to drive until it quits or spend about 1,500 or 2,000 on a upper end rebuild. Thats crazy I thought. These are hydraulic lifters right? Can they not be adjusted or is this the end of my jeeps life. Also this is a shop that works on a lot of jeeps. Any suggestions would be appreciated.

Thanks,
Ryan

Find a new shop.

AMC/Jeep hydraulic valve train is not adjustable. Some others (small-block Chevy, for example) are. However, valve train noise comes from the top of the engine, since the whole shebang sits on top of the head, not from down low at the front. Also, valve train noise sounds more like a light, rapid "ticking," not anything that I could possibly describe as a rumble.

Just for ha-has, have a good shop pull the inspection cover off the bottom of your bell housing and check to see if the bolts holding the torque converter to the flex plate are tight. While they're in there, have them look for an annular crack just where the reinforcing ring under the crankshaft bolts is. Either of these conditions causes a knocking noise that's sometimes mistaken for rod bearings.