Grinding noise during hard accelaration - 87 4.0L 4x4

[blazenzeek]

I did a search for this and found some posts that seem to elude to this issue, but I wanted to throw it out before I invest money to fix it to make sure I'm in the ballpark.

During hard, quick accelerations from a stop or trying to accelerate up a steep grade from a stop, there is a "Grinding" noise coming from "somewhere" (yea...I know...tells you alot, huh). During the acceleration, if I back our of the throttle slightly, it stops. As I roll back into it, it starts again as long as I am < 20mph and still accelerating. The amplitude of the vibration seems to be proportional to the how hard I try to accelerate. It makes no noise at highway speeds or under normal accelartion or if I try to accelerate hard @ > 20 mph. Towing makes it much worse. Even a normal acceleration with a tow load will cause the vehicle to make the "grinding" noise.

I don't believe it is an axel or wheel bearing because it does not do it all the time. I have replaced the front brake hub assemblies and rebuilt the rearend for other reasons. Made noise before and after although it does seem to get easier to get the vehicle to make the noise as time passes.

I cannot pinpoint a source. I can feel it in the gas, the seat, and the steering wheel so where ever it is is connected / mounted pretty solid. My first thought was the torque converted but the vehicle will not make the noise if it is not moving....no matter how hard I throttle w/ the break on. What else could it be? It has to be something that is effected only during acceleration....ie..like the torque converter.

Any help would be greatly appreciated.

Blazenzeek

[gtmud]

Check your u-joints in the driveshaft, also double check how much back lash you have in your pinion gear.

[cdlabelle]

Yep........smell's like u-joint, to me, needles gone missing in one of the cups from the yokes maybe.. Especially when I read that you feel it in the seat..
Check them out by physically removing driveshaft, a visual or pushing on driveshaft not always the best way to check them.....Good Lock

[Saudade]

Check your motor mounts too. Hard accel, especially under trailer loads will allow the engine to torque and could be causing something to "grind".

[Xendrea Jasper]

I think it's all in your head. As in valvetrane or camshaft. Spark knock? Try using premeum / high octane fuel with a "104 octane" booster. I know gas prices are up there but it's a cheep & easy way to find out if that is your problem.

[IdahoJeeper]

You know, I've had the same thing lately. It almost sounds like a tire rubbing, which I know isn't happening, but it will only do it on a hard 'from zero' accelleration, not at high speed. I too was suspecting something in the rear end, I hate to imagine what that would cost...

I'll look at the u-joints and see if I find anything there... I look forward to more posts on this.

[blazenzeek]

And the winner is........Saudade with the recomendation to check the motor mounts! That turned out to be the problem. Apparently this is an issue w/ alot of 87 and 88 XJs.

On the rear of the driver side motor mount, there is only about 1/4" space between the overlap of the bracket attached to the motor and the bracket attached to the chassis. When the motor torques, it causes the two brackets to touch causing the "grinding" noise.

There is actually a Tech Pub out on the issue. The propsed fix from Jeep is to replace the motor bracket with one of a newer design that allows more clearance.

Yours truley was able to slightly bend the edge of the chassis bracket to open it just a touch. I used a c-clamp to pull the front edges of the brackets together forcing the rear edges to seperate. In the gap I inserted a 1/2" wide slice of an old motor mount bracket. When I released the C-clamp, the piece of rubber became compressed betweed the two members. No more noise.

Thanks to all for your input. It is extremely appreciated. I hope to be able to return the favor in the future.

Zeek

[blazenzeek]

Actually...one more final update. I thought I was golden until late yesterday when I heard a noise under hard acceleration. Similar noise but a little less intense and seemed to take more accelearation to make the noise.

Ended up original 'grinding' was 2-fold problem. 1) Motor mount bracket....see above and 2) fan hitting fan shroud. I removed the fan shroud and saw where the blades had been hitting it. I'm planning on just leaving the shroud off unless it presents a temperature.

Hope this helps if anyone has a similar issue.

Zeek

[Saudade]

So what did I win?

Nice going. When mine broke, the fan kept hitting the shroud too.