Acceleration Thud Problem

[Salvagedcircuit]

My comanche's been a pretty great lil vehicle. The engine sounds good and idles smoothly at ~800rpm. The AW4 transmission seems to shift without issue and I'm pretty happy with it. I just replaced my muffler, tail pipe and window guides so the cab is quieter than ever.

 

However, I am noticing a thudding sound whenever I initially accelerate from stop. It's not particularly loud and does not occur with gradual acceleration. Only when I'm putting my foot to the floor when accelerating from stop. I put a gopro under my comanche and recorded the drivetrain a few times. There is a bit of movement in the yolk, but that may be normal. Oddly, the gopro does not seem to pick up the thudding sound in audio. The thudding sound is a 3 to 4 knock on hard acceleration and then vanishes. It's completely gone after initial acceleration and does not occur when accelerating from 20-->30 or 30-->60, just the initial acceleration. I originally thought it was my speedometer cable, but it does not change with speed.

 

 

 

 

I don't know what's causing this. I replaced the fluid in my transfer case 3 months ago. I replaced the coolant lines to my transmission since one was misshapen, I added transmission fluid to compensate for loss when removing my transmission lines. I replaced the transmission and engine mounts. I just greased the yolk fitting as well as the front / back drive shaft u-joints. I greased one u-joint in the double cardan joint as the other requires me to remove the cross member to grease.

 

Edit: I looked back through the videos and noticed my muffler is moving a bit. I didn't tighten it down super tight because I didnt want to deform the pipe. It's a pain to remove when the pipes are dented and rusted together. I guess I'll have to tighten them down.

Any ideas welcome. Thanks a ton guys!

[89 MJ]

Looks like you’ve got old ujoints. I’d check those for play first. 

[Salvagedcircuit]

12 minutes ago, 89 MJ said:

Looks like you’ve got old ujoints. I’d check those for play first. 

I checked my U-joints for play and they all seemed pretty solid. I'll check for play again though. Thanks.

[89 MJ]

55 minutes ago, Salvagedcircuit said:

I checked my U-joints for play and they all seemed pretty solid. I'll check for play again though. Thanks.

It definitely does seem to be a weird issue. Is the acceleration smooth? Or is it jerky when it thuds?

[Salvagedcircuit]

The acceleration is smooth when it makes the thudding sounds. Also, the thudding sounds are not present while decelerating. when driving 50 or 60mph there are no odd vibrations.

[Islander]

could your exhaust be bumping the transmission crossmember? I had some knocking sounds after I ax-15 swapped at lower speeds and mostly over bumps but it was also way more than 3-4 knocks. I was able to replicate the sound while stoped by lifting and pulling down on the transmission from underneath to get the exhaust to knock on the crossmember.

[Pete M]

I would pull down on the exhaust and see if it hits the crossmember

[gogmorgo]

Exhaust hitting the crossmember is a common enough occurrence.
Motor mounts in good shape? Are you accelerating hard enough it could be related to suspension drooping in the front or squatting in the rear?

 

Double check your lug nuts. Even if you don’t think that’s what it is. 

[Salvagedcircuit]

@Islander@Pete M@gogmorgo Somehow it's not hitting the crossmember. There's ~1/16in between the muffler and crossmember. There is a rust spot where it could be hitting. I'm going to grab my gopro and investigate again. I tried paint but it left no marks on the muffler

 

My motor mounts and transmission mounts are ~8 months old. I did not replace that rubber block that holds the catalytic converter hanger. Does replacing this rubber block solve the problem? Is there anyway other way to add clearance besides adding spacers between the crossmember bolts and the frame?

Edit: I ran across this guy's post:

https://there4.io/2021/07/26/Jeep-XJ-Crossmember-Exhaust-Hanger/

 

Maybe I'll try to add a half-pipe copper or steel pipe shim below the C-shaped hanger for the exhaust. That may give me some more clearance in case the muffler is rubbing. That seems to be the mos reasonable solution without having to spend 3-4hours removing the crossmember to get to the small lil rubber block.

 

 

[Salvagedcircuit]

I tightened down the muffler and troubleshooted some more. I put the MJ in the air on four jack stands turned on the engine and listened closely.

 

No thudding sounds in park or in neutral.

Thudding sound exists in drive and reverse. It is most noticeable going from 0-->5mph, any initial acceleration. If the tires are still rolling at idle and additional acceleration is added, the noise is not present.

 

I noticed that in 2wd drive and at idle my front tires were slowly spinning and my back tires were spinning a lot faster. I currently have the front CAD axle linked together and the vacuum disconnected. I investigated my transfer case linkage and it was seized. I thought it may be problematic because my MJ previously had a 2.5in lift on the front with 31in tires. It is now at stock height with 235/75-R15's. It took a while to wire brush the linkage but I was able to adjust it back to factory spec following the official AMC guide:

 

 

The proper linkage adjustment made the front wheels not spin while in 2wd. I did not put the MJ into 4H or 4L while in drive because I did not want to cause any damage from out of sync axles. I was able to shift between 4L and 4H while in neutral. The part time light came on in 4H and 4L too.

 

The results of my testing point toward the transfer case. I have an AMC 231, which should be an NP231. I inspected the inside of the transfer case and even though the back seal looks wet, the fluid level is still at the recommended level, right up to the bottom of the fill port. I put an endoscope camera in there and took some photos but I did not see something glaringly wrong.

 

I used a 3ft long screwdriver as a stethoscope and pressed it against the transfer case shell while the thudding occurred. I could hear the noise coming from inside the transfer case. At 270k miles, I'm guessing the chain has stretched. The amc 231 has a '88 build date with no rebuild tags, so I think it's original.

 

Any ideas welcome.

 

[Salvagedcircuit]

I looked over the video again and it seems that in 2wd the transfer case chain does not move. So that ruled the transfer case chain out. I'm confused, because I checked my drive axle U-joints and they are all solid and don't have play in them. Is it possible the noise I'm hearing is a "torque relief" sound that video mentioned? That wouldn't make sense though if the transfer case is not even being utilized.

I'm a bit stumped, going to have to investigate more.

[Pete M]

why on earth are your front tires rotating when in 2wd? 

[Salvagedcircuit]

36 minutes ago, Pete M said:

why on earth are your front tires rotating when in 2wd? 

Yeah, I think what happened is the transfer case gear selector did not fully engage into 2wd. It's possible the previous owner adjusted the tranasfer case selector when he put on larger tires but it has been seized in place since then. I of course had no idea I needed to adjust this when I went back to stock height. I do not know how long it's been like this. I am thinking the selector may have warn or just needed to be readjusted.

 

I just wire wheeled the rod, lubed it and readjusted it based on that amc factory video. The noise seems to have subsided, so fingers crossed it was just that and not a loose transfer case chain.

 

[MiNi Beast]

Just a random thought...have you checked your fly wheel bolts? Possibly loose or oveled out. Only saying as you seem to check most the common areas with no luck yet. Seems viable but I'm just guessing.