Riddle me this

[b3uhbp]

I have a vibration I'm trying to run down. Only started noticing it recently after I replaced the clutch and tranny mount. I feel it when I'm accelerating and sometimes when I hit a dip in the road. Never at idle or just reving the engine. Any thoughts?

 

[Posi]

Do you feel the vibration in the steering wheel or seat?

 

[b3uhbp]

No. Just a general vib. I'm thinking motor mounts but not sure how to diagnose.

 

[Torxhead]

When you replaced the parts you could have changed the driveshaft angle enough to cause that vibration. Might have to shim the pinion angle to correct it.

 

[Lumpy Grits]

I would first have your tires re-balanced.
LG

 

[b3uhbp]

When you replaced the parts you could have changed the driveshaft angle enough to cause that vibration. Might have to shim the pinion angle to correct it.

That did occur to me but the replacement parts were identical to original. Any change would have been extremely small. Could the mounts be under a bind after the removal and reinstall of the t-fer case and tranny? The mounts look good. Not sure how to diagnose.

 

[Posi]

Did you replace the pilot bushing?
Remove the rear DS and test drive, post results.

 

[b3uhbp]

Did you replace the pilot bushing?
Remove the rear DS and test drive, post results.

Yes, new pilot bushing. I did test drive without rear DS. Seems to me there were two types of vibration. I replaced the rear DS with a new one and that resolved one of the vibrations. What's puzzling is getting the vibration when I hit a dip.

 

[Torxhead]

Bad motor mounts can cause the engine to move to the rear. This can be spotted by checking to see if the rear of the valve cover is getting close to the firewall. Also the clutch linkage bellcrank should be perpendicular to the frame.

 

[b3uhbp]

Bad motor mounts can cause the engine to move to the rear. This can be spotted by checking to see if the rear of the valve cover is getting close to the firewall. Also the clutch linkage bellcrank should be perpendicular to the frame.

I just checked the engine and it's about 1.5 - 2" from the fire wall. The mounts do have some cracks. My mechanic is going to drive it and tell me what he thinks. I'll let y'all know what he says.

 

[Lumpy Grits]

Very common for the P/S engine mount to 'tear' from torque.
They are pricey for sure, but they will be the last mounts you will ever need. M.O.R.E. has the best.
https://www.mountainoffroad.com/bomb-proof-motor-mount-kit-pre-99-cj-with-amc-6-cyl.html

Been run'n a set since they first came out. No issues, ever.
I still say to have your tires ck'b for balance.
LG

 

[b3uhbp]

Okay, had the test drive with my mechanic, he said he was sure it was in the drive line not engine mounts. In discussion we determined that the new tranny mount could have raised the tranny and T-fer case enough to increase the angles. He suggested I use spacers to lower the skid plate about .5". There are no spacer now. It makes sense. I'll know later today if it works.

 

[Lumpy Grits]

The new t'mount only restored the height to OEM. That little bit(1/2" to 3/4" at most)of change should not because'n any 'vibs'.
Take a look at how squeezed the front engine mounts are. New engine mounts will raise the engine some, that in effect lowers the tranny's tail some what.
If you can loosen the 6 skid plate bolts(good luck). Do so, 1 side at a time(have a jack under the s'plate)put a 3/4" nut in between the s'plate and the frame and tighten the bolts down. Now see if the 'vib' is gone or not.
LG

 

[b3uhbp]

The new t'mount only restored the height to OEM. That little bit(1/2" to 3/4" at most)of change should not because'n any 'vibs'.
Take a look at how squeezed the front engine mounts are. New engine mounts will raise the engine some, that in effect lowers the tranny's tail some what.
If you can loosen the 6 skid plate bolts(good luck). Do so, 1 side at a time(have a jack under the s'plate)put a 3/4" nut in between the s'plate and the frame and tighten the bolts down. Now see if the 'vib' is gone or not.
LG

Putting the big nuts between the skid plate and Fran is exactly what I did. Unfortunately it had no effect. I'm still convinced it's in the drive line but it's a puzzle.

 

[mtnwhlr]

I'm going to throw this out there. I had a vibration develop with mine and it ended up being the waterpump. The bearing went out and the fan was wobbling. It was not leaking like every other pump I have ever had go bad. But it might be something to look at. Good luck.

 

[Lumpy Grits]

Yes a 'vib' can come from anywhere. That is why I will say for the last time.
................................Have your tires re-balanced.................................
Very easy to throw a weight.
LG

 

[b3uhbp]

When you replaced the parts you could have changed the driveshaft angle enough to cause that vibration. Might have to shim the pinion angle to correct it.

Torx, I think you may be right on my issue. I could have changed the angle when I replaced the tranny mount although I doubt I changed it much. My understanding is the two angles have to be the same. Do the angles have to be exact or just close. Also, when the 2.5" lift was put on they did shim the rear end to point the pinion toward the Transfer Case but the two angles don't look the same. This is a new area for me.

 

[Torxhead]

The pinion angle will vary as torque is applied also, so I will say just close to that angle. I think your mechanic had a good suggestion with lowering the Transfer Case , just make sure the bolts have enough thread left if you add washers. Better yet, get new bolts. This will decrease breakover angle clearance though. If you go to a different pinion angle that would decrease your rear lift a bit. Another option would be to get a constant velocity u-joint added to the rear driveshaft. When you start modifing driveline engineering things can get complicated. The engine sagging from tired motor mounts or even tire vibration could also be a factor here also.

 

[b3uhbp]

The pinion angle will vary as torque is applied also, so I will say just close to that angle. I think your mechanic had a good suggestion with lowering the Transfer Case , just make sure the bolts have enough thread left if you add washers. Better yet, get new bolts. This will decrease breakover angle clearance though. If you go to a different pinion angle that would decrease your rear lift a bit. Another option would be to get a constant velocity u-joint added to the rear driveshaft. When you start modifing driveline engineering things can get complicated. The engine sagging from tired motor mounts or even tire vibration could also be a factor here also.

I did lower the skid plate but it made it worse. I got longer bolts to make sure I had plenty of threads. The tires have been balance recently as well but thisbisna totally different feel from tires. I may have to give up and get professional help, and an expert mechanics too.

 

[Torxhead]

Well, now you know what does not work. So, you might as well put that skid plate back up. Other possibilities could be rotating the pinion angle, although there might be an issue with the motor mounts as this could affect the driveline angle. Searching the internet for "Jeep CJ driveline angle" or something similar might provide some good info.

Edit: "Tom Woods Driveline 101" provides some good info, but like I said it does get complicated.