Hey guys! Need some more help from the pros. So I haven't had my XJ long. When I bought it it was bone stock, since then I've done the 3 inch lift and replaced all the U joints for front and rear driveshafts. Here's my issue. When I get to around 35-45 MPH and let off the throttle, I hear this sound that feels like it's under my feet that sounds like a dull rotating sound, almost like there was slop in the front DC joint. It only happens when letting off the throttle and I feel like it's mainly between those speeds the most. I cannot recall if this sound was here before the lift unfortunately. I first thought it was my front shaft so I removed it and driving it without the front shaft changed nothing. So now I'm stumped. Any thoughts ideas as to what it is/could be or any other tests I can do to find the problem? Thanks!

 

Have you looked at the transmission mount and motor mounts?

 

Visually yeah I know for sure they are worn tho I can tell the most with vibs when in reverse and in 4th gear low RPM. You think that's the culprit?

 

Well, just thinking about when it happens. When you let off the gas, the driveline and/or motor may be settling if a mount is shot. Both are cheap and pretty easy to swap.

 

Indeed. I had them on my list to replace so I'll replace those and then report back. Was curious if anyone else had experienced it.

 

I bought mine lifted 3.5 in and it does the exact same thing still haven't figured it out.

 

Well, check your motor and transmission mounts! If they are shot it could be allowing everything to move around. Then you can move on to checking driveline angles. 3.5 is right on the edge of needing to correct driveline angles.

 

Alright will do! I'll post back with the results. Thanks!

 

Many XJ's require at least a transfer case drop kit with a 3" lift to avoid driveline angle problems, if so its a cheap easy fix. .......But its hard on motor and trans mounts also because of the drop, the correct way is an SYE install FYI, but costs are much higher.

 

Yeah I don't want to TC drop going to throw in the new mounts and see what happens!

 

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The tendency for vibes due to lift are variable depending on the XJ. Auto with D35 is most likely to vibe at 3". Stick with C8.25 most likely will not at 3". Do the mounts and then plan for an SYE.

 

Woops. My bad. Here's the post I was thinking of. Sub D35 for the C8.25

2. When should a SYE be used?

The stock rear driveshaft is a fixed length unit that relies on the slip yoke to adjust when the suspension travel. When lifting an XJ, the angle between the rear axle pinion and the slip yoke become much greater. The factory setup was not intended for these variations in lift. The angle difference will be most noticeable in the fact the slip yoke will be pulled further out of the housing than before the lift.

The differences in angle can be attributed to several things, but they all revolve around this , the length of the . You will hear various things from various people about what size lift can be ran without a SYE. The key here is to recognize the differences between their XJ and yours.

The worst possible setup you can have is a 96+ with an auto transmission, a NP242 transfer case, and an 8.25 rear axle. This combo makes for a long drivetrain and the 8.25 pinion housing is also quite long. These factors will leave you with a short rear driveshaft and even a 2" lift will see negative affects from it.

The best setup for less noticeable vibration would be a 84-95 with a manual trans, a NP231 transfer case and a D35 rear axle. The 84-95 slip yoke was shorter than the 96+. The Manual transmissions are shorter than the auto transmissions. The NP231 is shorter than a NP242 transfer case. The D35 has the shortest pinion housing, followed by the D44 and the longest is the 8.25.

 

[QUOTE=oldguy52;3094024]Woops. My bad. Here's the post I was thinking of. Sub D35 for the C8.25

2. When should a SYE be used?

The stock rear driveshaft is a fixed length unit that relies on the slip yoke to adjust when the suspension travel. When lifting an XJ, the angle between the rear axle pinion and the slip yoke become much greater. The factory setup was not intended for these variations in lift. The angle difference will be most noticeable in the fact the slip yoke will be pulled further out of the housing than before the lift.

The differences in angle can be attributed to several things, but they all revolve around this , the length of the . You will hear various things from various people about what size lift can be ran without a SYE. The key here is to recognize the differences between their XJ and yours.

The worst possible setup you can have is a 96+ with an auto transmission, a NP242 transfer case, and an 8.25 rear axle. This combo makes for a long drivetrain and the 8.25 pinion housing is also quite long. These factors will leave you with a short rear driveshaft and even a 2" lift will see negative affects from it.

This is exactly what I have. I played around with TC drops and 1 in. Is what I settled with until I can afford a SYE kit the vibrations are basically gone but the noise has continued I will check the mounts and see if that's the problem.

 

So just to provide a little update and revelation I had this morning.

Yesterday I inspected and can definitely see my trans mount isn't sitting completely where it should so it definitely needs replacing.

The other thing I realized is that yesterday after work the noises/vibes were particularly bad and this morning they were not that noticeable. This led me to believe that the mount is the culprit because it was hot yesterday after work and this morning it was cold so what I'm thinking is hot worn rubber mount = slack, play and vibes; vs cold rubber mount is a little stiffer. Also I tested my rear shaft at the slip yoke and noticed some play so definitely need SYE.

Will report back once mounts are done