Drive train rumble - Ideas welcome

[Freeborn]

Vehicle:
- 1978 FJ40
- original 2f rebuilt about 3 years ago
- DUI dizzy
- Sniper EFI
- original 4-speed
- fresh u joints

Issue:
My truck has a “rumble” that’s most pronounced around 2000 rpm in 3rd gear. It vibrates a little in every gear around 2000 rpm, but it’s worst in 3rd. It subsides the minute I let off the accelerator and comes back when in step on it again. It feels like it’s the motor, but it’s hard to tell with all the other noise in the truck.

Things I’ve played around with:
- fresh u joints (one was stiff), no difference
- timing: When I retard the timing it seems to get better, but it’s way retarded right now at about 2 deg BTDC and I live at 6k feet. I disabled vacuum advance as an experiment with no noticeable difference. The motor runs just fine - silky smooth even at idle and higher RPMs without a load.

So I’m looking for thoughts from the more experienced folks. Is it normal? Any tricks to diagnosing the issue? Any smoking guns in my description of the issue?

 

[duncanrm]

Is the rear tail shaft in phase? have you tried removing the rear tailshaft and driving on the front trail shaft to rule out the rear shaft? Are the front hubs unlocked?

 

[B y r o n]

What was the last thing you did before you noticed this issue?

 

[Paul_A]

So it happens only under a load? May be a drive line bearing gone bad. I would probably pull the rear drive shaft (you say the u-joints are new/good and in phase) and look for rear end pinion play, t-case output shaft play, etc. Just my .02.

 

[1969FJ]

Motor mount, maybe ?

 

[Freeborn]

What was the last thing you did before you noticed this issue?

This has been going on probably since I purchased the vehicle - maybe 3-4 years.

 

[Freeborn]

Motor mount, maybe ?

Maybe. They were replaced with the rebuild a few years back. I put a jack under the pan to see if anything moved and nothing obvious happened.

 

[Freeborn]

So it happens only under a load? May be a drive line bearing gone bad. I would probably pull the rear drive shaft (you say the u-joints are new/good and in phase) and look for rear end pinion play, t-case output shaft play, etc. Just my .02.

I’ll check that when I run it in front wheel drive. I know the rear diff has a decent amount of lash.

 

[Mr. Green]

What are the angles of output shafts relative to axle input? You didn’t mention any lift… is there any? If so How much? I would zero in on rear driveline. Remove and test sounds like good start.

 

[Freeborn]

What are the angles of output shafts relative to axle input? You didn’t mention any lift… is there any? If so How much? I would zero in on rear driveline. Remove and test sounds like good start.

It does have an OME lift - 2.5” maybe(? 33s fit). I measured angles before I replaced the U joints. The angles of xfer output shaft and the rear diff are within a degree or two. Drive shaft angle isn’t extreme.

 

[Freeborn]

Ok…Next step…

I took out the rear drive shaft and took it for a spin in FWD. Seems like the vibration at 2k rpm in 3rd isn’t there. The stick still vibrates if I put a little pressure on it, but overall things seem relatively smooth. Only disappointing outcome was a new noise from the drivetrain in the front - kind of grindy every now and then on deceleration. We’ll shelve that for the moment…

So now the question is where the issue lies. I’m thinking I’ll take the rear drive shaft to an expert to see if it’s bent or otherwise compromised. Beyond that, what’s the next thing to try and diagnose?

 

[Pighead]

I’m thinking I’ll take the rear drive shaft to an expert to see if it’s bent or otherwise compromised.

That is exactly what i would do. Perhaps have them balance it as well.

 

[MDH33]

So it happens only under a load? May be a drive line bearing gone bad. I would probably pull the rear drive shaft (you say the u-joints are new/good and in phase) and look for rear end pinion play, t-case output shaft play, etc. Just my .02.

^^^ this is what I would be checking as well. Have you checked oil lvls in the Trans, tc, diffs? Maybe drain them and look for metal.

 

[Freeborn]

^^^ this is what I would be checking as well. Have you checked oil lvls in the Trans, tc, diffs? Maybe drain them and look for metal.

Haven’t checked yet, but I will now.

Maybe related: Whenever I pull the rear drive shaft off the tcase there’s a little bit of gear oil between the end of the shaft and the ebrake. Eh?

 

[Goldbug]

I had a similar problem problem, but only happened when I was pushing on the gas pedal. Sounded almost like a truck engine braking and came with a rattle/rumble. Castor shims fixed it.

 

[peesalot]

The splines on the slip part of your rear driveshaft are worn, AFAIK they are NLA ( the slip yoke part ) maybe check with Kurt at Cruiseroutfitters.

 

[Mr. Green]

Also, need pictures. Can’t believe we made it this far without!

 

[Zjohnsonua]

Is the rear tail shaft in phase? have you tried removing the rear tailshaft and driving on the front trail shaft to rule out the rear shaft? Are the front hubs unlocked?

This. Can't tell you how many times phasing the driveshaft has been the fix...and how many times people suss out a driveshaft shop for what they could have done in 10 seconds at home.

 

[peesalot]

I dont see how you do it at home, u-joints yes, but replacing the slip spline, no.

 

[Zjohnsonua]

I dont see how you do it at home, u-joints yes, but replacing the slip spline, no.

More often than not the splines aren't excessively worn. Rather, someone took the DS apart and then put the splines together with the two halves out of phase. Given how coarse the spline is, getting the yokes out of phase by even one spline can cause this.