'bogging' down at about 55

[destiny71]

Last month, I started having a problem with my truck seeming to bog down at about 55.
It's doing it on a cycle.  Maybe 2 times a second.  Bogs, free, bogs, etc..  This weekend I noticed that when it bogs down, the speedometer jumps to about 75+ as the truck actually slows down.  Then, the speedometer drops back to actual speed, and everything frees up normal again.  Repeat.

After some driving, it starts happening at lower speeds, until I'm barely able to get up to 45 up a hill.

Possibly related, there's a 'growling' sound that's been happening since I got the truck a year ago.  Happens about 55.  Again, not continuous, but at a frequency that increases with speed.  Sometimes goes away about 65.

Raised the rear end, and spun the each wheel, and felt no grinding, nor heard any odd sounds.

Drained transfer case.  Nothing abnormal noticed in the oil.  Replaced with ATF (married to trans)

Drained transmission, dropped pan, and replaced filter.  Again, no flakes, chunks, or burned ATF.

u-joint to diff has side to side play of the center yoke of the u-joint in the cups.  Not much, but noticeable. With the wheels off the ground, and transfer case in neutral, spinning the tire, you can hear it 'clunk' as it rotates, and slides down every half turn of the drive shaft.  Will be replacing both u-joints this week.  But not convinced that is causing my bogging down.

75 GMC K25  350 to TH350 w/ NP205 & 14bolt

Any help would be appreciated. 
Thanks

[Jason S]

Is the engine bogging down like its running out of gas or does the engine rpm raise when the "bog" and truck ground speed slow when the problem starts? 

[destiny71]

Engine rpm seems to stay the same.  My tach is fried, so I'm only relying on sound.

After a while, and it get's worse, I'll floor it, and the trans will downshift.  The engine will rev up, but it the truck continues to bog down.

Yesterday, I removed the rear drive shaft, and drove it up to speed with just the front end.  Still does it.  But, the growling went away.

When I was putting the drive shaft back on, I noticed quite a bit of in and out free play on the output shaft of the transfer case.  It sounds a bit more noisy than it should be when spinning it while out of gear.   Possibly a bearing seizing up?

Checked on prices for a rebuild kit, and may go ahead and do that in a couple of weeks.

[Jason S]

The growling could be coming from your rear end since it didn't have a load on it. 

One quick question to eliminate any other possibilities: while the rear driveshaft was attached, were the front hubs locked or unlocked when you were experiencing the problem?  Otherwise, It sounds like you've narrowed the pending problem down to the transfer case... There shouldn't be much end play from the output and if the transmission is operating fine and the engine isn't showing issues then you're working the problem.

BTW, I think the NP205 needs gear oil and not ATF.

Good luck and keep updating.

[DnStClr]

There's been a lot of discussion about which oil should be used in the NP205 because it's a gear driven box and not chain driven. The general consensus is that tranny fluid is fine to use. (More viscous in cold weather). 

[destiny71]

The seals are leaking for the output shafts of the 205.  I suspected low oil, and in researching what to fill it with, I found AFT works, and is recommended for married t-case.  I drained the gear oil, and refilled with AFT.  It wasn't that low, probably got 2qts out of it.

When checking transmission level, I did find what looked like gear oil mixed in.  Did my best to flush everything out, and replace with fresh ATF.  And replaced the filter.

This problem started before switching oil.