GM 1970 GMC K2500 Driveline and Engine Running Issues (1 Viewer)

[BILT4ME]

We have a 1970 GMC 3/4T 4x4 350/4 sp /4WD (SM465 / NP205) with front lockout hubs and we are having driveline issues (as well as engine issues, but I want to address the driveline stuff first:

This truck lived its life as a construction truck and apparently had a flatbed on it for a large portion of the time, as it has weld spatter all over the roof and back window.

We have been working on this thing, getting it back into driving condition and have done the following:
NEW GM Crate 350 to match the stock factory 350
Rebuilt the SM465 4 speed (granny low transmission)
Rebuilt the transfer case (New Process NP 205)
Rebuilt the rear axle with new gears (4.11:1) and a new Detroit Locker, Dana 60
Rebuilt the front axle with new seals, Dana 44
Rebuilt front and rear driveshafts with all new yokes and U-Joints. (Inland Truck Parts)
Installed a Skyjacker 4" lift all-spring lift.

The original suspension had broken leaves in it and had a set of bolted-on overload springs on top of the original rear springs. This truck has had a lifetime of serious abuse. There is crap welded to it EVERYWHERE and we are trying to eliminate it as we can find it and get to it.

It apparently had a snow plow on it at some point.

We cut off about 150 LB of channel iron crap welded on the rear as a hitch. We left the 4" sch. 40 pipe bumper at this time.

Driveline issues:
When we first installed the lift, we reinstalled the original spacer blocks in the orientation that we took them out (doesn't mean it was right to begin with) and the thick side was to the REAR.
Skyjacker instructions instructed to place the thick part of the spacer on the spring pack to the REAR of the truck, consistent to what we started with.
Once all installed, the pinion was pointing UP in a "broke-back" configuration to the output shaft of the transfer case. It was giving weird axle-wrap issues and the clutch would not allow a smooth start, apparently due to axle-wrap.
We dropped the rear axle and turned around ALL the blocks and the pinion appears to be in correct alignment with the output of the transfer case (as in parallel).
We are now getting a vibration / bearing noise at the point of going from acceleration to deceleration. Possibly at the transfer case output.

So, we thought that MAYBE we were jamming the rear DS into the TC. So we measured the DS length at ride height, and full droop, and at deep compression.

I was surprised to see that at full droop the DS was SHORTER by 9/16" than at ride height.
When we compressed the rear (Added a LOT of weight in the bed), the DS never got longer than at the ride height. The splines were NOT protruding out the back of the yoke or anything.

This just seems backwards to me.

What driveline calculators do you all use and how do you confirm it is CORRECT?

Please help me with you experiences. I am calling a couple of you to this thread due to what I have read about you doing to your 80's, and I greatly respect that.
@nukegoat @jcardona1 @Tools R Us @inkpot

If you can't help or don't want to get into this, that's OK too. I appreciate your time.

 

[PAToyota]

Any chance you got the u-joints on the driveshaft out of phase when you rebuilt them?

 

[BILT4ME]

Any chance you got the u-joints on the driveshaft out of phase when you rebuilt them?

That was the first thing I checked.

They are in-phase front and rear.

 

[PAToyota]

Next thing I was going to say was that with the broken leaves there may have been damage to the t-case, but you said that was rebuilt as well.

I'm sure you know this, but how are the angles at each u-joint?

 

[BILT4ME]

Next thing I was going to say was that with the broken leaves there may have been damage to the t-case, but you said that was rebuilt as well.

I'm sure you know this, but how are the angles at each u-joint?

Immediately after we got the lift installed, following the directions, it looked like #4, the "Absolutely Not" version.

After dropping it and resetting the spacers so the wide part was to the front, it now looks like #2, parallel.

 

[PAToyota]

Do you have any pictures of the overall setup?

 

[numby]

Yes, rear blocks would likely have the wider/thicker end toward the front (though this does depend on where the center pin is located along the arc of the spring). This would tip the pinion slightly downward from parallel with the output of the transfer case. It would rotate slightly upward under acceleration to then come into parallel with the output from the 205. It will continue to tip upward the more load/torque you apply until the pinion goes past parallel and you get nasty vibration and short u-joint life. This may be your issue especially with cheapy lifts designed for on-road comfort or squishy off-road flex. The leafs are just too easily wrenched into an "S" shape as your right foot tries to get these heavy Chevys to stand up. If you start parallel you have nowhere to go but out-of-parallel once the truck starts moving. On and off the throttle will give a pulsation either way as the u-joints pass back and forth through copacetic at either end. Make sense?

Yes, the rear DS will lengthen at full compression because the axle is moving rearward as the spring flattens (measured in a straight line from eye to eye). The anchor point is the front eye (of the rear spring) with the rear shackle allowing the spring to lengthen/flatten under compression or shorten under droop.

Yes, you could still have t-case issues despite the rebuild. Crap happens. Brand new bearings fail out of the box. I'd also confirm your coupler splines are very, very good and the adapter is bolted up tight all the way around. Double check the trans mounts and the engine mounts.

You also said the rear end was rebuilt but what you've described sounds to me like r&p issues. HTH.

 

[BILT4ME]

Yes, rear blocks would likely have the wider/thicker end toward the front (though this does depend on where the center pin is located along the arc of the spring). This would tip the pinion slightly downward from parallel with the output of the transfer case. It would rotate slightly upward under acceleration to then come into parallel with the output from the 205. It will continue to tip upward the more load/torque you apply until the pinion goes past parallel and you get nasty vibration and short u-joint life. This may be your issue especially with cheapy lifts designed for on-road comfort or squishy off-road flex. The leafs are just too easily wrenched into an "S" shape as your right foot tries to get these heavy Chevys to stand up. If you start parallel you have nowhere to go but out-of-parallel once the truck starts moving. On and off the throttle will give a pulsation either way as the u-joints pass back and forth through copacetic at either end. Make sense?

Yes, the rear DS will lengthen at full compression because the axle is moving rearward as the spring flattens (measured in a straight line from eye to eye). The anchor point is the front eye (of the rear spring) with the rear shackle allowing the spring to lengthen/flatten under compression or shorten under droop.

Yes, you could still have t-case issues despite the rebuild. Crap happens. Brand new bearings fail out of the box. I'd also confirm your coupler splines are very, very good and the adapter is bolted up tight all the way around. Double check the trans mounts and the engine mounts.

You also said the rear end was rebuilt but what you've described sounds to me like r&p issues. HTH.

@numby thanks for the reply!

I need to take and post pics of the driveline and suspension.

The Skyjacker springs are the 52" springs and the pin is perfectly centered in the arch. As we load the suspension, the axle SHOULD rotate up and away from the TC as the springs flatten out because, as you stated, they pivot on the FRONT spring perch. However, I would expect that the DS would get longer, but it remained the same length as stock empty ride height as we loaded it.

Now saying that, I can see how it would get SHORTER when it droops, because the rear axle is still pivoting around the front spring perch.

There is a T-Case support block that all the forums say to remove (and we did because it was already broken) and we are considering fixing and installing it with new bushings in order to see if it will resolve any driveline issues.

So, I still don't have any clear answers. We tried to drive alongside while he accelerated/decelerated, but it was to difficult to match speed and actually SEE what was happening. I think we need to set a GoPro under there and get a good view of what it's really doing.

We also continue to find little things that need attention, such as a cracked spring perch on the front, front cross member that ALL the rivets are so loose that we cut them off, and installed grade 8 bolts and cranked them down. Also went back through and tightened EVERY bolt for engine mounts, transmission mounts, clutch linkage mounts and lubrication, and it has helped a little.

Many years ago, I had a custom built '69 Chevy K10 that was a Johnny Cash special and it had so much rear axle wrap that I built a set of ladder bars for it. Maybe I need to do that here, too.

I guess I don't even know what my questions are anymore.

We still need to address the engine running issues as well.

I will do a separate thread on that, though.

Thank you all for your input.

I will still gather photos and post at some point.

 

[numby]

There really isn't much change in rear driveshaft length on these longbed PUs unless you have super flexy suspension. Like maybe 24" or greater flex though that's just an estimate. The roughly 6' driveshaft only changes angle by a few degrees (out of 360º) when moving through spring compression so you're not going to see much.

I should amend what I said above about the lift blocks having the thicker end toward the front. That's not the way the manufacturers say to do it.They want you to tip the pinion UP so you won't have to lengthen your driveshaft after slapping on their lift kit. Installing them that way you will end up with something akin to PAToyota's "Absolutely NOT" image. Oddly, reversing the blocks from the way the manufacturers tend to say to install them (as I said to do) will affect your driveshaft length after the rear springs are installed.

A weird thought came to me though. Maybe your driveshaft slip joint is bound up somehow. Like maybe the grease has gone hard/dry and is creating a vacuum situation along the splines which isn't allowing any change in length at all. If that's the case then the driveshaft would be putting unnecessary pressure on your t-case output shaft and the pinion shaft when you load the suspension and would pull on both in droop. Remove the zirc fitting at the splines and see if that helps or, better yet, take the DS apart and lightly regrease it. Make sure you don't over grease it as that won't allow it to shorten when needed either. Fluids - i.e. greases - don't compress easily. If they did your brakes would always be spongy. Fun.

I wouldn't bother with the whole camera thing. It's not rocket science here. Everything follows basic physics. I figured out my similar vibrational issues when such options were completely unheard of. Trust me...If I can do it anybody can.

 

[BILT4ME]

There really isn't much change in rear driveshaft length on these longbed PUs unless you have super flexy suspension. Like maybe 24" or greater flex though that's just an estimate. The roughly 6' driveshaft only changes angle by a few degrees (out of 360º) when moving through spring compression so you're not going to see much.

I should amend what I said above about the lift blocks having the thicker end toward the front. That's not the way the manufacturers say to do it.They want you to tip the pinion UP so you won't have to lengthen your driveshaft after slapping on their lift kit. Installing them that way you will end up with something akin to PAToyota's "Absolutely NOT" image. Oddly, reversing the blocks from the way the manufacturers tend to say to install them (as I said to do) will affect your driveshaft length after the rear springs are installed.

A weird thought came to me though. Maybe your driveshaft slip joint is bound up somehow. Like maybe the grease has gone hard/dry and is creating a vacuum situation along the splines which isn't allowing any change in length at all. If that's the case then the driveshaft would be putting unnecessary pressure on your t-case output shaft and the pinion shaft when you load the suspension and would pull on both in droop. Remove the zirc fitting at the splines and see if that helps or, better yet, take the DS apart and lightly regrease it. Make sure you don't over grease it as that won't allow it to shorten when needed either. Fluids - i.e. greases - don't compress easily. If they did your brakes would always be spongy. Fun.

I wouldn't bother with the whole camera thing. It's not rocket science here. Everything follows basic physics. I figured out my similar vibrational issues when such options were completely unheard of. Trust me...If I can do it anybody can.

Thanks for your input!

This is a brand-new driveshaft. No old grease, no over-pressurization of the yoke.
We reinstalled the angled blocks the same way they came out when we removed the axle to prepare for the lift.
This does NOT mean they were in there the correct direction to begin with.
The lift manufacturer also stated that the lift blocks should have the thicker part to the rear.

We've done a few things to improve it. One was to improve the engine. It turns out it was having MASSIVE vacuum leaks on the intake manifold and by changing the intake gaskets and torquing it properly, we have made the engine run MUCH smoother so we get better and smoother clutch engagement, so the driveline is not being shocked so hard.

We also had a bunch of loose bolts on engine mounts and we have removed some rivets on cross members and replaced with bolts to reduce frame flex.

I had thought it was odd about using the same parking brake cables AFTER the lift as we had before the lift. Once we rotated the blocks and brought the pinion back down, we needed longer parking brake cables (about 6" longer).

On my 69, I had so much axle wrap due to the factory spring blocks that I HAD to install the ladder bars. I had 375 HP and just rolling in traffic in 2nd gear, I was getting so much wrap that the tires were walking forward about 3" and grinding on the fenders. Again, no lift, just a difference suspension design. I wanted to make sure we weren't having oddball issues here as well.

Over time, we will get this thing sorted out. It just gets frustrating having to correct so many things because the yahoos that owned and worked on this thing previously screwed up everything. We have been careful to place thing back the way we took them apart, or have tried to search out the "proper" way of doing them when encountering an issue.

Just last night we discovered that his LF axle housing seal is leaking diff oil. Now we need to determine why....

And the list goes on, just like the list on my LC, just a lot longer.......

 

[LINUS]

Just exploring some forums I never visit - @BILT4ME , did you figure out anything here (That driveline sounds bizarre on the measures).

We have a ‘84 4WD Camper Special (1t axles, susp under a 3/4t frame).

It’s got a Gear Vendors over/under-drive unit so it has modded driveshafts, so ours isn’t any good to use for trying to give you any measures on shafts.

I forget the front diff, RR is a FF 14 bolt I remember as I “helped”/learned w/ Dad swap gears in the R&P some ~25yrs ago.

We still have it, I have driven it plenty prior to my FZJ80 days when my s-box little cars were broke/in a teardown.

 

[BILT4ME]

Just stopping back in.

I was driving the GMC a few times in the last couple weeks, and another thing that it is experiencing is Bump Steer. Like LOTS of it. Every bump in the road (I'm talking city streets, not gravel or offroad) causes the steering wheel to jerk. It doesn't matter whether the new Rancho steering damper is installed or not.

Again, this is a Skyjacker 4" lift. We installed the offset steering arm (that raises the rear of the drag link) that came with the kit. As I'm researching bump steer, some folks are saying that the drag link must be parallel to the springs, not level with the frame or the ground.

So I looked at the new setup this weekend, and it appears that the new steering arm is about 1" to 2" too high because the rear of the drag link is HIGHER than the front, and it definitely is NOT parallel with the springs. I'm tempted the change out the steering arm back to the stock one just to see if it gets rid of the bump steer, but I feel there are SO many other things that could be causing issues here.

I know we need to do an alignment. I know all I can REALLY adjust is toe-in, but I would like to find out my actual camber / caster numbers in order to determine if there is another issue, such as a bent axle, springs not set up correctly, ........something.

Maybe I'll weld up the spring perches, swap out some rivets, change hardware to nylock nuts, then see where that leads.

Also, we have a ceramic puck style clutch installed to be "heavy duty". Through more research, I have also discovered that the ceramic puck style don't like to "slip" when engaging. They bite and jump. That can also be part of our issue of how we keep killing it at intersections, and it is NEVER smooth. This truck drives SOOOO much better when it has 1000 LB of stuff in it than it does when it's empty.

If anyone knows anything about bump steer, how to measure it, and how to correct it, I'm open to suggestions!

 

[Whiskerz]

The circle track and road race guys have lots of information about bump steer. There are some good articles out there. With a puck clutch you basically have an on and off switch. They will not like slipping. Here is an article on bump steer to get you started How to Correct Bump Steer - OnAllCylinders