Feeling of slack in drivetrain

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JohnBravo

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Hello all,

I don't know how better to describe this, but it's getting to be quite the annoyance. I pulled my '99 Corvette home 120mi, and while it was smooth sailing for the most part, I'm left pondering this issue - I'm hoping someone may have input that could lead me in the right direction.

Initial acceleration from a stop in 2wd feels sloppy...like the driveshaft is flopping around or something isn't grabbing. I couldn't break the tires loose if I tried (I'm not trying, but my point is I don't get full power from a standstill), so I've been under the impression it was some sort of torque management to save the transmission.

During the drive home a tractor pulled in front of me so I slowed to about 3-5mph. I couldn't safely pass until we reached one of the biggest/steepest hills on my journey (a 1/2mi pull). With a line of cars behind me waiting I put it in 1st and pinned the throttle. On an uphill with that weight behind me acceleration was 'sloppy' until I hit about 20mph...my wife asked if everything was okay and I nervously said, "yeah, it always does this when I first get into it, it'll smooth out in a sec."

Is this normal? Driving around empty that feeling is less-pronounced, and I haven't noticed torque converter shuddering. I used Lubrigard when I thought it could be the issue, and nothing changed. I drained and refilled ATF (and changed filter) 10k miles ago - fluid was prestine without burning odor, changed rear-end oil at the same time, engine oil is fresh 10w30, new shocks all around last year, two months ago I pulled the bed off and cleaned up the frame, rust-bullet the whole thing, put in new fuel pump and tank, cleaned/fixed all grounds I could find, and while the truck feels stronger (and no longer leaks anything), I'm still getting that slack-feeling.

The truck rockets away smoothly from a stop in 4wd...also confusing to me. One of my friends suggested a bad u-joint, but last year my mechanic said, "they all seem tight, it's an old truck, just drive it." He greased everything he could and on I went. I still wonder about that...additionally there's a vibration felt in the whole truck between 30-50mph, getting wheels/tires balanced didn't fix that so I've just lived with it.

Sorry for the book, looking forward to any and all input. I appreciate you guys!

Jordan
2000 K2500 Crew-cab 6.5ft bed, 5.7L 4L80e 3.73, 250k original miles, she's rough from derecho damage and the typical midwestern rust, but a solid truck nonetheless.
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Schurkey

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2000 K2500 Crew-cab 6.5ft bed, 5.7L 4L80e 3.73, 250k original miles

Initial acceleration from a stop in 2wd feels sloppy...like the driveshaft is flopping around or something isn't grabbing. I couldn't break the tires loose if I tried (I'm not trying, but my point is I don't get full power from a standstill), so I've been under the impression it was some sort of torque management to save the transmission.
If the driveshaft were flopping around, you'd have vibration. If it were torque management, it'd be smooth but gutless.

Is this a vibration problem or a power problem?

acceleration was 'sloppy' until I hit about 20mph...my wife asked if everything was okay and I nervously said, "yeah, it always does this when I first get into it, it'll smooth out in a sec."
GM has been known to delay proper power fuel enrichment until the pedal has been on the carpet for a certain number of seconds. I can feel this in my '98 Monte Carlo--gutless for awhile, then (I guess) the power enrichment fueling kicks in and it accelerates better. There doesn't seem to be anything wrong, it's built-into the computer tuning. I learned about this from this site--one of the computer-tuning wizards was discussing how he tuned that out of his programming.

Is this normal? ...I'm still getting that slack-feeling.

The truck rockets away smoothly from a stop in 4wd...also confusing to me.
I have no answer for that.

...additionally there's a vibration felt in the whole truck between 30-50mph, getting wheels/tires balanced didn't fix that so I've just lived with it.
First Guess: Driveshaft slightly bent, or U-joints failing. If that's the case, it wouldn't matter what gear you were in, it'd be speed-related not RPM-related.

My Trailblazer has had driveshaft problems twice. On that vehicle, it's worst about 72 mph. When it gets fixed, I notice that it's smoother from 25mph on up. But I don't notice 25-up having vibration until it's gone.
 

1998_K1500_Sub

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I pulled my '99 Corvette home 120mi, and while it was smooth sailing for the most part...

Initial acceleration from a stop in 2wd feels sloppy...like the driveshaft is flopping around or something isn't grabbing. I couldn't break the tires loose if I tried (I'm not trying, but my point is I don't get full power from a standstill), so I've been under the impression it was some sort of torque management to save the transmission.

If you've got a lot of weight on the back causing it to sit a$$-low, the angle at the pinion’s U-joint can become excessive and cause noticeable shudder on launch from a standstill.

If the torque converter stator was freewheeling (i.e., bad stator clutch), it would cause poor acceleration under conditions during which the converter slippage was high.

These are my random thoughts. Moving on...

On an uphill with that weight behind me acceleration was 'sloppy' until I hit about 20mph...

So, what happened at 20MPH?


The truck rockets away smoothly from a stop in 4wd...also confusing to me.

You're comparing "4-hi" to "2-hi", yes (I have to ask)?

It "rockets away smoothly", meaning... it accelerates quicker in 4wd vs 2wd, or rather it accelerates more smoothly in 4wd vs. 2wd?

Additionally there's a vibration felt in the whole truck between 30-50mph, getting wheels/tires balanced didn't fix that so I've just lived with it.

That's around where the torque converter clutch would be locking-up, depending of course on throttle position and speed. Hmm.

I had a mystery "vibration" in an S10 once that was due to the cab-to-frame mounting hardware (bolts and large washers, isolated with rubber biscuits) being so badly rusted that some of them had literally fallen off the truck (one actually fell off in my driveway... when I saw it I realized what was happening). With some of the cab mounting hardware missing, the cab would freely vibrate under certain conditions.


Other thoughts:

- Is the transmission evidently shifting between all four gears as it should? Are there any unusual transmission behaviors, otherwise?

- Is the torque converter locking as you believe it should? Are you certain?
 
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Orpedcrow

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Not being a smart ass but could it be you’re towing a car on a heavy af uhaul car hauler with a 5.7? I know corvettes aren’t heavy like dodges but it’s no Miata either lol
 

JohnBravo

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Thank you so much for your replies! I'll try to answer questions as concisely as possible.

Does the driveshaft have play? It does move around some, but not due to the u-joints as I can tell (it is a multi-piece driveshaft). I am going to try attaching a short video, but I'm almost certain I've discovered what the problem is (at least in-part) through your questions - not sure if it's considered a support bearing or what...I'll likely be dropping the driveshaft and replacing whatever u-joints I can access and the associated bearing if I can remove it.
Is it a vibration problem or power? I would say primarily a vibration problem that I had confused with a power problem.
First guess - driveshaft slightly bent or u-joints. Thank you for your thoughts, I am starting to agree with you based on what I've investigated.
What happened at 20mph? Acceleration became smooth, also I've noticed that with a load the 30-50mph vibration is less-pronounced.
Comparing 4-hi to 2-hi, yes? Yes indeed. It seems that what I perceive as slack tightens up when in 4wd because the truck willingly rockets away from a stop. It feels faster and smoother. My current working theory is that the connection to the front wheels is tight removing some strain from the rear portion of the drivetrain...probably covering up the perceived "slack."
Cab-to-frame mounting hardware - I will check that and replace if I can, I'm certain these are rotted out after 22yrs. I appreciate that thought, it had not crossed my mind.
Is trans shifting smoothly through all gears? Yes, it feels exactly how I expect from a 4L80e.
Is torque converter locking as it should? I feel it locks exactly as expected, smooth transitions between locked and unlocked.
Not being a smart ass, but could it be towing heavy with 5.7? Haha yes I suppose it's possible :) honestly though, the 350 has done great for me because I rarely tow heavy...I think the Vette (3400lbs) and trailer (2200ish) were safely within the rated capacity of the truck (though it is getting older so perhaps this was a bit heavy for the old beast). I still can't believe I managed just shy of 12mpg on that trip fighting wind and all...fits right in with my overall average of 11-13mpg lol I primarily use the truck to pull our UTV and occasionally my father-in-law's tractor or hay-trailer. I'm actually rather impressed with what can be done using the 250hp as rated knowing it's 'underpowered' against modern trucks.

I'm going to attempt to attach a video of my findings. It's currently transferring from my phone to computer via BT. It shows significant play in the driveshaft due to what I think is a worn-out support bearing. It looks like I'll need to remove the driveshaft anyway to replace this so I'll just knock out the associated u-joints at the same time.

Is there any reason to take the driveshaft over to a local shop to check balance while I have it off? I know of a place in town, but I'm told they are $$$expensive$$$

You guys are great, thank you so much!
Jordan

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RichLo

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Yep, thats what I was going to suggest but didnt see this thread until just now. I had similar issues with my dually and once I replaced that rubber carrier bearing bushing everything is fine now.

If the brearing itself is fine, you can get just the rubber insert and then you wont have to worry about pressing a new bearing on. Super easy repair.

I'm in the 'dont spend money if you dont need to' club and your driveshaft wouldnt go out of balance if it hasnt hit anything and the factory weights are still attached... fix the problem, try it out and go from there. You should be fine.
 

Intragration

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I'm a late arrival here. I had a problem with mine, I can't say it was similar, it was not during towing, but it also had some of the hallmarks of what you described. A "gutlessness", "sloppy" acceleration, low on power, but not apparent transmission slipping, clean transmission fluid, firm "normal" shifts. I tried for a long time to figure this one out. I thought it was engine, sensors, nothing made sense, and the problem would come and go, but when it was there, it was terrible and very noticeable. Powering down and restarting 30 seconds later would usually fix it temporarily.

As it turned out, it WAS the transmission. I had no reason to believe it was, it was supposedly a newer rebuild, and all the signs pointed, I thought, to engine, sensors, computer. But when I finally had the transmission built properly, all problems were fixed and it was like a new truck. I do not recall the findings of exactly what the internal problems were, but every symptom was fixed with the new trans (and converter). Again, I don't know if this is what your problem might be, and my situation is almost totally different (Suburban, no towing, 454, TBI) but some of the things you described sound similar, and you also have a 4l80e, so just throwing it out there for you or someone in the future.

On the driveshaft, if you're planning to keep the truck, I tend to agree with "not spending money if you don't have to", but when I take something apart, I tend to do the full job on that area, so I don't have to worry about it again. If I was taking the driveshaft out and doing u-joints, I'd have it balanced at the same time. Doesn't seem it could be THAT much, even at a crazy-expensive shop. Then you'd know that that one component is as good as it can be.
 
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