bad vibraton at roughly 2000 RPM, 98 6.5L 4L80E

[GM Guy]

hey all,

I was wondering if anyone could help diagnose mom and dad's 1998 chevy K2500HD EBSB 6.5L 4L80E 4wd.

ever since they got it, it has had a bad vibration at 2000 RPM. It has nothing to do with the truck, as you can pull it down to third and stay at the same speed, but with higher RPM, and it goes away. you can kick it in neutral and it goes away, but in gear or in neutral, at 2000 RPM it has a bad vibration.

the previous owner did stick an engine in not long before we bought it. is there anything that could be bent from the swap? (input on trans, flexplate, etc)

we were concerned enough to drop the crossover and pull the bellhousing cover/ bellhousing half and check the converter bolts for tightness, but nothing was loose or looked bad.

TCC will unlock when you let off the throttle, no engine braking, just down to idle. it will give a little engine braking if you pull it down while coasting up the offramp. It behaves very fincky, on any slight downhill it will unlock the TCC and rev up 50-100 RPM, going back up the hill, it will lock up solid and pull normal. some terrain it likes cruise better, some it behaves better on the foot feed.

any input appreciated, thanks!

 

[GM Guy]

I noticed my sig is wrong, the truck has more like 240K on it, not 280. I am not sure, but I think it is original everything, except for the engine. fluid has been changed, no difference, didnt smell or look bat at all. tiny bit of clutch "dust" in the pan, but only a little reside that settled out of the fluid. the fluid change netted no difference.

THe truck has a Fluidampr, that helped. grounds cleaned, that helped the engine run a tick smoother, but nothing has improved the transmission.

 

[Will L.]

Did you swap out the belt drive pulley when you put I the FD?
Maybe swap out fan/fanclutch with one of the others.

 

[GM Guy]

would un-hooking the belt for a less than 1 minute run time net the same?

 

[Will L.]

Yeah, pull the drive pulley off the FD also. Then fire it up. Only a couple minutes.

 

[NVW]

I would check the exh pipes, if they touch any metal you will get a vibration. I got one when I installed my ATT, I had to tweak my brackets to get rid of it.

 

[GM Guy]

this is a shuddering vibration, like a torque converter wanting to remove itself from the bellhousing, no noise or anything. do torque converters start vibrating/ shaking when they go south?

 

[Will L.]

350 sbc gasser
I threw a weight off a converter once, but the flex plate broke also. Sounded like a rod knock. It had a vibration but got worse the more rpm, all the way up to wot. When It didn't come apart after 10 minutes wot I decided to shut it off and fix it instead of scattering it. (can you say teenager?)

 

[billyg38120]

I will try to help but I need specific info. Did many @ GM in buy-back. First. I take it you bought it this way? Have you looked at the engine and trans mounts for collapsing and ground outs? Exhaust ground out on the frame? Broken Flywheel and/or trans case I need these cleared 1st.
Then if you put trans in park and run engine @ 2,000, is the vib there at the same level or close? If you allow the vib to run a bit at this steady RPM will the Vib cycle in and out? (Better then back to worse? Or does it remain?

Remove all the torque converter bolts and push the converter REARWARD. Have someone start the engine while you watch the flywheel. DO NOT PRESS on the throttle it revs up very fast. Does the flywheel move forward and rearward or BENT? A 1 inch bend in the flywheel WILL cause a first order engine disturbance or shake. I straighten the flywheels and use the starter nose to guage where to bend it. Try NOT to bend the flywheel AWAY from the starter bendix gear. Or just replace it if you don't feel comfortable doing this.
Reconnect the torque converter to the flywheel.
If the flywheel is straight and the vib remains I have seen missing torque converter weights cause this. Also I have had missing or WRONG flywheel on engine cause this. In extreme cases I have balanced the engine by adding washers to the torque converter bolts. I take a sharpe and number each area of the bolts like 1 2 3. Then add med washer to #1 and run engine at 2k. Better or worse? Worse, move washer to next bolt. You may end up with 2 small washers on 2 converter bolts to balance it.

If the VIB cycles in and out, it is MOST likely the turbine inside the torque converter OR the drum that is splined to the input shaft. In the case of the 4L80-E It could be the 4th Gear carrier or Forward clutch drum. Yes the forward cl drum. Look at your input speed sensor on the scan tool and you will see a higher RPM at idle(in park/Neutral) than engine speed. It does spin a bt faster than the engine. It being the forward drum.

I assume the vib is gone when you move the engine higher or lower in speed away from 2,000 RPM engine spd.

One more item. Is the rear prop shaft BOTTOMED out in the t/case or trans? Hit in rear and axle moved forward. I ask because I am not there to see this.
I saw reports on these dampeners @ GM from the Noise and Vib Lab. Use an OEM design OR a ATI rubber one. It does save the crank from the vibs that break them.