Vibration when downshifting at highway speeds (1 Viewer)

[Miljack]

I found a LONG thread that discussed "vibrations" and found some good ideas, however, I wanted to post a specific subject to get your thoughts.
My 100 ('00 LX) will cruise fine on the highway, when it needs to downshift and revs get over 3krpm (prob 3.5K and up is the problem), I'm getting a pretty good vibration. It's definitely RPM related, so I eliminating driveline issues (for the most part), my question, would a cracked manifold let the exhaust move around enough to rub somewhere on the chassis?
Also wondering if the fan clutch could cause this? Fan belt bracket bearing? I did the TB/WP but left the bracket as it was in really good shape. Any other ideas? Would a cracked/broken motor or trans mount exhibit this behavior?

 

[JunkCrzr89]

would a cracked manifold let the exhaust move around enough to rub somewhere on the chassis?

No.

Also wondering if the fan clutch could cause this? Fan belt bracket bearing?

No.

Would a cracked/broken motor or trans mount exhibit this behavior?

Possibly.

Any other ideas?

Is it a vibration or a shudder? Based on your description of when the truck does it, sounds like it could be a transmission-related issue (e.g., torque converter).

 

[Miljack]

@JunkCrzr89 thanks for the input. Ugh if it's the torque converter! If I have to pull the trans, is it worth updating to a non 2000 model year trans? I know the 5 speed is too much work to swap in...

 

[Eicca]

My parents’ 100 does this. It’s a 1999 and it’s been doing it for years. I don’t remember how, but I think I concluded that it was a front CV axle issue. Worn joints allowing slop, that sort of thing. Might have been from a combination of threads on here.

But it has the same symptoms. Higher RPM under load at higher speed makes the whole truck rumble.

 

[aljollano]

Check your driveshafts.

A.J

 

[Miljack]

I had the rear prop(drive) shaft balanced and replaced the Ujoints. I haven't done the front, nor the CV's. How would driveshafts start vibrating when the engine RPM's increase? Maybe the extra torque?

 

[FxFormat]

Sounds a lot like CVs to me, when you put a load on the engine, the angle of your CV changes, and if it's crap CVs you'll feel the vibrations. Check to see if CVs are oem or not.


Edit: Do you feel vibrations in the steering wheel or the truck overall? What kind of vibrations, small constant vibration or oscillation vibration. Tough to diagnose unless i drive the truck myself. Do this, find a flat, smooth road and cruise at 35mph, then drop the hammer and see if you feel the vibration as soon as you accelerate.

 

[AlpineAccess]

Remove the front driveshaft, lock the CDL and go for a drive. If vibration changes its something in the front end.

Front DS or CV's most likely. If CV's OEM, then I'd focus on front prop shaft.

 

[Miljack]

@FxFormat @AlpineAccess I just thought of something, I recently had the rig aligned by a shop that knows LC's, he raised the front a little at my request. Even with a light trailer that I was towing, the front end was raised higher than normal. I'm beginning to realize maybe the front CV's are the culprit. They are original as far as I can tell, and I have 280k miles on my rig now...
I'll check this out and post an update.
thanks,

 

[Robertcladner]

I found a LONG thread that discussed "vibrations" and found some good ideas, however, I wanted to post a specific subject to get your thoughts.

My 100 ('00 LX) will cruise fine on the highway, when it needs to downshift and revs get over 3krpm (prob 3.5K and up is the problem), I'm getting a pretty good vibration.

It's definitely RPM related, so I eliminating driveline issues (for the most part), my question, would a cracked manifold let the exhaust move around enough to rub somewhere on the chassis?

Also wondering if the fan clutch could cause this? Fan belt bracket bearing?

I did the TB/WP but left the bracket as it was in really good shape.

Any other ideas?

Would a cracked/broken motor or trans mount exhibit this behavior?


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The most common reason for a car to shake is related to tires. If the tires are out of balance then the steering wheel can shake.

This shaking starts at around 50-55 miles per hour (mph). It gets worse around 60 mph but starts to get better at high speeds.

 

[Eicca]

@FxFormat @AlpineAccess I just thought of something, I recently had the rig aligned by a shop that knows LC's, he raised the front a little at my request. Even with a light trailer that I was towing, the front end was raised higher than normal. I'm beginning to realize maybe the front CV's are the culprit. They are original as far as I can tell, and I have 280k miles on my rig now...
I'll check this out and post an update.
thanks,

The aforementioned parents' 100-series that also has the same vibration issue also has been lifted about 1.5" inches in the front. I wonder if a diff drop bracket would help.

 

[AlpineAccess]

Diff drop bracket would likely help somewhat. I'd measure the CV shaft angles if OP has an angle finder.