Well,, just a suggestion. I have done this and got good results with a rear issue on my SRT truck.

Use an action camera, and mount under the truck to look at the suspension points while driving. Get up to speed and get the vehicle to go in and out of the vibration issue. You may pick it up.

Also,, have you had the wheels and tires balanced while they are still on the vehicle?? This may pick up on your issue as well. Have them take the machine to full speed.

Good luck

 

Have you checked the rear end at all?
Tried removing the rear driveshaft and driving in 4HI?

 

No have not. But would that cause a steering wheel vibration/shake? Not feeling any vibration in the seat.

I like the thought though, as it seems it's worse on coast vs acceleration, which could be wrong/worn set up on the pinion gear. I may have to tear down the front diff to see if something is going on there

 

May have to give the camera trick a try, I've got a couple of them.

As for balance on the truck, no, but did try indexing the wheels and rotors on the hubs to see if that made any difference.

Going to stop at a buds shop today, I found references to this type of vibration via Chevy TSB's on the Z71 suspension. Only the reference but no info, so I'll see if he can pull up the bulletins.

If anyone has access, these are the numbers I found. 3453A, 3107B, 3039, and 1655777

 

May have to give the camera trick a try, I've got a couple of them.

As for balance on the truck, no, but did try indexing the wheels and rotors on the hubs to see if that made any difference.

Going to stop at a buds shop today, I found references to this type of vibration via Chevy TSB's on the Z71 suspension. Only the reference but no info, so I'll see if he can pull up the bulletins.

If anyone has access, these are the numbers I found. 3453A, 3107B, 3039, and 1655777

Just read an email from a GM guy. Seems this problem is more prevalant than you might expect. The reason for this unknown, and hard to locate vibration was traced back to the Torque converter. The official term that was used was fluild degradation. The transmission fluild is going bad, and causing the torque converter to shutter, and vibrate. They have a test that requires the use of a picoscope to actuually see when it occurs, like you don't already know that. I don't want to misquote the gentleman, but what sets it all in motion is some slight firing distrubence, and yes coasting does make it worse. I am going to try an locate the email, and will post it when I do.

 

My thought is simply that you have tried basically everything up front, but nothing in the back.

 

Long story short, my current truck had a similar issue and when the dealership did some engine work for the timing chain actuator, which involved removing the rack-n-pinion. When I got it back, I noticed right away the vibration was greatly diminished/almost gone. coincidence? Maybe check those rubber mounts/bolts up front? You can use google advanced (https://www.google.ca/advanced_search) and narrow the search field on “last update” to “past year” and search “truck vibrates when decelerating” for the most current ideas on where else to check.

(long story lol) At 70k miles I had a similar problem with my silver z85 2wd. I changed all suspension parts with OEM, 4 new tires balanced twice, 4 shocks, 2 front coils, 2 rear leafs, complete 2-piece driveshaft/hanger bearing, had rotors/drums turned, new brake pads, checked mounts, put 500lbs in the bed, clamped Hellwig 550 EZ spring helpers to the rear leafs. Nothing seemed to help, so I traded the truck in. Just curious, does the steering wheel isolate back n forth? Reason I ask, is the only place I did not check was the rack-n-pinion. This sounds crazy, but my current (blue) Colorado had a vibration close to what was happening to my silver Colorado, and I thought oh crap not again.

 

Sorry work called.

No the steering wheel does not shake back and forth, like you would get with something like a warped rotor. It's a vibration in the steering wheel that builds as the speed climbs above 50 mph. Probably the easiest way to explain it is if it had a weak motor mount and was just allowing the two metal halves to rub each other barely.

Like said I'm not feeling it in the seat, it's in the steering wheel that it's most prominent.
But as you get up to around 70 mph, I can see the center rear view mirror view getting blurry from the vibes. Neither of the outside mirrors exhibit the same problem at all.

I'll crawl under the truck tomorrow and check the rack mounts, and let you know what I find/see.

Thanks to all for the suggestions.

 

I have a similar problem but its on my Civic not my truck. The Honda just needs the rack & pinion adjusted to tighten it up. Adding a little tension to the adjustable shoe is pretty common on the Civics and I'm sure ours are adjustable too but access can sometimes be a problem. The teeth always wear in the center of a rack first so the result is a loose spot when driving straight ahead. Add some fairly heavy 4x4 tires and its going to shake.
With the key on, engine off, and the front wheels pointed straight ahead just wiggle the steering wheel back & forth without moving the tires. You said the inner & outer tie rod ends are good so if there more than just a little play adjust or replace the rack & pinion.

 

That's a great point - does anyone know if/how to adjust the preload on the rack and pinion?

 

Because most driving is basically straight ahead with minor corrections right & left a couple of teeth in the center of the rack + a couple of teeth on the pinion get the majority of the wear while each end of the rack will remain like new. That means we need to take as much play out as possible but not so much that it is too stiff at the right & left ends of travel.
I have an '04 2wd 3.5 and any adjustment would have to be made at the large female hex plug pointing forward and horizontal on the front of the rack and directly under the A/C compressor. I am not sure if it is adjustable or if maybe installing some shims is possible. However it doesn't look externally like it is made to be adjusted. No promises but I will check it out a little tomorrow morning when its not so hot, it's over 100F now. If it proves to be adjustable I will share my adjustment method.

 

I did some more searching and I think it may be adjustable. The trailblazer has a similar rack and pinion and there are adjustment instructions floating around the web. As Russ mentioned, there is a 1.5'' hex nut on the front side of the rack. It has a lock nut on it. If it's similar to the trailblazer all you do is remove the lock nut, tighten the large hex nut until it bottoms out, then back it off one flat (about 60 degrees). Replace the lock nut.

 

Well I did check the rack mounts, even loosened up the bolts and lubed the bushings with white grease.

Had to make a run up the interstate today and yep it's still there.

BUT, an odd occurrence, or twist to the tale. Just for S&G's I ran the truck past 70 mph.
And at 80 the problem disappeared COMPLETELY! It went from the droaning vibes, to like it was running on glass, riding smooth as silk.

Around 75 mph I could feel the problem lessening slightly, and like said at 80 it was gone.

Let off the throttle and and as it crept down to 70 the problem reared it's ugly head again.

So what ever is causing it, it speed sensitive. You can drive into and out of it.

 

I'm following this thread with interest. I had nearly the identical problem on my 2004 CC ZQ8.

I no longer have my ZQ8 but my son does.

 

I still have not checked out adjusting my Canyon steering as I promised but I did do more testing on my Civic which does indeed have a badly worn R&P. Driving straight ahead it shakes badly especially @ 65-75 MPH but in curves at the same speed it is rock solid. I'm sure that our trucks will respond exactly the same way so using curves as a R&P wear test should be a valid test. The remaining question is are any, some, or all of our rack & pinions adjustable?

 

A wheel slightly out of balance can do silly things. A wheel weight might have fallen off.

 

At Rock Auto there are three different pack & pinion part numbers for 2004. The Z85, Z71, & ZQ8 are each different. My Z85 doesn't appear to be externally adjustable and after a 100 mile drive yesterday I determined that it is still in excellent condition so I will not be experimenting on it. You will have to determine it yourself to find if yours is adjustable.

 

UPDATE: My son bought some used Cadillac rims off of Craigs list. He had new tires put on the rims and had them installed on my old truck. The wobble is completely gone. He says there is zero wobble/bounce from zero to 90 MPH to zero.

His buddy owns the tire shop and he wants to take a close look at the original ZQ8 rims to see if he can find a bad one in the set.

I lived with this for 12+ years and 170K+ miles.

 

Congratulations!!