Vibration during deceleration

[TNTahoe18]

Hey y’all. My name is Chris, from Nashville and I am new to this forum, and to be honest to forums in general so I apologize in advance for any improper etiquette and appreciate any assistance or wisdom anyone might have for me. I came across the forum when researching some issues with my Tahoe and was taken back by knowledge and experience on the forum and was hoping someone might have some insight for me.

I have a ’07 Tahoe Z71 with 250k, and had a new motor dropped in about 20K ago. The issue I’m having is that when I decelerate, there is a pretty bad vibration. It's pretty consistent with the speed, typically starts when I hit around 25 mph and stops abruptly around 20 mph. I wouldn’t necessarily say it’s a violent vibration, but it’s bad enough that I need to look into it. It’s not brake related, or at least it occurs whether I’m on the brake or not. A friend of mine suggested that it may be something to do with the “smart technology” where the engine switches from 8 cylinders to 6 and then 4 based on the need. I can’t say I know enough about it to diagnose the problem, and before I take it to a shop I was hoping the collective knowledge you all have might be able to shed some light on it.

I had been working just around the corner and driving > 30 miles a week so it wasn’t something on my radar. I recently started commuting 30 miles one way (additionally, if anyone has tips on MPG conservation it would be appreciated) so I’m finally getting around to addressing the issue. This was occurring before the new motor was put in, and I’ve had new rotors, pads and alignment recently and it hasn’t changed anything.

Any insight would be greatly appreciated!

Thank you in advance.

 

[swathdiver]

Welcome aboard Chris. Tell us about your tires, how old are they and when were they last rotated and balanced? FYI- 4-6-8 was Cadillac's old displacement on demand (DOD) technology. Our engines only switch from 8 to 4 cylinder. Interestingly, the same guy who was involved in the Cadillac design, also did Active Fuel Management or AFM.

You'll also want to inspect your u-joints and engine mounts. More will chime in as the day gets going.

 

[TNTahoe18]

Thanks! They're in decent shape. Maybe 40k on them. They we're rotated and balanced maybe 5k ago.

 

[gpracer1]

U joints.
Possible trans or motor mounts.
Drive 70 and put it in neutral and see if it vibrates, if it does, u joints or driveshaft bent or out of balance.

 

[Doubeleive]

ya sounds like it could be a bearing or u joint, bearings could be pinion or carrier (rear end), might want to check your front wheels hubs for play also if your up that high in mileage and they haven't been done yet. you can get under the truck while parked on a flat surface so it's not bound up and grab your driveshaft and try and give it a good shake and wiggle and see if it moves around at all I would check the driveline at the front and the back. front wheel hubs can be checked by jacking up either both wheels or one at a time, while the wheel is off the ground grab the tire at 12 & 6 and see if it has any play (should have none), then check at 9 & 3, 9&3 will have some minor play because of the steering system but shouldn't be a lot.
while inspecting the driveline look for any grease coming out of the output shaft (transmission or transfer case if 4x4) or input (pinion) at the rear as grease leaking will be some indicators of possible bad bearings having worn causing some leaking from the seals.

 

[inmypassatlife]

Aren't NNBS notorious for bad motor mounts?

 

[Tonyrodz]

Aren't NNBS notorious for bad motor mounts?

From what guys have posted, yes. They're fluid filled which is supposedly the problem.

 

[inmypassatlife]

From what guys have posted, yes. They're fluid filled which is supposedly the problem.

Read some other forum, LSTech I believe where they used H2 motor mounts that are solid. Could be wrong, but ran across it a few weeks ago

 

[Tonyrodz]

Read some other forum, LSTech I believe where they used H2 motor mounts that are solid. Could be wrong, but ran across it a few weeks ago

Read that too.

 

[TNTahoe18]

I appreciate the feed back. It seems that U joints are the most common thought. I haven't had a chance to climb under and check the drive train but will try to this week.

In researching U joints, it seems that the vibration starts when you let off the gas. When I do this there's no vibration until I coast to around the 25-30 mph range. Not sure if this changes anyone's thoughts, not sure if I was clear.

Is this something that a novice could handle? I've done the basics (brakes, rotors, power steering line) but nothing too extensive .I know enough to get myself into a project, not necessarily enough to get me out.

Thanks again!