Vibration / shake when shutting off... WTF?

[db131994]

My wife sends me a message at work today "my truck is making a weird sound when I shut it off..." We all know how that goes! I get home and take it for a drive. Besides the apparent wheel hub that sounds like it's going out AGAIN (I just replaced them both this summer...), I don't hear anything unusual. I pull into the garage and shut it off (2005 TB 4.2L). There it is, a nice little "diesel" type rattle and shake on shut down.

I jacked it up to take a look, while I had the wife start it up and shut it down. Thought a motor mount might be bad, or a tranny mount, but nothing was apparent... When I had her rev it up a few times, I didn't see the motor jump up or the tranny twist too much. Not saying it isn't the mounts, but my quick inspection didn't make me think that is the issue...

It idles smooth and runs smooth as usual. The only indication something is wrong is when you turn the key off, and it shakes and rattles as the engine is winding down. It acts like my Duramax when it shuts down!

Any ideas of what to look at? It has about 80k on it.

Appreciate the help!

 

[808_LS_EXT]

My wife sends me a message at work today "my truck is making a weird sound when I shut it off..." We all know how that goes! I get home and take it for a drive. Besides the apparent wheel hub that sounds like it's going out AGAIN (I just replaced them both this summer...), I don't hear anything unusual. I pull into the garage and shut it off (2005 TB 4.2L). There it is, a nice little "diesel" type rattle and shake on shut down.

I jacked it up to take a look, while I had the wife start it up and shut it down. Thought a motor mount might be bad, or a tranny mount, but nothing was apparent... When I had her rev it up a few times, I didn't see the motor jump up or the tranny twist too much. Not saying it isn't the mounts, but my quick inspection didn't make me think that is the issue...

It idles smooth and runs smooth as usual. The only indication something is wrong is when you turn the key off, and it shakes and rattles as the engine is winding down. It acts like my Duramax when it shuts down!

Any ideas of what to look at? It has about 80k on it.

Appreciate the help!

Sounds to me like it is "Dieseling"...

Sometimes, dieseling (aka pre-ignition) is caused by carbon build up on the pistons.
The build up absorbs/traps liquid gas, and also raises the compression.

Even with the key off and no-spark, the higher compression can combust the fuel trapped in the chamber, like a diesel does.

Try a combustion chamber cleaner like SeaFoam.

 

[triz]

Try some Techron concentrate in the fuel

 

[db131994]

It shuts off, not really dieseling, just "shakes" as it shuts off...

You can start it and shut it off repeatedly, and the issue is very consistent. Unlike dieseling, where it may run for a few seconds, or not.

I will throw some fuel treatment in it to see, but I don't get the sense it is a pre-ignition issue.

 

[808_LS_EXT]

It shuts off, not really dieseling, just "shakes" as it shuts off...

You can start it and shut it off repeatedly, and the issue is very consistent. Unlike dieseling, where it may run for a few seconds, or not.

I will throw some fuel treatment in it to see, but I don't get the sense it is a pre-ignition issue.

"rattles as engine is winding down"...

Maybe timing chain tensioner loosening as the oil pressure drops?

 

[Mooseman]

I can tell you categorically it is NOT the timing chain tensioner. I just did mine and it still has a shake on shutdown. I think with "rattle" he means a shake. Mine also has a mild vibration on idle so I figure it's the motor mounts for both issues.

The timing chain tensioner is oil pressure fed but has a ratcheting mechanism that holds the tension when the oil pressure is lower, like at idle. It was the ratcheting mechanism that failed on mine.

 

[db131994]

Yeah Mooseman, it gives you a low frequency vibration when you turn it off. Maybe 3-4 jolts? Besides that it idles smooth as silk, and runs just fine... I'll give it some more time to see if it gets worse...

 

[Mooseman]

Sounds like old age

 

[triz]

That makes me think your motor mounts are on their way out.

 

[db131994]

I noticed its doing the same on start-up... Any advice on checking the motor and trans mounts? I'll replace all three if that is the issue, but I would like to make sure before I throw parts at it.

 

[Mooseman]

The usual is a 10Hz vibration while idling but this might be another symptom.

 

[HARDTRAILZ]

Lay under it and let someone hold brakes while going reverse to drive and you can see trans mount move if bad

 

[KNBlazer]

pour a lrg bottle of techron fuel treatment to 15 gallons of fuel... if this doesn't help, then I would suspect your motor mounts, passenger side most likely will be the culprit....

 

[TB2004]

I've found on mine, fuel system cleaner will takecare of it. maybe 2 tanks if I let it go too long between treatments. Does sound like a diesel shutting down.

 

[offroadrider12]

My truck had a similar issue and it was the ignition switch in the steering column going bad.

 

[BlazingSaddles]

I know this is an old post; but db131994, did you ever get this resolved? Mine is doing the same thing.

 

[Mooseman]

Chances that he answers are slim to none as he hasn't been on here in over 2 years.

 

[AzTruckGuy]

I started having this issue a 2 months ago I notice a vibration on start up, new motor mounts installed last year and I use Chervon gas, just chalked it up the many annoyances that happen to the trailblazer