Low Frequency Vibration

[HammerWerf]

Folks,

My 1999 Suburban has developed a low frequency vibration. It is especially noticiable at idle.

Here's what I know:

The vibration can be felt/heard at idle, and at very slow driving speed.
Frequency fluxuates over time
I have noticed that my Tach is reading about 150rpm lower then it use to ( 500rpm vs 650)
When the key is turned to off, the engine shut down is hard, like something is stopping the rotation (clunk type sound )
Fuel economy is not really changed

I have put on a new rubber drive pully, no change. Inspected previous one, NO cracks seen.

Pulled the serpentine belt and ran the motor.
Low Frequency not present.
Shut down returned to normal ( No clunk type sound)


From the above information, it appears that the vibration is associated in the belt drive accessories.
Using a stethascope, I have not really heard anything on the accessories I can reach easily. Vacuum pump was to listened to.

I'm at a loss as to what can drag that engine down by 150rpm on the belt drive.

Any guidance. Don't just want to start throwing parts at it.

Thanks

HammerWerf

[NH2112]

My first instinct is a AC compressor seizing up.

[DmaxMaverick]

My first instinct is a AC compressor seizing up.

That was my first thought. I'm sure he doesn't want to go down that road again, so soon.

Water pump/pulley/fan/clutch
Bent PS pump shaft or pulley (quite common)
Vacuum pump getting ready to let go
Bad belt

[Robyn]

With the serp belt off, inspect each accessory to see what if anything is wrong with it.

Turn the water pump by hand as well as the power steering pump, alternator AC pump and the vacuum pump.

The vacuum pump will turn then bump over center as the diaphram is compressed (Normal)

If the issue is not there with the Belt off, its has to be one of the accessories causing the problem

Also check the belt tensioner pulley

A power steering pump with an issue can really drag the engine down.

Good luck and keep us posted.

Missy

[Hubert]

Is the lower idle with and without the serp belt or just with belt?

Can you tell in difference in sawing on the steering wheel? Is the speed vs variable steering assist different ie now easier at low speed? Drive in a parking lot at idle or just off idle and brake while turning at idle and see if rpm's change appreciably or is different than before maybe press AC compressor on and off and see if rpms change around.

See if you can increase idle a bit back to normal and then shut down and see if the clunk follows. With less flywheel rotation inertia (lower rpm) it probably just shuts down quicker with more "cam over" compression causing some different sounds.

[HammerWerf]

Thanks for all the good ideas. All of them were not the main cause.

The cause of the clunk sound at shut down, and the deep vibration at idle was:

The motor mounts had gone flat. The rubber was all destroyed and the bolts were resting on the safety cage.

Thanks for the ideas.

HammerWerf