Ok here is the deal. 99 cummins auto. When I'm sitting at idle. 900 rpm. This vibration comes and goes comes and goes. Like clock work. Now if I raise the rpm too 1200 and hold it nothing happens. So I don't think its a bad nalancer. Any ideas?

 

Bad torque converter or broken or cracked fly wheel if auto or a bad clutch or clutch disc if manual?????

how many miles and when did it start

 

could be it seems a little weak. its stock, but weak. its got 230k on it. not 100% sure when it started. Its my moms truck. I borrowed it a few months ago an didnt remember it. I was just siting in walmart and noticed it.

 

any chance it was cold out and you just started it,(engine does not have to be cold, just cold out side) and it was the grids being cycled???

 

Nope it was warm. Truck seems little weak but it sits 90 percent of the time. Not sure what to think.

 

is your A/C cycling on? if its on defrost it will cycle too. my 90 has a vibration when the A/C kicks in