The 99 with the 6sp trans is not able to hold a steady speed with the cruise engaged while running light. put 12,000# behind it and it is as smooth as can be. While running light, you can set the cruise and it will be great for a while then the thing starts accellerating and dropping off at first slightly then as it goes longer it does it with greater lntensity. I can not see any difference in the speedometer while this is happening, but it can clearly be seen on the boost gage, as much as 15psi increase. It can be felt easily by the seat of the pants, It is anoying as all get out. If left to go a while I can see the GPS speed rise and fall.
This will go on for hours, I have stopped to pee and let it run, only to come out and the problem persists. I have stopped for a meal, shut it off for an hour, and come back to the pulsations.
Hook a load to it and it stays right where it was set, all day and night long.

Any Ideas?

Thanks

 

Check the cruise control vacuum pod and connecting lines for leaks. Unless you dont have the vacuum controlled cruise. Usually timing/fueling boxes cause the cruise to do this but I didn't think you have any.

 

You are righ in that I have no timing or fueling box. All the changes were done in the Cummins computer. This has just started in the last 10,000 miles or so. I will look for leaks but I cannot understand why it does it when running light. When loaded it is as smooth as you could want

Thanks