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cruise control 'pulsating'

5

540s10

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Shrewsbury,MA
in my truck when on the cruise control you can feel like the truck is hitting bumps or 'surging' (if you want to call it that) when going downhill.

my dads 8.1 truck does this too, but its slightly more noticeable on the 6.5.

I'm not to worried about it, just wondering what causes it.
 
The gas and diesel engines are apples to oranges. The comparison made is "So what?" as I explain below. The diesel may need service where the gas engine is 'normal'.

A diesel engine is expected to compression brake and cut fuel to decelerate. Release the throttle on a diesel at high RPM, anything over idle, and no fuel until the RPM drops (Idle governor kicks in ) or the operator presses the throttle. A gas engine generally doesn't cut fuel until they came out with computers and DFCO Deceleration Fuel Cut Off that is only used for extreme circumstances or cruise control.

The 8.1 - watch the tach. The RPM will drop 200 RPM as the computer finally cuts fuel, DFCO, to slow it the hell down. You got one of the last rigs to do that. Yet, it doesn't stay in fuel cut off very long on the gas engine. A diesel will cut fuel so long it can cool off below 160 degrees with no fuel for an entire long downgrade when compression braking. Thermostat may be closed but there is still 6.5L of air going through it without fire...

There is some curse to cutting fuel on gas engines that engineers, GM, or whoever avoid like the plague. Good luck getting a modern GM engine like a Tahoe to release the throttle as GM would rather eliminate the drive line clunk and keep the throttle slightly on.

Maybe it is because of concerns like this that 'GM' does this.

Anyway I think your diesel has an issue and I know the 8.1 is normal. (My 2008 Duramax and 1993 don't surge to decelerate...)
 
Matt Bachand said:
Did you optic bump? Some report 'trailer hitching' effect in cruise when bumped.
Click to expand...

nope, I was wondering if it could be my timing chain, it was pretty loose when I had the pan off and it almost sounds like a change in cam timing (which would also be fuel timing). Odd thing is though that it won't do it with with my foot held steady. It also is less prominent at lower rpms and higher rpms, only really notice it between 60 and 65
 
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