Engine surging while cruising at low speeds.

[DenaliHD66]

When I'm coasting at about 10 mph, cold or hot running temp, the RPM's fluctuate between about 850 and 1000. It's a constant and consistent pace that it goes up and goes down, goes up and down. Anything to do with transmission doing something it shouldn't? Throttle body and other air intake components are always clean.

 

[gmcman]

Pull the selector into "1" and see if it still acts the same. Does this happen under a load or just on a level surface trying to hold 10 MPH with barely any throttle input?

Could be that you are increasing speed just enough to load the TC and not really loading the engine up. Try this up a steep hill and see if it acts the same.

 

[DenaliHD66]

Pull the selector into "1" and see if it still acts the same. Does this happen under a load or just on a level surface trying to hold 10 MPH with barely any throttle input?

Could be that you are increasing speed just enough to load the TC and not really loading the engine up. Try this up a steep hill and see if it acts the same.

Yeah was gonna try it in neutral also. It happens on a level surface trying to hold 10 mph with zero throttle input. Once I step on the throttle, it responds accordingly.

 

[gmcman]

Was the A/C on? Sounds almost normal but hard to say without driving it. Can you post a vid?

 

[DenaliHD66]

I have noticed a bit of... rough operation or noise coming when the AC is on... nothing extremely noticeable, but if it was there before I never noticed. Also is a diesel sound coming from the bottom of the engine... its not the regular diesel sound, this sound is when its idling even fully warmed up.

 

[gmcman]

Also is a diesel sound coming from the bottom of the engine... its not the regular diesel sound, this sound is when its idling even fully warmed up.

Have you checked the water pump for play? Believe me when they are worn they make the oddest sound, almost like a rod knock. When mine took a dive it sent a chill up my spine and happened abruptly, I thought for sure it was a rod when I first heard it. If you are getting this knocking or dieseling sound it could really mess with the knock sensors and jump the ignition timing all around.

 

[DenaliHD66]

Yeah its got that old engine knock sound to it. Inspection Tuesday!