RPM Oscillation

[magic carpet pilot]

I have recently noticed that there is an oscillation in the engine RPMs. This only seems to happen after the truck has been running at freeway speeds for an extended period of time. When I take the exit ramp, and shift into neutral the engine RPMs will oscillate from idle to idle plus 200-300 RPMs. It cycles through the oscillation about 2-3 times before it settles back to normal idle RPM.

Any ideas?

2004.5 3500 quad cab long bed 6 speed manual.

 

[rashwor]

I have noticed this on my '05 when idling along in 2nd gear every once in a great while. Haven't worried much about.

 

[Tuesdak]

Get a scanner and watch rail pressure - both desired and actual. I would suspect the FCA first, but, any high pressure leak will surge esp at idle.

Lift pump needs to be checked. Fuel filter and condition of fuel? Water/gasoline contamination?

 

[AH64ID]

I have recently noticed that there is an oscillation in the engine RPMs. This only seems to happen after the truck has been running at freeway speeds for an extended period of time. When I take the exit ramp, and shift into neutral the engine RPMs will oscillate from idle to idle plus 200-300 RPMs. It cycles through the oscillation about 2-3 times before it settles back to normal idle RPM.

Any ideas?

2004.5 3500 quad cab long bed 6 speed manual.

It sounds pretty normal based on the several HPCR's I have driven. The fueling cuts to zero and has to then stabilize at idle speed, which is higher when you are not stopped.

Different tuning, injectors, etc and it still does it.

 

[HHhuntitall]

.... I'm curious if it isn't a safety feature to prevent stall when braking hard in neutral or clutched..... Every standard shift HPCR I've driven does it. Perhaps because it still has a speed sensor reading, yet no APPS reading?

 

[AH64ID]

My guess is the computer doesn't start adding fuel until you're at 750 rpm's, but when you dropped to 750 from a higher rpm you have a deceleration rate to overcome the causes the ossilation.

 

[magic carpet pilot]

Thanks guys for the ideas. I will start to look into them, but sounds like this might be "normalish"?

I have never noticed it when in anything other then neutral.

The lift pump got replaced with the in the tank style about 25,000 miles ago. When that thing went it went fast. Of course I was on a long road trip, away from home when it went out. But when it did it went from intermittent ruff acceleration to limping at idle speed off the highway in about 25 miles. Fun times! I would have replaced it with a FASS System, but on the road and limited with what I had.

The fuel gets filtered to 2 microns. I have noticed this issue spanning the change of fuel filters and several tanks of fuel, plus the most recent filter only has about 1000 miles on it. So this makes me think it is not a fuel condition issue.

I can see it being an issue with the change in fueling going from highway speed to idle in short order, but will look into rail pressure. Never done that, anyone have a video?

Thanks

 

[magic carpet pilot]

I started to pay a lot more attention to it this week and seems anytime it goes from a High RPM to a low RPM quickly it goes through the oscillation. I am starting to think it is a normal thing, but will test rail pressure when I get a chance and post back what I find.

Thanks for everyones help.