Surging at Idle in Gear

[Badunit]

Months ago my engine started surging when idling. It has gotten progressively worse. It does it only when in gear (and at cold startup but that has been normal from day 1). It smooths out when I shift to neutral or park and often will stay smooth when shifted back into gear. Below are some charts of fuel rail pressure vs commanded pressure. First one is when in gear idling and surging then shifted into neutral where it smooths out. Second shows some spikes when I was driving around. It appears the pressure jumping around at idle is my surging problem but I have never looked at this when there were no problems so I don't know what is normal. It looks like the actuator is not following the commands well but is it that the best the FCA can follow commands that are not quite steady? I don't understand why neutral vs in gear makes such a difference. Thoughts on what is causing it? I have not tried anything yet. I did not want to load the parts cannon without a better idea of what it might be. MAP, FCA, limiting valve, cascade overflow valve, fuel filters?

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[Lordsman]

I put a new map sensor on mine and it really helped. I always had a rough idle on cold starts. Man there was so much soot on it how could it even read!

 

[sag2]

My vote would be the FCA. Can you swap one from a friends truck?

 

[Badunit]

I wish I could do a quick FCA swap but I cannot. I will have to take the hit and get a new one if that is it. It has been my first suspect but I have been scratching my head about why the pressure is stable idling in neutral but not in gear. I cannot think of a good explanation.

 

[Ozymandias]

If unsure buy a cheap aftermarket one for a test, if it corrects it then get the real deal.

I'd also point to the FCA.

 

[p-Bar]

well your are putting a load on the engine in drive . Could explain it acting differently.

 

[Axel]

Stupid question what does FCA stand for? My son's 15 did the same thing only when it is really cold outside so.

 

[p-Bar]

Fuel control accuator.

You can unplug it for a quick test. Some guts guys have put some 2 stroke oil in the fuel to loosen it up.

 

[Badunit]

Replaced the FCA. Seems to have cured it. Hopefully it will remain fixed. Not an inexpensive fix but sure was easy. Thanks for the help.

 

[C M McCormick]

@Axel I did a little cut and pasting.
People also ask
What is a FCA on a Cummins?
Basically, the FCA is the metering device for the CP3 injection pump. ... Presuming the truck starts, immediately turn it off as you will have full pressure running to the fuel rail, but at least you have fairly high certainty that the FCA was causing the issue.
ETA: Fuel Control Actuator

Craig