C 15 cat running a little rough

How long has it been doing this? If it just started try changing fuel filter first.
If it has been going on a while I would change engine speed sensor first, since it isn't throwing a code.

 

I had a flutter on my E model but it would throw a code . It was the wiring hardness.
There would be days that mine would purr smoothly and there will be days it would run a little bit rough, but would be the same mpg. It wouldn't miss, just idle a little rough. I guess it has its good and bad days.......

 

Sounds like a crook injector. Getting on and off the gas or splitting gears de-energizes and energizes the solenoids and if one of these are slow to re-energize it will cause a missfire/shudder under load until the solenoid comes back to life. Not sure how long the fault has to be in it for the ecm to log a code.

 

Change fuel filters and see if it cures it. If not check fuel pressure may be a weak check valve or transfer pump or primer pump may be bad. If that don't cure it then I would run an injector cutout test to see if you have a weak injector. Little cheap things first before you start throwing big $$$ at it.

 

I used to change the fuel filters on my cat about every 10,000 miles.

 

Mine gets changed at 12k when I change the oil. Before I had NASTC fuel card and I was filling up where I could find the cheapest fuel instead of the bigger truck stops, I would have to change fuel filters before truck was due for oil change.

 

After eliminating the filters as the problem i'd check your return fuel pressure. what you describe is what happened when the fuel transfer pump started going bad on both of our BXS engines and a 5EK.