I have a 05 2500 4wd auto that I’ve owned for about 4 months. Routinely getting 13mpg. Stock height. The engine is stock still has a muffler and a cat. Trans has about 20k on rebuild. No blow by. Recently had Yukon free spinning locking hubs installed which had absolutely no effect on fuel mileage. Intermittently the engine stumbles at idle. Is the poor fuel economy related to the engine stumble? Any suggestions?

 

First thought is plugged cat and or muffler.
My fuel economy went up when my stock muffler rusted out years ago. I looked in it and it is very restrictive. Add a bit of soot and it is even more restricted....
A suggestion is to go with any of the aftermarket see through muffler systems. Yup, the exhaust is noisier...

Second item would be the pressure relief valve on the common rail for the injectors. My truck would get 19MPG (empty) no matter how fast/slow I would drive. I noticed an improvement in economy after the replacement.
However, after replacing the relief valve, the injectors started bypassing too much fuel so those needed to be replaced. At 300,000km they were nearing the end of their "average" service life anyway.

The stumble could be the Fuel Control Actuator on the back of the CP3 pump.

Neither the pressure relief valve nor the FCA are terribly difficult to replace, nor require exotic tools or procedures. Clean any loose dirt first, 24MM deep socket for the pressure relief valve. and a Torx T25 for the FCA

Pressure relief replacement (I'm not connected to the video in any way)

FCA replacement

 

But it's a good noise...13 MPG is about what I get with my 2009, I just live with it......Ben

 

Thank you for the response and advice. I will replace the FCA and relief valve. I will also discard the cat and muffler. 19mpg sounds much better I hope I can get it. I will keep you posted.

 

Try not to go cheap on the parts...

Your saving labour and shop parts markup, so use that to invest in quality parts.

Merry Christmas!

 

Will do…

Have a Merry Christmas too.

 

How did you work it out ?

 

Something to try with the intermittent miss at idle...

My truck would intermittently "backfire" when cold at idle after having the injectors replaced.
The googler turned up other people (some on this forum) back in the day with the same issue.

The suggestion was to disconnect the intake heater grid.

Sure enough, that appears to have resolved my "backfire" (actually more like a small pop out the exhaust)

Perhaps that would resolve your intermittent miss at idle?

 

My '03 began stumbling, like a miss in a gasser. Ended up being a bad injector. I sent them to F1 for testing and rebuild. Fixed the issue. (I had them rebuild the whole set) Truck ran another 100k+ miles before I sold it at 295K miles.
Back to morning brew . . .