Posting this question for a friend who has a CC 3500 Dually, LBZ Allison, with ~3K miles.

He recently had the glow plug control module update done on his truck after he recieved the notice in the mail. Before that, he was getting 17mpg consistantly at 72 mph on the highway. After the GPCM update, he is getting ~13 at 72 mph on the highway. Same stretches of flat Florida road, if anything a little cooler ambient temps, and fuel from the same truck stop as usual.

Our first thoughts are the dealer screwed up the update and did something to the PCM inadvertantly. Any techs out there that could way in on this possibility? Another thought is his fueling station switched to ULSD?

Any help would be appriciated.

i was getting the same MPG before and after the reprogramming, and I am a srw, only thing to check is it winter fuel?? the stations are switching up here as we speak, would love to get 17 @ 72mph tell him to slow down..

Florida speed limit is 70 in most places. Although if you are doing 70 you get run over.

I went from 20mpg @ 70 down to 16.5 after the GPCM reprogram. But the milage will return. Mine came back after about 1 month.

Someone correct me if im wrong, but I believe this is b/c they flash the ecm and it looses its memory. Then its got to relearn the entire system, and get confortable again. Its just like when we bought these trucks and they were brand new and we were all getting 12 mpg. Give it time, and the mileage will return. :ro)

I think it's more of a placebo effect...

I think it's more of a placebo effect...

Might be but the DIC is feeling it too.

My EGR had to be replaced. There was a period of time just before I got a continuous CEL that my MPG fell from about 18 to 13-14. I do not have the DIC but I have kept a log for all of my 15K miles. The drop was noticable even without the log, but it was nice to have data to confirm my gut feeling. Since the EGR has been replaced my MPGs are back to 18. I even had a stretch of slow highway driving (about 400 miles) where I got a little better than 20 MPG.

Remember when the recall first came out lots of people were saying their mileage improved after the GPCM was reprogrammed? My mileage didn't change but the light didn't come on as long.

Edit: I forgot, they also reprogrammed my ECM at the same time. Thats why i lost mileage. Maybe i should think about something more before i post. :lol: