My sisters 92 dually 5 speed has been doing something strange, thought id ask your opinion,

after accelerating to speed in any gear, but more noticible in 4th and 5 th, if you hold the gas pedal in the same place the truck will lose about 5 mph, you can mash the gas and it will take off, on a few occasions its done it on a slow steady climb to speed,

only mods to it are a straight pipe and pump tweaks done quite some time ago,,,thanks

 

I don't really know but has she changed the fuel filter in a while?We have a old ford idi diesel ranch truck and it is very sensitive to how new the fuel filter is.If it hasn't been changed in a while it will barely go at all.

 

was my first thought changed it

 

Check the fuel pin at the bottom of the AFC housing. Remove the cover on the AFC and move the throttle. The pin should pop out easily. Some times they stick and wont let your pump fuel enough. If that is working fine I have heard in rare cases that the throttle shaft going into the pump will slip causing the simptom you describe. This happened to my buddy's truck, but Im not sure how he fixed it. Hope this helps.