hey, my friend has a 1998 2500 12valve , he is getting really bad milage. like 500-600km per tank he has a exaust brake and i was woundering is the 12 valves have a IAT like the 24valves.. if so where is it and the map sensor located? also the truck has 130km not miles on it, how often should the valve lash be done? one one last question the rims off a 2001 wont fit on the 98 cuase they have drums on the back right?? thanks for all the help in advanced..
Pete

 

The IAT sensor on a 12 valve has nothing to do with poor mileage, it's only purpose is to control the grid heaters.
No MAP sensor on a 12 valve.
Best to adjust the valves every 24k for the first two times. After that every 100k may even be a waste of time as after the engine breaks in the lash seems to stay put.

 

in know on the 24 valves it makes a difference i wasent sure about the 12valve.. the reason i ask is cause i pulled a IAT out that was covered in soot from the exaust brake.. cleaned it and the map sensor , put it back in and saw a steady 3mpg inprovment. thanks for the info

 

Check fuel pressures first, assuming you know the fuel filter and pre-filter to be clean.

Low fuel pressure is a big cause of poor mileage.

I think you have it backwards on the rims, the 2001 should go onto the 98 but perhaps not the other way around. I think there may be a difference in the offsets.

 

he tryed to put the rims on today.. they fit on the front but the hub in the rear are too big for the 2001 rims to fit on... whats the solution if it is low fuel presure? replace the pump r get a helper pump?

 

it could be the overflow valve on the pump too. Replace filter, drain and clean the prefilter housing.
Does it exhibit any starting issues? longer cranking to start?
best bet is to test the fuel pressure