Engine/Transmission (1994 - 1998) Good Power/ Poor mileage

[Rustymule]

I have a stock 97 3500 ( I lost the cat somewhere, can't find it). Truck has 102,000 miles on it. Injection Pump timing was check by the dealer at 15,000, it was retarded. Over the last 40,000 or so, my mileage gone to about 16. 5 to 17MPG. Use to be 19. 5 to 20 MPG. I've checked the easy stuff, loose turbo hoses, valve clearance, fuel filter is changed every 7000 miles, and I checked the prefilter. I have good power (I have replaced the hose from the turbo to the diaphragm on the pump, but I realize that only helps the acceleration not the total fuel). What does the overflow valve do, where is it and how does it effect fuel mileage? Is the injection pump timing something that I can check or does it have to go the shop.

 

[Got Smoke?]

Your timing could have slipped again. I know Will @ BMD was having a lot of trouble with his timing on Dyno Day. Think he finally fixed it though. Find a TDR member near you or a Cummins Dealer that will do timing and have it set at 15. 5* or 16* BTDC and make sure they torque it to spec where maybe it will stay. I did mine with the help of a fellow TDR member and I went from sometimes high 16's to mid 17's up to 18. 5 in the city and 18. 3 on the interstate if I can keep it @ 70 or below!

 

[chrleb1]

sometimes the overflow valve loses its ability to keep up the pressure in the pump. You can buy new ones, some people run upwards of 40 PSI.