Engine/Transmission (1994 - 1998) Fuel Mileage slightly degraded, culprit?

[Gifford]

My 97 12v auto used to go 505-525 miles on a tank till it ran out, since my fuel guage crapped out long ago, I just fill it up at 480 miles or so. Now the truck runs outta fuel at 475-490 miles. (the odometer is slightly off due to the 255-85 tires, so my actual mileage is slightly more) What does one check in a diesel as far as a "tune up", I put a new air filter in, checked tire pressure, but is there some maint. I should do on the injectors/fuel pump?? Truck has 147k miles, mebbe wheel bearings???

 

[Blakers]

Well that's only 30 miles different, which could easily be accounted for by filling the tank to a different level, due to foam, the angle the truck was sitting or the sensitivity of the filler nozzle.



Lots of things could give you that difference, including load, wind resistance, driving habits. There could be something wrong with the truck causing problems, but I don't think wheel bearings would have an effect unless you have one locking up or something.



Checking your air cleaner and tire pressure is a good idea. What modifications do you have?



Your mileage seems awful low unless you are carrying a load. I can go about 325 miles on 1/2 tank of fuel.



Good luck in finding the problem.



Blake

 

[Joe G.]

Gifford,



I experienced a drop in fuel milage. I'm told that the CARB formula for diesel was changed a little. That was a couple of years ago. I got the milage back by advancing the timing.

 

[drees1]

Before we discuss possible engine problems verify that the following has remained fairly constant.



1. Your driving habits.

2. Aero dynamics of the truck, ie. no new accessories that could add drag, like a brush bar, etc.

3. Same fuel source and topping up tank in the same manor.

4. Truck on a level ground when topping up.

5. Tires at same air pressure.

6. no significant extra load or weight in it.

7. You are not comparing summer to winter. (The heating value of fuel changes a lot in some states)



Ok, there is little that will effect the diesel in terms of fuel economy, and usually there is another complaint to go with it.



Check aftercooler hoses and clamps. I lost a hose at your mileage and was surprised how loose the clamps were. They need to be very tight. Loss of boost will also result in loss of power. How's the power?



A bad or weak injector will reduce economy, maybe power, and you will notice it running unevenly. How smooth is it?



Fuel leaks, anywhere. Usually you can smell it or see it drip.



Turbo OK? Easy to check the inlet side. But you would have to pull the exhaust housing to see if something went through the exhaust side. You would notice a drop in boost, maybe.



Timing could have slipped. I do not hear of this much on our engines, but it is common on others that use the key less taper shaft method to drive the pump, like ours. You can pin the engine on #1 TDC, and remove the timing plug in the pump to see if the blade is centered in the hole. If not, then further investigation of timing by measuring the plunger height is warranted. Retarded timing would cause power loss, fuel mileage loss. Opposite for advanced timing.



Air entry into the fuel system on the suction side. Usually accompanied by hard starting or excessive white smoke on start up and low power. Plugged fuel filters or screens also.



I think that covers it. Since your mpg change is so slight I would continue to monitor it carefully before doing anything. Other than the timing slipping, most of the other problems will tend to get worse as you go farther, and it would be hard to find such a small loss of economy at this point.



One more thought. If the valves have not been run in a while, that could make the difference too. Good luck