Engine/Transmission (1994 - 1998) Fuel economy issues

[SRay]

Hello fellow Dodge Cummins owners, I have a 1997 Dodge 12v Cummins with a missing cpl plate so no engine info other than the basic 180 hp pump. This truck used to be auto, now nv4500. The odometer is 2,000 k shy of 500,000 k.



The issue I'm having with it, it's got a bad fuel mileage that I've been trying to address that's been going on for a year almost.



This truck used to easily reach 850 k on a tank by the previous guy that recorded and was decent for me as well.



Pump was adjusted 4 years ago with a #100 plate, it was good on fuel as well when I drove nice. When I had fun, the truck would run out by 650 k. Right now the goat is literally running out of fuel, on the side of the road with the worst mileage ever at 598 k for 127 liters. (I have fuel gauge issues to be addressed soon)



Timing setting? Don't know, have not verified yet but can't be off that much.



New ddp 50 hp injectors have been installed. Engine runs smoother, smokes less and more responsive, especially with the bhaf filter installed as well. But no difference in mileage yet as I'm on my 3rd tank. Sure 3 tanks may not be much but I went back to my trucking duties after the install and have not noticed any improvements other than the truck running better.



I'm beginning to wonder if somewhere on this truck there is a hidden fuel leak between the tank and injection pump. I don't smell any diesel but it could be leaking and spreading out or something.



Any info would be greatly appreciated.



I'm looking at dropping the tank, replacing lines, fuel sending unit, etc.



Thanks,



Shawn

 

[STeichert]

It seems to me that if it were leaking fuel you would see it on the frame, axles, or on the ground when parked. I would look for drag or resistance of some sort. Wheel bearings, dragging brakes or things like this may be part of the problem. Do you have a boost & pyro gauge? If so are they reading different than when you were getting better mileage?

 

[SRay]

Steichert: wheel bearings are good, pads are new, front calipers have new bushings, new calipers on the rear with new pads.

Just had boost and pyro installed 3 weeks ago. Cruising boost is between 2-5 psi, egt is 500-750°f.

Right now I'm averaging 14-16 mpg (cnd) when truck used to do 20-22.

There is no fuel leaking other than the filler neck which is gonna be replaced once the tank is dropped. I'm wondering if the fuel is leaking while the truck is running and wha the common leak areas are.

Shawn

 

[RSchwarzli]

Have you considered things like Slipping clutch, more vehicle weight than you had before, waste gate issues, or type of driving?

What about engine things like timing and all?

 

[SRay]

Bschwarzli: as far as I see, truck hasn't changed in weight since the Dana 80 was installed. Wastegate is seized permanently closed. Been that way several years.

Waiting to hear from friend when he can check the timing with his tools. I'm hoping this Saturday.

Engine itself? No idea what the engine is doing other than its running very smoothly since the injectors were installed. My friend Jessie's truck runs rough but gets better mileage although not much better.

 

[fest3er]

Be sure tires are correctly pressurized. Be sure the fuel filter and pre-screen are clean. Be sure all the fuel lines are in good shape and don't allow air to be sucked in. Then drop the fuel tank, remove and disassemble the in-tank module, and clean the screen on the bottom of the basket and the one inside the basket. Be sure the pressure regulator on the backside of the p-pump is working correctly. If that changes nothing, treat the fuel with an ordinary cetane booster.

My mileage had been steadily dropping 'til I was getting 17. 5 or so on the highway. Once I replaced/rebuilt the in-tank module ( and re-assembled it correctly, I got an astounding 20+ MPG (estimated*) going to and from MDTDC's Fall Brawl back in Oct. with one bottle of Diesel Power cetane boost (old-formula). That's the first time in 15 years I ever broke 20 with that truck.

The in-tank screens were well coated with black goo which likely greatly restricted fuel flow.

* Estimated because I *still* haven't refueled since then. A week or so after returning, the trans let go (the teeth were stripped off the input shaft. 8 years with ATS TC/VB, 215 injectors and stock plate shoved back? I'm not complaining). It's been one thing after another. I *might* get the truck back in a couple weeks. At least I had the motorbike to ride all winter (when the roads were clear and dry).

 

[fest3er]

Be sure tires are correctly pressurized. Be sure the fuel filter and pre-screen are clean. Be sure all the fuel lines are in good shape and don't allow air to be sucked in. Then drop the fuel tank, remove and disassemble the in-tank module, and clean the screen on the bottom of the basket and the one inside the basket. Be sure the pressure regulator on the backside of the p-pump is working correctly. If that changes nothing, treat the fuel with an ordinary cetane booster.



My mileage had been steadily dropping 'til I was getting 17. 5 or so on the highway. Once I replaced/rebuilt the in-tank module ( and re-assembled it correctly, I got an astounding 20+ MPG (estimated*) going to and from MDTDC's Fall Brawl back in Oct. with one bottle of Diesel Power cetane boost (old-formula). That's the first time in 15 years I ever broke 20 with that truck.



The in-tank screens were well coated with black goo which likely greatly restricted fuel flow.



* Estimated because I *still* haven't refueled since then. A week or so after returning, the trans let go (the teeth were stripped off the input shaft. 8 years with ATS TC/VB, 215 injectors and stock plate shoved back? I'm not complaining). It's been one thing after another. I *might* get the truck back in a couple weeks. At least I had the motorbike to ride all winter (when the roads were clear and dry).

 

[SRay]

Fest3er: is the fuel sending unit and everything all one unit? My fuel gauge doesn't work and the filler neck leaks while filling so I'm either gonna try lifting the box which I've got to get the heavy tool box out or dropping the tank. Looks like dropping the tank is gonna be easier once its near empty.



I'd like to replace the lines anyway as I've had the return line spring a leak last year, causing me to run out of fuel completely in only two hours driving. I installed a short rubber piece over the leak.



If air is getting in the lines going to injection pump, would it cause hard starting? Truck still starts good.

 

[STeichert]

Have you changed tire size? I have a '98 12v at work with a little bigger tires than stock, and now the speedo is off a little, and that affects my odometer readings. Your boost and pyro numbers are similar to my '96 numbers.

 

[SRay]

STeichert: I've been running 315/75/16 Toyo M/Ts for 4 years now. I have an iPhone app called Road Trip which has 3 years of fuel data in it. I've already compensated for the tire size in the app which is part of the great features it has. So every time I fill up, it automatically compensates tire size which is 10% over 245/75/16 factory on the door. The GPS also verified the 10% in speed difference between the speedo and GPS reading.



Also after compensating, it showed the bigger M/Ts to be better on fuel than the smaller tires as its like adding another gear.



Fest3er: the M/T tire pressures are at 25 psi rear and 37 psi front. It's the only way to get the tires to sit properly on the road. When I had them filled the first time 4 years ago, I put 50 psi in and only 5" of the tread touched the road which is not good for wear. The side walls are extremely stiff which also holds up the load too.



This air pressure set up I used got 90,000 k out of the last set.



The fuel issue only happened last year.



When I went to cummins plant in 2011, truck got 22-23 mpg hwy. Robert Schwarzli was getting 24-25 mpg and his truck is aired 80 psi all around and 1,600 lbs lighter. His dad's 2008 mega cab got 22 mpg with factory size tires aired up to 60-80 psi as well.



Sorry for the long article on tires but for others reading this as well so as to keep confusion out.

 

[RSchwarzli]

I actually run 60 but that's not really relevant to the topic... .

 

[HEMI®Dart]

I know you don't want it hear this but my '94 4x4 2500 3:45 ring & pinion rear. Manual trans I can hit almost 23MPG on summer fuel.

 

[SRay]

Bschwarzli: thanks. I knew your tire pressure was up there somewhere but not sure.

Hemidart: you're not the only one getting that mileage. My friend with a 1998 12v 5 speed gets 24 mpg with a #100 plate and afc adjusted for mpg.


I have just purchased a fleetguard filter, fuel heater filter and fuel elbow from geno's garage. On Monday I'm going to get 3/8 fuel grade hose from the hardware store, then once fuel level drops to where tank would be manageable to be removed, I'm gonna replace all the fuel lines, clean the filters in tank and find out why my fuel gauge doesn't work.

 

[RSchwarzli]

Valid point. Have you taken into consideration winter and summer fuel?

 

[fest3er]

I'm gonna replace all the fuel lines, clean the filters in tank and find out why my fuel gauge doesn't work.

The in-tank module is a single unit. You may find the hard lines outside it rotted.



Beg, borrow or steal a portable ATV lift. A half-full tank is easily managed with the left side axles on jack stands.



And be forewarned: the quick-connect fittings are *not* quick-disconnect after many years of salt and rust.

 

[SRay]

Fest3er: sounds like its better to buy a whole new unit. Might be able to source a used one but likely go with new. I don't like surprises that much which would end up with my truck down for the count.

 

[ENafziger]

Did you ever find out what was causing the poor mileage?

 

[SRay]

Ednafziger: hmm... A one year old post..

The truck has gotten better like I did a lot of towing through last year and for the power the truck is making now, the economy hasn't changed much.

It's still 2 mpg below my best summer mileage.

In the winter, the mileage hasn't really changed much even with extra idling.

I'm thinking the extra pressure of the pop off of the ddp50 is making the truck act like the timing is retarded more.

 

[ENafziger]

Alright, thanks! Ya, I know, a year old...but I figured that gave you some time to get things figure out

 

[SRay]

Fun to give old threads life again.