Hello Fellas,

Our daily driver '98.0 quad cab has been keeping me stumped for quite some time now. I used to be able to get 22-24mpg on a hwy trip. I am now getting 16-17mpg all the time. Changed almost overnight.

I have a fuel gage monitoring lift pump pressure. I recently replaced the piston lift pump as a matter of chasing the trouble. The gauge usually reads steady around 25psi.

I have replaced the IP with a good used pump and the results stayed exactly the same.

Ran the valves a long time back, but few miles. (considering revisiting this)

The motor just doesn't sound as smooth as it used to. When I did the lift pump and strainer. The heater in the strainer was toast and looked like it may have had a small leak. Eliminated the heater and did the 1 gasket routine.

I'm not a mechanic or a mechanical idiot... but this is stealing my confidence. Is there something simple to do to diagnose this? I've spent way too much time and money to not solve this. I even replaced with brand new bosch injectors.

A vacuum gauge tells about the health of a gas engine. Compression check. Obviously vacuum doesn't work with diesel. Could someone's fancy computer help? Maybe there is an air leak in the fuel issue, between the LP and tank. Haven't done anything to the tank or lines. Maybe a clogged pick up filter in the tank? Small air leak in a fuel line? Crap! This is beating on me. Please help if you can...
Thanks.
Greg

 

Boost leak maybe, or timing might have slipped. Auto or manual? Converter not locking will decrease mileage.

 

Winter fuel will cut your mileage starting around October every year...I lose 3 to 4 MPG every year because of it.

 

I wondered if it could be fuel quality. I also had an incident with biodiesel that seemed to be the beginning of my troubles.

I put some sort of bad fuel containing water in the truck and drove it to work 40miles, before I could pump the tank and put in clean fuel. So I thought surely I hurt something in the fuel system. Lift Pump, injector pump, or injectors. Failing all of these things, I wonder if there is mechanical damage. Pistons, rings... anything else?

I should re-run the compression test, I guess. I know the last time I ran the compression they all built pressure well.

The truck also runs rougher then it used to. Maybe I could post a video from youtube of it running and see if anyone else thinks it isn't as smooth as it should be. I don't think there is much blow-by. I don't think it would be too hard to drop the tank when empty. I could check the pick up screen and maybe inspect the pick up and return lines.

I thought that I could bump the plate back in the pump too and see if that improves mileage. I did advance the timing a bit when installed the used injector pump. The plate would not have anything to do with the quality of the idle, though. Kind regards and thanks for your input, guys.

What do other good condition 12v of this vintage get for mileage? Mine is a long bed 2wd Q-Cab with auto and 3.54 gearing.

 

16-17 is a little low for a 2wd truck. Lots of little things can add up...sounds like you've replaced most wearable parts that can contribute. Positive the timing is set right via chart for your pump?

Make sure the brakes aren't dragging. Any gauges to see cruising boost and egt's? How many miles on truck? Try synthetics, dose of power service? Sounds funny but my truck seems to like a good beating now and then, heavy tow into the hills etc.

 

Mine gets approx. 20-21 in summer 18-19 in winter (mostly highway) I've got a 4x4 with oversized tires. Check your overflow valve and air filter too. Both of those will cause mileage issues.

 

Now that I think about this a little more, these two make me think the overflow valve alone could be causing this issue.

 

I haven't replaced the overflow valve, yet. I do, however, have a fuel pressure gauge in the cab. I also have a boost and EGT gauge in a pillar along with trans temp. At this point, replacing the overflow would be simple and easy. I used to know where everybody would source them, not anymore. Any suggestions? Thanks

 

Is your fuel pressure around 25 at idle and raising to 32-35 at WOT?

If so, doubtful it is the OFV, but if you want to try it, Larry B's is a good source, get the torkteck unit.

Also how many miles on the truck? Any Injector work done?

 

The truck has around 260K on it with a rebuilt tranny. I put brand new bosch injectors from the diesel guys on facebook. I had them pop the old ones too. They said that they weren't too bad. Implying that they weren't the root cause of the mileage drop. Anyhow. Yes, I do have at least 20psi at idle and then it rises. Maybe to 28 to 30. I don't often do WOT.
Thanks for the replies.
I saw LarryB had OFV for like $28 or other were like $75. I also need a shut off solenoid boot. Could do both at the same time.

Thanks.
Greg

 

Dont forget about your air filter. That will drop your mileage as well.

 

look for a boost leak. The intercoolers corrode and get small holes hard to find unless u remove inlet and outlet plug and charge with air with a gage.

 

I would use the 28 dollar Bosch one for a relatively stock truck.

I don't think you will change anything though, your pressures are close enough to good to indicate it isn't the OFV.

If you do decide to change it, you might also want to pick up a new primer pump rubber boot as well, cheap and if yours isn't cracked, it soon will be.......

Larryb - Overflow Valve for Dodge Cummins Diesel with Bosch P7100, 94-98.5

Larryb - DODGE Diesel Cummins 94-98 Lift Pump Primer Boot Only

 

I was experiencing an almost identical issue when I bought my truck. It is a 1996 all stock with a Banks twin ram kit. I replaced the injectors, replaced the overflow valve, and put in a #100 fuel plate. It ran and still runs like a brand new truck. It was like night and day, hope this helps. Edit: The overflow valve I bought was the tork tech unit.

 

It is interesting that you mention the fuel plate. When I installed the new IP I slid the fuel plate ever so slightly forward. I marked it, or rather you could see where the washers previously sat. So I returned it and maybe a bit further to the rear with it. Air filter is good etc. I noticed a slightly less snap in turbo response. Maybe a pound or 2 less boost. Anyhow, I put new tires on it and filled the tank. I will clock the mileage again and see if it helped. Was it the plate that finally pushed your mileage back to where it is supposed to be? Thanks.
-Greg