OT 5.9 Cummings diesel

molinebob · Dec 12, 2011

Buddy at work has a 1998 dodge with a 5.9 Cummings in it, awhile ago he changed the fuel filter in it and the mileage dropped from 18 on average to 14-15, he figures he got a bad filter from Napa, and that maybe he scoured the injector pump on it? Question is can it be fixed by adding an additive or replace the pump? He"s changed the filter now two or three times and the mileage is still at 14-15, to make it tractor related we just drove the truck to Missouri, down by Saint Joseph, from Rosaeu, Minnesota to deliver a tractor I had sold. We left on Friday night at 8pm was home by 1030pm sat night, little over 1500 mile round trip, on the way down we averaged little over 11 mpg and the way home around 15 mpg, all driving on interstate 29... thanks for any all all replies...Bob

BCnT · Dec 12, 2011

something aint adding up...was he getting 18 pulling empty trailer before or 18 with just truck?
15 pulling a empty trailer dont sound like much to gripe about.
on the other hand the old filter could have been partially clogged causing the truck to run overly lean

LJD · Dec 12, 2011

The story as told makes no sense to me IF it's an early 1998 with the 6BT engine. If a faulty filter actually DID cause the pump to get scored, it would become very hard to start when hot. It wouldn't make the fuel mileage lower.

If the filter was plugged and the pump wasn't getting the fuel it needs, power would be lower and fuel mileage - if anything - a little higher.

Now if it's late-year 1998 with the ISB engine - I have no idea what silly things the electronic controls might do.

S2710 · Dec 12, 2011

When he changed the filter was that about the same time as the stations went from summer fuel to winter fuel. I know mine drops in milage when you go from summer to winter fuel. JMO

Bob

MSD · Dec 12, 2011

I've noticed with my 02 that where you fill up can make a difference. Went to Wichita one time and was getting 15 pulling a trailer. Filled up down there and the next tank I got 9. Next tank I was back to 15 again.

GordoSD · Dec 12, 2011

Don't want to jack your thread, but has he made the fuel plate adjustment procedure? When I did that I think i got at least 50% more torque, and the mileage stayed the same. Best I ever got was pulling a an 18 foot Lund boat to Canada with a nice tailwind. 20.4 mpg.

Anonymous-0 · Dec 12, 2011

Something is not right. Is it a 12 valve or a 24? I have a 96 12 valve and get around 24 without a trailer and pulling a loaded 20 footer get around 17-19.
Manual or automatic?

cumminspuller · Dec 12, 2011

I may have missed this but has he changed the filter again and got the NAPA filter out of it? It could've had trash in the filter housing that got into the line when he change it. Does it have the spin on filter or the drop in filter? Is it a 12 or 24 valve. Did it smoke more out the exhaust after the filter was changed? Has the air filter been changed in recently? Personally I would only run Mopar or Fleetguard filters. (only 2 that have the smallest micron filtration available)

fixerupper · Dec 12, 2011

My 98 one ton 24 valve gets 14-15 putzying around and 16-17 on the road. The pump is newly rebuilt. I keep track of every gallon.Jim

molinebob · Dec 12, 2011

its the 24 valve and yes hes changed filters twice now with ones hes bought from titan machinery..havent noticed any more smoke out the exhaust either..

molinebob · Dec 12, 2011

automatic

molinebob · Dec 12, 2011

no idea, i'll have toask him, hows that done anyways...

Anonymous-0 · Dec 12, 2011

Is the converter locking up? If there is no miss in the engine and it does not have low power...beats me.
Never heard of this by just changing the fuel filter.

nickg · Dec 12, 2011

Ive got an 01 with auto trans 4.10 gears. Ive got a cold air intake kit and an edge programmer. best Ive gotten so far is 16.5 in town. Dad has bone stock 99 with 3.55s and get 16 in town. There could be a few things but filter bein clooged will first ruin the lift pump and then ruin the main injector pump. Have your fuel pressure checked if he doesnt already have a fuel pressure gauge. He might be loosin a lift pump or have a bad injector or one that is clogged. Could also be a difference in the fuel. I know my cummins seems to use more fuel in the winter months. I would also check and make sure it has a good airfilter. If he has a k&n filter I recommend he takes it out. Those are not very good filters to run in a cummins at all. There is also a cummins forum that has a lot of knowledgeable guys. Its www.cumminsforum.com. Ive gotten alot of good help from it. Also some outthere stuff to just like any other forum.

RodinNS · Dec 12, 2011

Fuel filters do not effect mileage... at least not in a negative way. If the filter was plugged and the pump starved of fuel you'd quickly notice a lack of power and if severe enough... missing and surging. I'd expect similar results from a bad pump or as JD suggests... poor starting when HOT.
Assuming that it seems to be running right... I'd start by changing the air filter and checking the boost pressure against spec. If that checks out I'd either wonder about the transmission getting ready to take a dump... or mabey you've got fuel that's been heavily cut with Kero... Nothing like kerosene to make the mileage go away.

Rod

OT 5.9 Cummings diesel

molinebob

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LJD

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S2710

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MSD

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GordoSD

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Anonymous-0

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