Engine/Transmission (1994 - 1998) Poor mileage woes

[JeppinHackSaw]

So, I just picked up my first cummins, a 98 12V QC 4WD dually w/ a slushbox. 176K miles on the ticker.



So far, there's 4" exhaust to the turbo, K&N element in the air box, afc slid, plate slid about half way forward of stock, starwheel adjust slightly forward.



Now, before I did anything, AFC, plate and starwheel adjustments, I drove it about 400 miles (300 free way, 100 town, roughly) I was not to impressed with the 13. 5 mpg. I've read on here that people are getting upwards of 22 MPG with these things.



After I did the adjustments, the mileage was the same on a 300 mile freeway trip (I even took it easy, 65 ish most of the way) to see if my MPG would improve, they didn't, exactly the same @ 13. 5.



Now, where should I start? Now, I do notice that there is a bit of a haze at higher RPM's. I can't really see it during the day, but if there is a car behind me, I can see it plain as day after dark w/ their headlights on me. Somewhere I have read that this could be due to the timing being retarded... I don't know if that is correct.



I think I am going to try to change the fuel filter, even though the previous owner told me that he just changed it with the oil @ 175K miles. Its hard to tell when buying a used vehicle what they are bieng honest about or not.



Now for the good. I absolutely LOVE the truck. It sure beats the hell out of the ol 6. 5TD GM that I got rid of to get this truck... WORLDS of difference.



I would like to take care of the KDP and adjust the timing,l but I've never dove into a diewsel motor like this before and am somewhat intimidated... I'm no slouch when it comes to holding a wrench, but, for saome reason, this has me somewhat worried...



So far, I love this site, there is a wealth of knowledge here. . best 35 bucks I've ever spent



Thanks!



Truck

#ad


Project (looks like I have parkinsons from the picture)

#ad


Gas mileage ride

#ad

 

[bdcraig]

Nice looking truck!! Need more info to see if your milage is in check, such as gear ratio and transmission. My 3500 gets 12-13 at 65 mph with the 4:10 gears. It will get 15 to 16 at 55mph and in town. Mix the two together and it is around 14mpg. I put a GV overdrive on it and saw no changes in town at lower speeds but did get between 16 and 18mpg interstate. My 2500 auto with 3:54 ratio gets around 16 to 18 in town and 20-22 on the interstate. Your milage may be normal for the ratio.



brian

 

[Joispoi]

what's the rpm at 65mph?

 

[JeppinHackSaw]

I dont recall and am just going by memory, I think its around 21-2200



I know it's got 4. 10's tire size is 235/85/16. I just figured that there was something that I could do to boost the mileage a bit... although im not going to complain to much because its going to be GREAT when towing a jeep or 2 on a trailer.



Does my assumption about the timing slipping due to the white smoke make sense?

 

[McLaughlin]

From everything I've heard yo should get the best fuel mileage when running between 1700 and 2000 rpm.



The problem you have can be difficult to troubleshoot because there are quite a few things which can cause the symptoms you are seeing. If you have continuous white smoke it could have air getting into the low pressure fuel lines(you won't necessarily see a leak), a defective lift pump, a defective overflow valve, a defective injection pump, injection pump timing, or bad injectors.



Suggest starting with the less expensive things and working up to the more expensive things. Checking/replacing the fuel filter is a reasonable place to start(check the prefilter screen also a lot of times it doesn't get checked/cleaned when the filter is replaced), verifying the lift pump and overflow valve are fairly easy and inexpensive (especially if you can do it your self), if those are all good then I'd check the injectors as it usually isn't to expensive (the injectors can be rebuilt or you can go to an after market injector). I'd leave the injector pump until last because of the cost and because they are pretty reliable.



Unless it is difficult to start, lacks power, or runs rough, I suspect it isn't a timing issue but if you know how (and have the equipment) it isn't that difficult/expensive to check. Since you mentioned the KDP, I'd suggest checking/adjusting the timing at the same time since you are going to have the cover off the truck anyhow. It makes setting the timing easier.



If the original rubber fuel supply and return lines haven't been replaced you may want to consider doing that sometime in the near term. The fuel lines have a reputation for developing air leaks without visible indications. If you replace them go with a quality diesel fuel line/hose.



My mileage fell to about 12mpg and I had a light hazing problem just as you described Also had about 180K on the truck. It turned out the (original) injectors where not within specifications (releasing at to low a pressure) and dumping fuel into the cylinders.



For the last two years my truck's mileage has gone down over the winter. I'm getting about 14 in town and 16 on the highway; during the summer I get about 16 in town and 19 on the highway.

 

[Bob V]

unfortunately you have the worst combo for mileage a 3500 4X4 auto W 4:10s. A little more timing might help, but that combo will not get you much better that the mid teens, 16-17. You can get better by slowing down to about 60, top mileage will be with the engine at about 1600 to 1800 RPMs.

 

[Buck88]

I feel your pain. I'm seeing about 12-13 in town and about 15-16 freeway. I've tried everything... fuel filter, checked for leaks, blah blah blah. I've come to the realization that it's the gearing. 4:10's suck, unless you tow of course.

 

[WDixon27]

man that positively makes me feel good about my mpg, i have 4. 10s 235/85/16s and being a cab & chassis i have all of the aerodynamic problems of a 4wd (without actually being a 4wd) and a flatbed, i flog it fairly hard and get 16. 5 on my combined driving, i have not done a tank of highway where i wasn't towing something and the worst of the worst of that trip was 12 going 70 mph hauling a 1973 f350 camper special on a 1500# trailer and a slate pool table on the bed and the best tank with that load was 14. 5 once i hit flat ground, about the only thing i do to help mileage is try to keep it under 70

I feel your pain. I'm seeing about 12-13 in town and about 15-16 freeway. I've tried everything... fuel filter, checked for leaks, blah blah blah. I've come to the realization that it's the gearing. 4:10's suck, unless you tow of course.