Engine/Transmission (1994 - 1998) Fuel Mileage 3500 vs 2500?

[P-38]

Just returned to FL from LA pulling a small boat (about 2k lbs) got 10mpg at 78-80 mph and 12mpg at 72-75. Does this sound right?



Is it possible to increase the boost at this speed? Would more boost improve mileage?



I'm a new owner of the #6 plate slid all the way forward with 3k springs and K/N filter.



Does the TST elbow need adjusted or replaced with one that better matches my all-the-way-forward #6 plate?



Timing is coming next



John

 

[Missouri Mule]

John,

Depends if you had a head win. But mileage sound low, a lot 2500 and 3500 get that mileage pulling max weight 10,000 to 12,000 depending on year and rear end.



I would check for fuel leaks or air being introduced into fuel lines/system.

Fuel return line can start cracking and suck air but not leak fuel.

Transfer pump could be getting weak.

Fuel overflow valve(return fuel valve) on injector pump can cause low fuel pressure.

If you check out high pressure fuel lines remember to use cardboard to check for leaks.

Quick disconnect connection on fuel module(on top of fuel tank) sometimes leaks or the fuel lines coming from there.



I thought the tst elbow was adjustable?

 

[illflem]

The more boost the worse the fuel economy. Sort of like a vacuum gauge on a gasser. Your goal should be to keep the boost low if you want higher mpgs. I've also found using cruise rather than your foot will drop your economy up to 2 mpgs. With the cruise on your throttle setting moves way too much, diesels do better with a steady throttle.

 

[SEmerson]

I have 2 3500's autos. speed killes the mileage. 75 to 80 gives me 15 on my 24v and 13 on my 12v (12 is 4x4 thought). And that is empty. I am hopeing that advancing my timing will have the 12 v though.

 

[BCFAST]

Yup... your mileage sounds way low... . although, like it was stated above speed MURDERS your mileage. The fueling curve in the pump isnt linear. At about 2050 engine rpm, the fueling curve goes sky high at a very steep incline! You will find your engine happiest at 2k, regardless of mph. Check my signature below for the combo I'm running, but with the tall tires and 3. 54 diff, I'm turning about 1760 at 70mph. I keep my eye on the boost guage to tell me if I should back off a bit or pour the coal to it. On flat ground with no wind, I run 72-73 and get over 23mpg!! Just filled up yesterday and with combo driving, got 22. 46mpg. When I try to push just a little bit... say, 75mph... my boost goes from 1-2lbs to 5-6lbs and the economy goes bye-bye.

 

[shendren]

"Fuel return line can start cracking and suck air but not leak fuel. "



how does the fuel return line suck air? just curious. scott.

 

[P-38]

I'm new to working on this Turbo Diesel, what would be the easiest way to check for leaks other than seeing or smelling fuel.



I have a very keen nose and have had leaks before that I fixed. I'm quite sure I am not leaking fuel out, however, I'm not so sure about it sucking in air.



Are there any simple methods for checking this?



John

 

[illflem]

When the return line goes bad it can suck air but not leak fuel when the engine is off, this results in hard starting after the truck has sat overnight. If the line was leaking enough to drop your mpgs it would coat the whole underside of the truck, you would smell diesel from a hundred feet away. Hard starting is what you get with a sucky line.

 

[P-38]

Starts exceptionally fast!!! absolutly no fuel smell.



Thanks

 

[ewcmr2]

Just got back from Kansas, towing a 12K pound enclosed extra height gooseneck trailer (a/k/a a sail) with the 96 truck. WORST mileage was 10 mpg at 75-80 with cross winds. Best mileage was 14 (wife driving at 65 mph). Most of the trip was 70-75, with 11-12 mpg.



When we towed our race car on an open trailer (~5K pounds), it got anywhere from 16 to 21 mpg at 65-70 mph.



So... yeah, your mileage seems low.



Eric

 

[Missouri Mule]

P-38,

If you park it on level service it will start like always.



I looked at my hose for 6 months never saw it actually leaking.



It was only hard to start when truck was facing up hill (in my driveway and I would have to prime it) when it was in street it started with one crank.



Where the hose is close to the head I could see if it was moist, months later when I parked in driveway I could see that my starter had been slimed by diesel fuel. (Maybe slimed is to strong of word for some turbo heads I appoligize in advance)

I wipe it off and it would be there the next morning. Finally went in after it.

The side of the hose facing engine head was cracked (like crow feet on paint job).



Diesel leaks look like oil leaks it attracks dirt/dust and what ever else is blowing by.



Do fuel pressure check this will tell you a lot.



The parking up hill with hard start can point to three things, return fuel line, tranfer(lift) pump, Overflow valve.

 

[P-38]

Well, everything I have read so far tells me that I need to get my rpms down in the 2050 range. 70mph = 2200rpm, 75mph = 2400rpm and 80mph = 2600rpm +/- a few.



In my opinion this SUCKS!!!!



I want to cruise at 70-75 with my rpms at or below 2050.



Anyone with a 3500 / 4:10 gears automatic experiencing the same thing??



I might be going to the junk yard soon looking for a 3:54 rearend.



John

 

[P-38]

Will sliding my #6 plate backward a bit (closer to where my stock plate was) give me better mileage?



I actually replace the two braded fuel lines about a year ago, they were leaking (drip drip drip) everywhere.



John

 

[illflem]

Moving the plate won't help. A two speed gear box like US Gear offers wil get your rpms down and may turn out cheaper than a rear end. Larger tires will also get your rpms down. I would suggest you check the timing and valve adjustment first.

Cruising at 75 (2000 rpm) 3. 54 rear, 285 tires, AT, 4wd gets me 22-24 mpgs empty.