Engine/Transmission (1998.5 - 2002) Erratic Fuel mileage

[KennethJohnson]

I sure hope someone can help with this. I am experiencing very erratic fuel mileage with my 99 2500 4x4 diesel 24v. It is ranging from 10-12 mpg to 16-18 mpg. We have tried different fuel and have changed the oil, air and fuel filters. we have 265x75x16 tires on it at 70 lbs. Is there something we can do to bring the mileage up and get it consistent? The truck has 111,000 miles on it. please help

KennethJohnson

 

[ToolManTimTaylor]

Hey Ken I am going to Touch this up and move it for ya Now it's "FUEL" Mileage How are you getting your figures? Long hand style or by the overhead trip meter. That overhead deal is waaay too unreliable and cannot take into the equasion real world scenereos. Also don't rule out that your fuel level guage may be erratic as many a 99 has had issues with them as well. Mileage can depend on



Outside temps

Fuel system condition

Valve train

injector condition

driving conditions

Tires

Alignment

intercooler cleanlyness / intake plumbing condition (No leaks)

 

[Pinion Twister]

Might try cleaning or replacing your IAT senser.

It's on the drivers side at the back of the block.

Hope it helps!

Keith

 

[KennethJohnson]

Hi Thanks for the reply and the fix! My wife typed all that in and I told her to change gas to fuel but hadn't gotten around to it yet. My truck has 3. 54 gears 4x4, fuel figured longhand, intake clamps all okay, truck is stock except no muffler, all filters are new, got 265/75/16 tires going to 255/85/16as soon as 265s wear out, truck has power and runs good the mileage is just bad, truck is driven easy on the throttle, almost never sees 2,000 RPM. My buddys 94 auto, 354 gears, 245/75/16, 4x4, gets 21. 5mpg stock, He drives his truck harder than mine, I know he has less power andaand he has a mechanical fuel injector pump. But shouldn't I be getting 17/18mpg constantly? thanks for your time and the wife is still typing so go easy on her okay?



Kenneth Johnson

 

[KennethJohnson]

I already cleaned the IAT sensor, didn't help, but thanks anyway.



KennethJohnson

 

[ToolManTimTaylor]

well at 2000 rpm and under you are actually using MORE fuel than not. If you notice EGT's climb when the truck is "Lugging" itself creating boost. Boost = more fuel being burned than nessecary. On my way home from work the last mile I run 2nd gear locked up using my smart controller. The reason is I am just off the highway and running the truck @ 25 - 2700 rpm steady and OFF BOOST the rest of the way and my egt's drop and stay @ or around 450 degrees until I get home. Dow if I drive it normal the rpm's stay about 1500-2000 and egt's are about 700-800 and I am on boost for the ride. (Remember boost = more fuel being burned = more heat).

 

[McCRam]

I think the tool man is right. I have found that when I keep the rpms up, and do not let the turbo spool up, Ill get better mileage. Its hard to do though cuz I love the sound that turbo makes. I think that this could give you the numbers you are seeing. You may have already done this but, fill up and calculate after each type of driving, city, highway or towing. This will help to find where you are getting the bad mileage. Different drivers can make a difference too. My uncle gets better mileage driving his 98. 5 then my aunt does, he runs it much harder.

 

[KennethJohnson]

Thank you very much for your time. We really appreciate it. We will try your suggestions about higher rpms and let you know what happens. Have a good day and we'll talk to you soon!!



KennethJohnson(and wife)

 

[KennethJohnson]

This may be asking too much but does anyone out there have the time and patience to explain what EGT's are and how these diesel engines work to me (the wife) who drives it most of the time. Do we need to be driving with Higher RPM's all the time or just at slower speeds? What exactly is boost and how do you know when its on or not on? thank you ahead of time. Susan Johnson

 

[ToolManTimTaylor]

EGT's (Exhaust Gas Temperature) Too high SUSTAINED is 1300 degrees for a rule of thumb. in SHORT BURSTS Higher temps are o. k but not reccomended. Me I have hit 1680 but that was a short run 1-od converter locked. (I. E under a minute) Over a minute or longer then 1300 is the Safe max.



A Pyrometer is your best friend especially when towing. you can see with just a pyro and boost guage what may be ailing your engine. For example Low boost High Egt may indicate an intake leak or very dirty air filter (Yes other but I am tryin to spit up the basics)



Now as before when I made mention of you "Lugging" the engine. That is a condition that is LOW RPM and Heavy fueling. Usually 1600 rpm on down but I include 2K. Theese things can rev out with no ill effects. D/C Made sure of that one with a 3400 rpm limiter. Theese trucks have seen 4-5000 rpm (5K is with alot of work) So fear not to let her sing. You get IDEAL fuel mileage when Boost is at a minimum as fuel is needed to feed the fire to spin the turbo to feed air into the fire to burn more fuel to spin the turbo to feed air into the fire to burn more fuel to spin the turbo to feed more air into the fire ... ... ... . etc.



Keep a higher RPM you are feeding fuel to keep momentum going ... . . with little boost which consumes less fuel ..... blah blah. I find 2000 and up to be good for this.



There is a wicked host of knowledge ..... waay more than I can possibly describe better than I ever will be able to too in the new to diesel area. However do not ever hesitate to ask.

 

[ToolManTimTaylor]

Boost BTW is pressure created in the intake via an exhaust driven compressor. It is accompmanied by a high pitched whistle and a smile or a naughty grin :-laf



Also check out



https://www.turbodieselregister.com/forums/forumdisplay.php?f=68

 

[KennethJohnson]

What is the best set of guages to get when you are running stock?

KennethJohnson

 

[ToolManTimTaylor]

The Basic three are

EGT

Boost

Fuel Pressure



Fuel Pressure being A#1