New to the forum. Just bought a 1984 f250 HD 6.9 idi with 116,000 on her. Regular cab, long bed, 2wd, Dana 70 3.73's and stock 32" tires. Mechanically, it's a dream. Runs tight and makes plenty of power down low. I'm consistently grabbing 20+ mpg at around 55 mph. Last trip I logged 23 mpg on a 200 mile trip. Keep in my mind I let her sit just above 2100 rpm in the right lane. Anyway, I haven't messed with the pump due to the lack of an egt gauge but I think i may be running lean to get these kind of mileage numbers. Thoughts? Also, I heard people run these engines against the wall for pretty good periods of time. Am I asking for trouble to run 3000+ Rpm on the highway?
Thanks for your time,
jack

 

You really can't run a diesel "lean" it doesn't work the same as gas...

Are you happy with the power? Does it smoke at all? You might be able to turn the pump up some, but without an egt gauge you could be asking for trouble, and without a turbo, you might not get any gain.
Any reason you're limiting yourself to 55mph? I'm not saying thats a bad thing, by the way. Its a good way to keep fuel costs down and baby the engine. If you're not in a rush to get there, whatever right?

Honestly I'd love to get mileage that good on my truck, so not sure why you'd want to crank up the fuel.

 

i try to keep it around 55 to reduce fuel costs as you were saying. I'm 17 and this is my first truck. I waited a long time and spent a pretty penny to get a truck of this caliber. I pay for both my truck maintenance and fuel so I'm trying to be smart about the way I drive it. No smoke at all unless I really jam my foot in it after an up shift. I try not to lug it in fourth for obvious reasons; However, it usually blows white Smoke If at all. Not sure if it should be black or not? The HD axle and auxiliary leaf springs bump my 250 to a 1 ton rating. Can't beat a 1 ton truck pulling this kind of mileage. Although it rides pretty rough empty.

 

Since you're footin the bill the first thing I'd invest in is some good gauges for the usual suspects, #1 being EGTs. You can never go wrong having one of those in a diesel no matter how "stock" it is.

Turning up the fuel on an idi is not going to net you much of significance unless you have a way to feed more air to burn that fuel, ie a turbo. You'd probably benefit more from the tools to properly time it and maximize what you've got.

And as usual, no pictures, it didn't happen.

 

Thanks for the advise. I'm relatively new to these Diesel engines and I really appreciate any and all advise I can get my hands on. I'm glad I can pick your brains about this stuff. Learning the hard way is too expensive!

 

Typically white smoke indicates retarded timing(provided you aren't leaking lots of coolant... you'd know it if you were).

 

Welcome to the forum. It's the best place on FTE.


Do you have a manual tranny? If not, then I'm not really buying the 23mpg unless it was 200 miles all downhill with a tailwind.

I do lots of ecomods on my IDI, and to break 19mpg is VERY hard without a turbo, manual tranny, OD, or other mods. But the one thing you are doing very right is keeping it at 55mph. That'll save you lots of $$$$$

 

yes, but no overdrive. i have the 4speed borg-warner T18 wide ratio tranny. basically a 3 speed since 1st gear is at 6.32 to 1.

 

Bone stock you can run these engines right at the governor all day long, it's how they were built. A non turbo tends to loose efficiency around 2500-2600 rpm's though so fuel economy will definitely drop if you try to go 70, you'll get 15-17 at best. Which really isn't all that bad for an 80's model truck that weights what a double cab dually now a days weighs.

I would not recommended running the rpms at 3500-3800 if you have no egt gauge though. A lot of people like to put the 7.3 injection pumps on a 6.9 thinking they'll get some extra ponies. It may add 5 but it's really only enough fuel to raise exhaust temps on a hill. Flat ground will normally be just fine.

I get 15-18 mpg on a regular cab single wheel with 3.55 gears 4x4 leveled with a maxed turbo pump. So 20ish mpg on a 2wd seems doable. I can push mine to do 90 but the egts get a little hairy working er that hard. I can see 1200+ at the end of third and you definitely don't want to hold it there!

I would stick to stock until you get a gauge at least, if you're happy with the power leave it there. The extra fuel does give me a bump on low end but it's not really any faster. Mainly just black stacks after 3/4s throttle.

A slight white haze is pretty normal with aged injectors and the crank case vapors your intake sucks in from the cdr. If you paid a pretty penny and want to keep the fuel numbers I would leave her stock on the fuel end and call it a day. Maybe hunt around for a second-hand overdrive unit so you can run 70 with decent numbers. they bolt to the back of your tranny or in front depending on what company it is. Easy enough to do on a 2wd.

 

I have a 86 F-250 single cab, long bed, 4-spd manual gear box and 3:55 Diff.
I can vouch getting 20mpg and even 22+ if I keep her 50 - 60 mph.
Go over that, then the MPG drops a bit.
Now this is an empty Truck.


Lately I have been in the 18mpg range, probably do to the winter fuel.
She needs Injectors and IP replaced, get to that this summer, and it might improve, IDK. We`ll see.


When I fill the fuel tank, I fill her to the end of the neck. This give me an extra 3 gallons. This way I get it exact when figuring out the mileage. Stopping the fill when the handle clicks, could give a slight difference when figuring the mileage. Foam etc....


That extra 3 gallons saved my rear one time. Fuel gauge does not work, so I write down the mileage and subtract etc... Told the Wife I think we need to get fuel. Put in 21.221 gallons.




Charlie