Fuel Mileage--need your input

[Michael Fowler]

My first IDI ( '84 w/ C6, completely stock) got high teens to low 20s mpg when empty, and mid teens when towing my 5th wheel.
New IDI is a '94 extended cab, 8' bed with factory turbo, and E4OD on 265/75 X 16 wheels and tires gets 13 empty and 10 while towing the same 7000# 5th wheel. Driving mostly 60-65 mph.
I checked the odometer against measured miles for 20 miles and the error is under .05%. Speedo checks out to about 2-4% slow when compared to my GPS. Truck can maintain 50-55 mph on the uphill grades on I 70 in western MD, and PA while pulling the fiver- not blowing a lot of smoke at WOT, just a hint of visible smoke.
Injectors and IP are of unknown age, truck is showing 237,000 miles. Not excessive blowby. The return line kit appears to be relativity new.
I am leaning to replacing the injectors and IP, but what do you think? Am I unrealistic to expect better fuel mileage?

 

[Ironman03R]

Im gonna say that sounds normal. My truck has gotten a best of 15 empty but usually is 12-13. Towing is normally 10-11.
I've noticed the 6.9 gets better mileage, but also noticed that the early 7.3's seem to be better than the 92-94 7.3's

 

[Silver Burner]

Still seems way low to me. I get 18 empty and 14 or so towing a 6000lb trailer over Mt Hood.

 

[gandalf]

You haven't mentioned the axle ratio of either truck. That can account for quite a difference. The difference between a 4.10 vs a 3.55, or even, say, a 3.08 (is that number right?) will affect you mileage quite a bit.

What axle ratios do/did your trucks have?

 

[Michael Fowler]

You haven't mentioned the axle ratio of either truck. That can account for quite a difference. The difference between a 4.10 vs a 3.55, or even, say, a 3.08 (is that number right?) will affect you mileage quite a bit.

What axle ratios do/did your trucks have?

Thanks for pointing that out!!
I even crawled under the truck to check the axle tags, and forgot to include that information.

3.55 in both trucks.
'94 is also 4X4 with the hubs running "free"
One more observation--the '94 is considerably more "subdued" and refined feeling. You can still hear the engine, but it is much more muted than the '84. My ears don't ache after 3 hours on the road--the '84 was not kind on the ears.
One more thing---timing has been properly set by Mel, himself, so cross that possibility off your list.

 

[The Warden]

I know the timing's good, but how's the pump and injector health?

Are you sure you don't have any brakes dragging or anything on those lines? Also, have you done anything about the crushed factory downpipe?

 

[Michael Fowler]

I know the timing's good, but how's the pump and injector health?

Are you sure you don't have any brakes dragging or anything on those lines? Also, have you done anything about the crushed factory downpipe?

I have noticed no excess heat that would indicate dragging brakes. I haven't even looked at the down pipe.
Injector health and IP health are unknown--that's the reason for the thread--to see if the general opinion is that I might benefit from replacing them.

 

[Diesel Max]

I had a 92 F250 7.3 turbo 4X4 E4OD with 3.55's that got about 19-20 on the interstate running empty consistently.

I have a 93 F250 7.3 Turbo ZF5 with 4.10's that gets between 15-16 on the interstate running empty consistently.

 

[Brianedwardss]

My 6.9's I've owned have always got comparable or better fuel economy to my 7.3's, even tho the 6.9's never had overdrive, all but my current 7.3 did.

 

[jhnlennon]

I've owned 6 ford IDI's now and the mileage was consistant between all of them, what ever the combination was. 18mpg's on a long high way trip, 15-16 mixed driving, and 12-14 towing around 8-10k. Never once saw the elusive 20+ mpgs, no matter what or how hard I tried. Now the 2 cummins trucks I had were a different story. 20+ mpgs on a long high way trip was the norm running empty. They semed to average about 2 mpgs more.

 

[The Warden]

I have noticed no excess heat that would indicate dragging brakes. I haven't even looked at the down pipe.
Injector health and IP health are unknown--that's the reason for the thread--to see if the general opinion is that I might benefit from replacing them.

Assuming you still have the factory downpipe, the restriction there could be choking your exhaust enough to hurt economy and performance. And, if you don't know how many miles are on the IP and injectors, I would seriously consider at least pulling the injectors to test them, and it may be worthwhile to summarily replace the whole set depending on what your financial situation is.

Unless you have a lead foot (and I doubt you do), I don't think 13 mpg out of an empty truck with 3.54's is anywhere near "right". For that matter, the worst I've ever seen out of my truck with 4.10's and the original T-19 was 12.5. I'm pretty sure there's an issue, and while I don't want to spend your money my gut tells me it's in the fuel system...if the timing's happy, then parts are worn. With that said, taking care of the downpipe will certainly help as well.

Hope that helps...good luck

 

[1994IDI]

I am in the same league. The truck in my sig gets a best of 13 unloaded, and will only pass 16 mpg's on the highway if I keep it under 55. I don't have a lead foot, I don't have any breaks dragging, and my fuel system is in good health. This has come to seem the norm to me. However, you and I are running different transmission and tire size. I am running 285/75/r16 with a ZF5, and a 3" downpipe. I am also running a 4.10 rear.
It looks like "Diesel Max" is in about the same boat with me. Hope it helps.

 

[Michael Fowler]

Warden,
Your summary is about where I am.
Hope to have a few $$ to spend over the winter for matched and tested injectors, and a pump, maybe a baby moose.... and some real gauges.

 

[The Warden]

Hope to have a few $$ to spend over the winter for matched and tested injectors, and a pump, maybe a baby moose.... and some real gauges.

That would be my next step If you can afford it, I think a Baby Moose would be a worthwhile investment...

 

[Brianedwardss]

14 mpg is the best my pavement princess ever gets, even with my very light foot.

I had a 2wd 6.9 std cab that would top 20 mpg consistently, so I know it's possible. But I've never had an idi that got over 18 mpg since then.