I've got a 1991 F350 CC/LB with the 7.3L IDI, and while not under load its great, however when i put a load on there it struggles to maintain around 60 on the highway. it seems to struggle any time there is a load, no matter how heavy. i pulled my 20' GN with a Dana 60 front and 70 rear, and it had troubles making it to 60, and with a '75 CJ5 custom built into a rock crawler (probably weighs close to 4500, and about 1500 lbs of misc weight on the trailer) it also has issues with hills, even the smallest hills i go over lug it down to 35-45mph.

is there anything i can do to help improve the highway speed while under a load? I dont have the cash flow to turbo it, so i need to try other routes if i can.
tramsmission is the E4OD, tires are TOYO M/T 285's (33x12.50)

Thanks for the help

 

I have an 85 6.9 and when I pull my 24' 5000 lb travel trailer I lag on the hills also... New trucks pulling longer 5th wheel trailers blow by me.. But, I think I'm getting everything out of the engine that is there. It is what it is. All my RV boards say don't go over 55 when pulling the trailer, and I think they are right... On level I can push it up over 55 but its unstable and just burns extra fuel. I do feel that even though I'm lagging, the engine is fine wide open on hills and could do it all day... I don't think I would feel that way about a gas engine.....?

Is this something new to your 7.3 or has it always been a dog on hills? If its new could it be a fuel filter.

Or, I think Ford added a turbo for a reason....

 

Your gearing will play a big part... the lower the better. 3.73:1 minimum, or 4.10:1 ideally. Next, as pointed out above, the lack of a turbo is telling. You're only dealing with, what, 185 hp and 380 lbs-ft. You have to multiply what you have with gears. If you already have 4.10s, I wouldn't recommend lower just because it will start costing you solo fuel economy. As was perfectly said above;

"It is what it is!"

Always remember you are driving old tech. It's never going to be a match for the new trucks. The driver must adapt to the equipment. Learn to master the Zen of the slow lane.

"****** the injector from my hand, Grasshopper!"

 

right now its got the stock gearing, whatever that happens to be. I stay in the slow lane pretty much all the time, just moderately that i've only got about a 10k load and it struggles to pull it even up the smallest hills. flat land it'll cruise at around 60-65, but with the slightest increase in angle on the road it drops to 50-55. I've toyed with the idea of getting my hands on a turbo, but with 300,000+ miles on the truck i'm not sure how it would handle the increased power.

to answer Redbug's question, i bought the truck in Oct. of 2008 while on leave from a deployment, so that trip was the first time i've drove it long distance and pulled a load. the PO used it to pull their 35' bumper pull trailer, and i'm going to take a guess and say that they experienced the slow speeds too. the truck was very well taken care of, regular services, new glow plugs, injectors, (have 4 extra that are new) air filter, fuel filter, and a couple other small things here and there. it runs like a dream, just struggles with weight.

 

Only 10k load? To me thats a big load. My manual says 5k is about my trucks limit, even with the 4.10 gears. Not surprised you are lagging on the hills... With that load my truck would be crawling up the hills.

 

One thing that may help slightly is to get a smaller tire size. That may help a little but i would say that 10k pounds is pretty tough for that N/A. The only problem with the smaller tire size is a small decrease in fuel economy. Maybe try advancing the injection pump a little? Or, hell, swap in a 5 speed. You would have a lot less transmission drag on the engine allowing you to get better fuel economy and more useable power from the engine. But that may be quite a job to do. Anybody else have anything to say?

Aaron

1975 F-250 390fe Eddybrock performer package, headers, 4 speed, 35s

1994 F-250 Turbo Diesel idi, reg. cab, 5 speed, boost and pyro gauges, 4inch stacks

 

Dude, get your hands on a turbo. Its the best thing you can do. The rest is just going to be small gains that you most likely wont notice.

Buy it one piece at a time if you have to. If you dont need to go fast just learn to enjoy the scenery.

I bought my turbo since I could not stand to be passed going up hills unloaded.......

 

My original 1986 owner's manual states up to 12000lbs trailer weight depending on the gearing and transmission of the truck.

I second Jim's remarks about gearing (brave words for a guy that tows with 3.08s LMAO). Basically, you need to keep the RPMs higher up if you want to move that much weight at those speeds. What is your RPM at those speeds? 2300 or 1800 or?

To really properly tow 10000lbs with a N/A IDI, you want to keep it close to 2500 RPM. This gives you the higher end Hp and also keeps the air flowing through the engine to control EGTs.

How much do you like this truck? If your injection system is old, then replacing it might help performance. A new turbocharger is steep in price, but used ones can be found for much less and they work just as good despite the marketing spin. If your head gaskets have never been replaced, you may want to do that now, as a precaution before adding a turbo, but its not required and there are many high mileage IDIs out there that get turbocharged without blowing up.

Mild fuel settings and turbocharging will actually help the engine live longer thanks to lower EGTs for the same amount of work.

 

Show me a gas engine that is even still running after towing duties for 300,000 miles.

 

stock gearing is 3.55's or 4.10's ... but i'm pretty sure the 4.10's were reserved for the dually's. average RPM for the truck while towing are around 2400-2800, and when pulling really steep hills over distance it spikes to just over 3000. temps are generally stay pretty low. The injection system is pretty old, although the injectors themselves are new. I've got 4 brand new ones to replace bad ones if any are going bad (on a side note, whats a quick way to tell if they're going out?)

I'll probably see if i can get my hands on a used turbo, or get a setup piece by piece. i just dont want to push the envelope on it and try to run too much boost to the motor.

 

When you are loaded, have you ever weighed the entire rig?

I am guessing you are moving far more weight than you think.

Truck has to be close to 7000 pounds, more if it is 4x4.
Trailer is probably over 3500 pounds.
Then say 4500 for the Jeep.
Add in another 1500 for you misc category.
So that puts you at 16,500 pounds.

Now figure 185 HP at the flywheel moving that weight.

Add in the fact if you have 4.10 gears, at 60 MPH in OD I have you turning 1800 RPM with 33" tires.
The taller wider tires are increasing the frontal area of the truck, that is another drag on the HP.

That said I have many miles towing much more weight with my 6.9.
I was never the first one there, but I always made it where ever I was going.

Give me a speed and RPM in OD, then I can figure out what gearing you have.

 

truck is a 4x4, weighed in at 7280, thought about hitting the scale on the way down, but never remembered on time. loaded i was hitting 60mph on flat surface at 2800-2900 rpm

 

Check the axle code on the door jamb with the codes on this site. My 350 is a SRW and it has 4:10 gears...

 

33" tires with an E4OD and the over drive locked out works out to 67 MPH at 2800 RPM with 4.10 gears.

The 285/75 tires have your speedo off by about 5 MPH.

Also the extra width and height from the tires is increasing the truck frontal area, which increases wind resistance, which can be a serious power drain.

All in all, considering the weight you are moving with a 185 HP engine, you are doing rather well.

Are you still running the stock exhaust?
Have you made any changes on the air intake side of the engine?
Do you see any black smoke while pulling the hills?

 

it is running stock exhaust, however there is a very large hole in the muffler (going to be replaced soon), I havent made any changes to the intake, i was thinking about getting some of the dryer exhaust tubing and running that to create a ram air.

it does pump out black smoke while pulling hills, but not a huge amount, no more than some of the newer psd's when under a load.