I bought my well cared for 154,000 mile 1991 F-350 in early 2007 and its never pulled like it should.Its had new injectors and now a rebuilt injection pump and its did nothing for it.Its still gutless and gets 7 mpg pulling a trailer no matter what the load is..It acts like it has less than 100 horsepower.My old 1979 Chev K-30 with a 350 and 4 speed would way out pull this truck.

I've played around with the pump timing and fuel settings with no results.In mid April I was on Hwy 60 west of Van Buren,MO and basically ran out of horsepower in 2nd gear pulling my trailer with a 5000 lb tractor on it.It was a very steep grade and due to road construction I couldnt take a run at it.There was a pull off halfway up that saved me just as it was about ready to kill in 2nd gear.I let traffic clear and just stayed in 1st gear until the top.I was in 4th gear most of the rest of the way home at 50-55 mph except for a couple of steep grades between Mt Grove and Springfield,MO.

The truck starts and runs decent and isnt an oil burner.It has to be way down on compression,has some bad valves,flat camshaft,or something? Quite a few around here have told me that these trucks never pulled good when new but nothing could be as sick as mine.Friends that ride in it cant believe how poorly it pulls.

Yes,I'm aware of intake and exhaust mods but they arent going to give me what I need.As much money as I have poured in it,I hate to give up on it,but I'm close to reaching the limit.

 

wow thats really bad.
i would check the compression,the trans,the rear axle,the brakes,and wheel bearings.
seems like you have a plugged up exhaust or something.whens the last time you replaced the muffler?
does it run ok without the trailer? do all the wheels spin freely on the truck and trailer?

 

When the IP was replaced did you remove the timing/oil fill cover with the IP....

Do a check of the timing.......

The timing mark is found on a little plate that is on the front of the engine on the passenger side. The plate is mounted so it is very close to the main pulley on the lower front of the engine which is mounted on the vibration damper. On the side of the vibration damper, is a line, which when lined up with the line marked (O) on the plate, tells you that the #1 piston is at TDC(top dead center) You can see the line on the damper, and it points to the line on the plate with the (barely visible)O mark on it.You'll have to find a way to turn your engine over by hand (Crank/harmonic balancer bolt). Make sure you remove the power wire to the injection pump(so the engine won't start)and then rotate the engine until you can line up the marks.

The engine rotates in a clockwise direction as seen from the front of the truck, and for every 1/4 turn of the engine(crank) each next piston will arrive at TDC. So, When the line on the vibration damper is at the 2 oclock position, cylinders 2 and 5 will be at TDC. When the mark is at the 5 oclock position, cylinders 7 and 6 will be at TDC, 8 oclock has cylinders 3 and 8 at TDC, and back to 11oclock, 1 and 4 will be at TDC.

Cylinder numbers are assigned 1,3,5,7 on the right (passenger USA) bank from front of truck to back, and 2,4,6,8 on the driver's side (USA), front to back. Firing order is 1,2,7,3,4,5,6,8.

OK all the above is true...... to ensure the #1 piston is at TDC compression remove the inspection cover on the oil filler/gear cover. You will see a dowel pin from the IP if it is at 4 oclock that is TDC compression stroke, if it is at 11 oclock that is TDC exhaust stroke.......

If the dowel pin is NOT at the right location of 4 oclock with the #1 piston at TDC on the balancer mark and the lines in alignment. The one could assume there MAY be a problem with the timing gear(s) alignment.

Note if a prior owner removed the timing cover and gear with the IP it may have been put back together 1 tooth out in either direction????? What do you think any possibility?????

 

The truck has all new brakes front and rear,repacked front and new rear wheel bearings,new rear hub seals,new straight thru muffler 1 year ago,several new fuel and air filters,new tranny and rear end oil,new u-joints,all new belts and much-much more.The brakes arent dragging and it rolls free.The trailer bearings were recently packed and roll free.Everything has been checked and rechecked.

It does OK empty and gets 13-14 mpg but 98% of the time its on a 25 ft gooseneck trailer.A 5000 lb empty trailer is a moderate load for it and it gets 11 mpg.Put 5-6000 lbs on the trailer and it absolutely works it to death and 6.5-7 mpg is all you will get.The slightest grade seems like Pikes Peak.

PLC7.3,I'm sure that the truck had the original pump and injectors when I bought it.When I recently installed the rebuilt injection pump I pulled it off the correct way by the 3 bolts.The cover stayed on.The truck has never had any power since I've owned it.

6.9 and 7.3 IDI's werent very popular in my area and mine is one of the last ones left running..20-25 years ago most everyone around here that pulled trailers with their F-250's and F-350's ran 460 gas burners and I think I know why.Several friends even had old 390's and 428's that pulled like freight trains.

 

what are the rear gears? 3.55 or 4.10?
what does it have for trans?
have you considered adding a turbo?
if i hauled a 5k lb trailer with a 5k lb tractor with a 7k lb truck for a total of 17k lbs often,i have to admit i would swap to the cummins as the most powerful recorded turbo 7.3l idi on the dyno showed around 200 ponies but the torque was much more modern numbers at over 450 ft lbs.this combo means she will pull it no problem at all,but with low hp,she's going to crawl on real grades/hills with that weight.
but short of this,adding a turbo and a good 3" exhaust will wake it up.but thats a LOT of weight.
if you have 3.55's i would swap to 4.10's in a heart beat for this kind of hauling.the difference will be huge.
still.your empty running MPG is weak.somethings not good,unless your at a high elevation? -which of course will make the turbo addition much more drastic.

if i where you,i would do three things right off quick:
1.load up this combo and hit the scales.see what you really have total.
2.have the engine professionally timed (and not by someone who does it by ear lol.)
3.compression test.

 

My injectors and pump look original too, but I know for a fact they've been replaced cause of the rebuild tag on the pump and the totally mismatched injectors (I got some BB-, some D-, and some E-code injectors all at once) - point is even if it looks original it may not be, so even tho you replaced the pump the right way there's no guarantee someone hasn't messed up the timing well before you get the truck...

 

My 86 with 4.10 gears used to pull well over that weight up mountains and not have to slow to first gear. 5000# was nothing for it, I hauled well past 3 times that weight with it. It has the 4 speed as well, which seems to make a big difference over the C6 my dad had with his 84 6.9 with 3.54. It was gutless from the factory, a good IP change made a big difference for it. I doubt it could run with my 86, but it helped. I don't get quite the mileage you do, but it doesn't drop that far when I run the loads that I do. I have a two car trailer, then my sister's horse trailer, which weigh much more than that tractor of yours.

 

I'm in the same boat. Low power and poor mpg. With 16K lbs combined on a 6% grade, I'm down to 2nd (5 speed), maybe 3rd at 20-25 mph. Running around empty, it has decent power. But the best mileage I can get is 12-13.
New pump and injectors made little difference.
I have a couple of things working against me. Oversize tires, curb weight that's over 8K lbs, and being over 4000' elevation.

Jason

 

12-13 would kill me, I currently get 15 mpg at 55 mph with a 10ft tall slide-in and overall aerodynamics of a brick wall. This is with the tired IP and the clusterf*** of injectors, so I definitely have room for improvement, no financial resources for it tho.

 

Its a 4x2,5 speed,with 4.10 rear gears and I live in fairly level country with an elevation of 900-1000 ft.The late gentleman that bought my F-350 new at my local Ford dealer often pulled a large travel trailer to the west coast.It must have done lots better back then because it would have trouble going thru the mountains empty now.

If I could find a cheap turbo I'd try one.With the purchase price and all that I've spent on it,I've got $7500 in it but I have put 16,000 miles on it in 3 years.I'd say I weigh 17-18,000 total most of the time.My guess is its way down on compression.I'll try and limp it thru the rest of this year and look for something else this winter.

Lots of my friends run Dodges with Cummins and love them.One has a 128,000 mile Dodge 2500 and pulls a trailer identical to mine.He hauls a 10,000 lb pulling tractor and gets 12-14 mpg loaded at 70 mph.He weighs over 20,000 lbs going down the road.

 

Two thoughts. Check your air cleaner, from manifold all the way to grill. Awhile back, had an issue like that. Discovered an old plastic grocery sack had been sucked into the air intake by the grill, had snagged, and was blocking portion of the intake without going all the way to the air cleaner. Second, compression. If it has new injectors and the timing is right, it could be low compression. If the injectors were old, I would suspect the spray pattern wasn't fine enough, meaning incomplete combustion. Consider finding a chassis dynomometer you can put the beast on to see what you are getting for wheel hp.

 

Its really easy to miss a tooth on the timing gear, the mark is just deep enough in that hole under the doghouse that its a real bugger to see. Of course id def do the easy stuff first like checking for intake or exhaust blockage. I drive a old 7.3 for a work pickup it has 245,000 miles on its origional engine ive never pulled with it so dont know what to tell you there but it runs great pulling itself, though the compression is def down cause it does crank a couple turns extra to start . Every vehicle on the place has had the spurs put to it by some cowboy somewhere in its life and been to thru 9 lives, and when the rest the vehicle is hammered out those old 7.3s are all thats left in em thats any good.

 

i really hope you continue your search for the poor performance and mileage problem SWMO.
its a shame to think of the IDI has a total dog like this before you move on.
compression check,fuel flow/pressure test,accurate timing done using proper tool,and check of the throttle linkage etc.
you have the hauling gears for it(4:10's),but id also be pulling that diff cover for a close inspection of the gears as well.
i cant help much,as i dont haul this kind of weight.but empty fuel mileage seems wrong with a 5 speed,even with 4:10's.
are you running stock tires? have you verified the odo/speedo to be accurate,if so how? via gps or another vehicle etc.
iv done in theory 7k truck/2k chipper/4k load for a 13k total,and no issues at all with 3.84 gearing to get moving nor breaking any speed limit.not instantly of course lol but only slow on real hills.by "slow" iv NEVER seen first gear,only 2nd,and when i hit 2,she talks and starts climbing until i back off it.

edit,
actually i got to thinking (this doesnt help your poor fuel economy problem) that with 16k in 3 years and a manual shift,you may just not be familiar with how the IDI works yet.
note: HP = momentum. peak HP is up at 3k stock.
this means,with this kind of load,you want to be sure your down a gear well in advance before you meet the hill at,at least 2500 rpm.otherwise your gunna fall on your face.
i meet the hills when loaded always @ 2500k i hit the OD button for 3rd/direct,and then floor it way before she starts to fall off to greet the hill in 3rd well before i get there.by the time she asks for 2nd,she's still rolling and talking,know what i mean? dont be afraid to work it.she's made to work this way.

 

If you are looking at the turbo equipped diesel and expect a naturally aspirated diesel to pull with it, you will be disappointed no matter what name is on it.

 

I also put brand new Injectors and an Injection pump on my 6.9 last summer, replacing the original fuel system at 95k miles.
I felt no difference in power, it just started easier because I replaced the return lines at the same time.
Diesels LOVE air. Don't deprive them of it, or they'll be a dog.
Funny you talked about the 390 FE's...I've got a 352/4 speed in my '65 F350 and I swear the FE will run circles around my IDI.
But, 6 mpg with a big block FE or 16 MPG with my IDI? Wallet says IDI wins.

Basically, I've learned you have to drive the diesel hard to be happy. I used to be miserable on the hills, but then I got SO fed up with the gutless pig that I just floored it before the hills. Let me tell you, if you start the hill at 2600 RPM, you won't lose much speed. These things are hard to bog down at high RPMs.

The turbo will build more boost when the engine is under load. More boost = less boggind down = happy driver.

What kind of smoke do you see?

And without a running start at a hill, I would expect to be in 1st or 2nd gear pulling that hill as well. get that RPM up around 2600.
Floor it, don't be scared.

Smell anything sweet? See any seepage around the head gaskets?
That would indicate head gasket issues, which drops compression, which loses power.