89 7.3 idi 4x4 69,000 miles. can i have a list of things to check/diagnose to see if they are working correctly. diesel is $4.30 right now and rising and cant afford this truck anymore at this cost. my temp is at 165deg which is odd to me but not sure if it effects mpg that much like gas motors.... a list of possible things to check would be nice. thanks!

 

i have never heard of mpg calculations this low before.twice that is pretty bad for a srw/pickup bed setup.
whenever fuel economy looks outright horrendous like this,the first thing to check is for fuel leaks,and or fuel returning to incorrect tank.


your fuel economy will indeed suffer if she's not coming up to proper normal operating temps of at least 190-200ish range.
locked up brakes,shot wheel bearings,poor transmission condition,bad ring and pinion,are also things that come to mind with such horrible economy.
im not sure she would have any power or even start if the fuel injectors were so far shot.so id look in more obvious places rather than the engine for starters.
what does it have for a transmission?
is the odo/speedo accurate?
is the rear axle full of old sludge rather than fresh lube,due to the truck sitting and hardly getting used for so many years? is there even anything in it? something very serious is up like this.
perhaps her fan clutch is shot,and is locked up solid.that old energy robber locked up could explain a lot.they tend to fail the other way around,but this does happen too.would be very loud.then again,if you never heard it off....

 

I have to agree you have something way wrong to be that poor of mileage. My 88 7.3 4x4 gets about 15-16 mpg on highway, and that is me running about 75 mph. I have slightly larger tires which also gives a bit of drop in mileage. The ZF-5 and ATS turbo help, but being a 4x4 and sitting up in the air it will never get as good as a 2wd that is lower and less air turbulence underneath.

On your truck, have you checked the timing? Where did you get the 165 degrees from? Factory gages can be off, but a new thermostat is fairly low cost to change. Any leaks from your return lines? Not just on top of the engine, but also on way back to the tank. Can't get mileage from fuel dripping on the ground. Tires at full pressure? Changing fluids will never hurt and mostly lower cost. Do you haul a lot of weight around for no real reason?

Just checking that you are calculating the fuel MPG correctly? Fill it up, then drive and record the miles (example 225 miles), then fill up again (say it takes 15 gallons), your mpg is 225/15 = 15 mpg. You really need to do it over several tanks to get a good idea of the avg mpg, as wind/road/weather/load conditions will affect a single tank.

 

motor seems to run fine. tranny was flushed including torque converter. never checked diff. i turboed it. all parts were off factory 94 motor. doesnt leak any fuel because there is never any drops on the driveway. i know it runs at 160 deg because at my work (excavator rental company) we have a laser heat gun thingy lol dont know real name for it. i opened the radiator cap 2 min after driving 20 miles and it said 160

 

oh i doubt thats the issue.drive it for 5 miles,then point it at the upper rad hose.that will get ya your 195ish reading.
id verify the odo/speedo.if the odo is correct,and your fuel isn't returning to the wrong tank,then you have a serious lead foot,a shot injection pump,and injectors that dont stop leaking fuel,and a majorly retarded timed engine.even then i think you need a couple bad wheel bearings,and at least one brake locked up to get 6.5 mpg.

 

can you actually watch you fuel gauge move. how many miles do you go on a full tank before you have to fill up again?

 

I have an 88 4x4 7.3 ZF5 4.10 that gets 16mpg and an 89 set up the same but with 3.55 gears that gets 20mpg. These are at 65 mph or less and with 235/85/16 and 265/75/16 tires.
How does the truck pull?? Does it start right up or do you have to crank for a while?? Do you have a tach?? What rpm's at 65 mph??

My Western Star gets 6.1mpg pulling 86,000 lbs over the Rockies.
Something is very wrong with your truck.

 

x2 with what everyone is saying, something is seriously wrong with your engine....transmission and drivetrain, it could be as simple as your fuel is draining back into the wrong tank...the FSV is known for this(fuel selector valve=FSV)

 

i dont understand why fuel would be returning to the wrong tank.... wouldnt it not take as much fuel on fillup then since its going back in? it still costs alot to fill up i would imaging that if it was returning to the wrong tank then it would only take a few gallons

 

Take for example your set on the rear tank. If your pulling from the rear but a faulty selector valve returns it to the front tank, its going to make it seem like your sucking fuel like mad. If only one of your tanks gets used you may never notice (the unused tank will eventually fill). Switching the valve should tip you off to this but if the tank isnt used because of a leak or faulty gauge, who knows?

 

I agree with the others, over half of what youre getting is horrible for an IDI, i drive mine like i stole it and get 17.7, towing around 7k i still manage 12-14. Is it down on power? Unlike it seems from the above posts, it sounds to me you have a whole bunch of little things working in conjunction to produce such bad mileage. I would start with checking the brakes, ie: set your rears, pull and grease the calipers/sliders. Check the trans, t-case, rear-end for proper lube. Grease your front end bearings and hubs, make sure the hubs arent staying in. Look for worn/binding u-joints. Get a 180 or 195 thermostat. Have the timing checked. Make sure your return lines are returning to the tank and not the ground. Are you sure she is running on all 8 cylinders? Does your oil smell like diesel, any smoke, black, grey blue or white? I hate diagosing things over the internet, but hopefully some of these get something figured out.
One last one, is your converter locking up? if not they can lose ya 2-3mpg