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6.9L Diesels Technical discussion of topics related to vehicles powered by the 6.9 Liter In-Direct Injection Navistar engines.

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Old 06-01-2009, 02:02 PM   #1 (permalink)
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Info on 86 F250 6.9L Needed

Hello,

My grandpa-in-law has an 1986 F250 6.9L diesel with a large 3500lb camper that he let us use last weekend. The truck had been sitting for 2-3 years. New tires and batteries, and off we went.

I may be buying the truck off him; excellent condition with 94k original miles, however I do have a few questions/concerns about it.

During pulling, especially with the first tank of gas (first tank of gas was good, put new gas in it before I left), it didn't want to get over 55mph. On the way home, I could just barely touch 70mph (WOT), but it was a struggle. So it seemed to get better the more we drove it. Now, I do NOT need to go that fast, I'm just generally concerned about performance. I only had 3 gears I noticed (automatic); is that normal; am I missing a gear or did these come with 3-speed automatics?

Also, I got about 8.5mpg on my trip (was a 4-hour trip each way on the highway). I know it had a 3500 camper on it, and the camper kills aerodynamics, but what can one normally expect out of these engines? Perhaps a tune-up would bring things up a bit?

Speaking of tune-ups, any recommendations? A general Diesel noob, but have been reading quite a bit. Truck is in great condition; headliner perfect, AC Ice cold, seats in great condition, carpet spotless, so if I can get this motor going well I think I'm set. Note: The engine actually ran great, was just a bit surprised by the lack in power, fuel economy and speed (and that's with some already low expectations). Just want to know if that's below par for the engine, or if I need to reset my expectations!


Thanks!
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Old 06-01-2009, 05:16 PM   #2 (permalink)
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8.5 mpg on the modern diesel fuels with a slide in camper trying to run 70 isn't too far from normal. The gear ratio in the truck will have a huge effect on MPG, as well as the aero drag from the camper. If you have a 4.10 rearend ratio, 70 mph was probably running the engine at maximum governed speed (3300 RPM). The 4.10 will be the hardest on fuel too. The C6 transmission in it is a 3-speed with no overdrive and no lockup torque converter, so this is also a bad combination for fuel economy.

For comparison, I have a 3.55 rearend and a turbo and only manage about 13 mpg empty. Even with the turbo, mine is no land-missile or anything. They are only 175 hp stock, and that is at 3300 rpm.
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Old 06-01-2009, 05:34 PM   #3 (permalink)
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Ok, that helps a lot. Yes, it was at about 3300 rpm (and it's a 4x4). Note though, I wasn't going that fast for more than 5 minutes on the 4 hour trip. Was usually 55-65mph, unless I came to a hill then it was 35-45mph.

Now, I hear of people on these boards getting 20mph on the highway empty. Is this even attainable with the rear-end that I have? Any tips here?
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Old 06-02-2009, 09:34 PM   #4 (permalink)
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You better drain that GAS out of the tank, before you ruin the engine!
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Old 06-07-2009, 12:21 PM   #5 (permalink)
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i just pulled a 8900 pound camper up and down the mts here in vermont for 6 hours got 15-18 mpg. thats on waist oil with acitone. maybe try changing your fuel filter and air filter. or add some fuel conditioner/cleaner. and yes these trucks are as slow as a old style horse and wagons going up hills and will probable blow black smoke too!
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Old 06-11-2009, 11:48 AM   #6 (permalink)
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I used to have an 85 f250 6.9 and I could pull 18 mpg, yes it is possible particularly depending on what your altidude is, I would say you should be able to hit 20+

The secret is let it get all of the coolest air that it can. Get rid of the cereal bowl in the air cleaner, upgrade the exhaust to a straight 4" from the manifolds back
It is worth it, being that with economy, comes horsies.
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Just finally lost the wheels off of my 85 F250 6.9L. (actually that credit goes to the semi that t-boned and got that done )
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Old 06-17-2009, 05:42 PM   #7 (permalink)
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i got 25 mpg in mine on a 9 hr trip at 75 mph average. truck had 3.54 gears, 35 inch tall tires and a stick(4 speed no overdrive).

iwould change all the filters and as stated open up the intake/exhaust.
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i'd rather be 6.9'n than cummin or strokin'!
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