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With it being diesel though I am clueless as to what is consider too high of mileage to buy. If I get to the dealer and find out it was maintained would this truck be good for another 100k?
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Depends on proper maintenance, warm-up, and shut down procedures - and luck. /ubbthreads/images/graemlins/shocked.gif
With proper maintenance and proper driving habits, the engine should last at least 300,000 miles, and probably 400,000 miles. For warm up, don't allow a cold engine to idle with no load on the engine. Crank the engine, and as soon as you have oil pressure put it in gear and move out. Baby it until the coolant gauge moves off the peg, then be nice until the engine gets up to normal operating temp. For shut down, never turn off the engine until the exhaust gas temp (EGT) falls to 300º or less - that takes anywhere from a few seconds to a few minutes, depending on how hot the engine was when you stopped.
I bought mine new, maintained and drove it right, and it now has 152,000 mostly-towing miles on it. I can't afford to replace it, so I expect it to last until they throw dirt in my face or 400,000 miles, whichever comes first. /ubbthreads/images/graemlins/cool.gif
The stock auto tranny in "your" pickup probably won't last much longer, unless it's already been replaced. So plan on a trip to Arkansas and have a BTS installed for about $4,000. The BTS will then outlast the engine. (I had a BTS installed with about 112,000 miles on mine, and I expect to have no tranny problems "forever".)
A rebuilt Ford automatic tranny and torque converter with a 2-year warranty costs less than $2,000 for the parts, but it will probably cost you about $2,400 or a bit more out the door. If you want it to last more than 100,000 miles, then never let it get over about 225º, use synthetic ATF, and change the ATF every 30,000 miles using the procedures in the '99-up FAQ.
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if it makes a big difference, the Toy Hauler is 13000lbs max weight and will be towed about 1-2 times a month.
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Your GCWR is 20,000 pounds, so when towing the toy hauler you'll probably be slightly over that limit. But the diesel engine will certainly handle that load a whole lot better than any gasser engine.
For that load, you need both a tranny temp gauge and a pyrometer. The stock tranny temp gauge is almost useless, so get a "real" gauge with clear markings around the 220º to 230º temp area.
Install the sender (thermocouple) for the pyrometer before the turbo, then use 1,250º as the red line. When climbing a grade with the trailer, drive by the pyrometer and don't let it go over 1,250.
Install the sender for the tranny temp gauge in the pressure port on the side of the tranny. Normal tranny temp is anything less than 200º. Slow speed and high torque can cause the tranny temp to zoom over 200, so watch the gauge and don't allow more than about 225 to 230. Backing the trailer up a grade is the most likely cause for overheating. Also, towing up a steep grade at less than 40 MPH. Even stuck in a traffic jam and crawling along at 5 MPH can cause high tranny temp when towing a heavy trailer.