I have a late 99 f-350 DRW 4x4 6 spd. 285 tires w/70 lbs of air, Motor is stock with exception of cold air intake and straight pipe exhaust. Truck has 187000 miles on it. I run rotella synthetic and change fuel filter every oil change. I have owned the truck for a year now and the best mpg has been 14 empty. Pulling my 24' car hauler I get 10 mpg. I recently pulled a mastercraft 22' boat across Oregon and only got 7.5 mpg. When pulling almost anything I have to downshift on even the slightest inclines. This is the strange part. Last month I pulled my car trailer with a car inside and a half tank of fuel through a 98 mile long stretch of the Colorado mountains winding back and forth the entire way. When I got to the first town out of the mountains I filled up and hit the interstate. I noticed my truck was blowing a little black smoke in between shifts and had A LOT more power. I pulled the next upgrade at 70 mph without downshifting. This sudden increase in power only lasted for about 50 miles and then was gone. I have read hundreds of threads about poor mileage and power and have not encountered anyone that has had a similar problem. The truck seams to run great. No unusual noises. It does have a little turbo lag but other than that everything sounds normal. I have also changed all the rear-end bearings and front hub assembly's. I am at my wits end. ANY help would be greatly appreciated.

 

Do you have a boost gauge to monitor psi? Also check to make sure vac line is connected to wastegate

 

No boost gauge. vac line is connected. I dropped the tank yesterday to check for trash hoping the pick up screen was plugged up, but it was all clean. Pulled fuel filter and blew all the lines out and still no difference.

 

Any slop in turbo impeller shaft move in and out? Little side to side movement is ok

 

Also any check engine light codes?

 

No play on impeller shaft, No engine light

 

Have you checked air filter and fuel psi? Id also check for any boost leaks feel for air around pipes boots and intercooler

 

New donaldson air filter.Trying to find a shop that will test fuel pressure. I am still baffled why after driving through a curvy mountain road it came to life and had so much power for a short period.