I have a 99 F-450 with 7.3l diesal engine. We driving home on the freeway running around 60 and any incline in the road would make it hard to pull the hill even though the gas petal was all the way down. The speedometer would drop too. I had to pull over 4 times. The 4th time I pulled over, blueish white smoke came out from underneath the truck, thats when i turned it off. I turned it back on and the engine sounded like it was knocking. I turned it back off immediately. I had to get it towed home after that, I didnt want to mess anything else up. I had just changed the oil, oil filter, transmission oil, glow plugs, and the glow plugs relay. I dont know what it could be, I need this to be fixed as soon as possible as i use the truck to pull my takeuchi trackoader. Thanks for the help.