I should have mentioned that the number is off the block, not the tag, sorry. I had a guy look at it and he said its an older type cummins, probably a 290 (it has three rectangular, flat valve covers). We just bought this old truck, trying to get it working right now. the engine seems to be running perfect till under load, then cant keep rpms up. had a couple different guys try it, seems like its either fuel pressure or turbo. So we changed turbo, no difference, then we started at the tank and checked all fuel lines and fittings, no obstructions. then we changed fuel filter, still no change. Throttle is advancing all the way. Fuel pump is fairly new, i guess it still could be the problem though, any suggestions on what to check next ? bear in mind that im not a mechanic, even the most obvious suggestions appreciated ( I do have a diesel mechanic coming on friday to have a look though) the engine is running smooth as can be until you go up hill, then just got no pluck. Not blowing any smoke, no hesitation, fires right up.