My 03 F650 had a 3126 and a 6 speed.
While it was a total dog, you could pull a freight train.
Most of the truck was gearing. Get the gearing right and you can do whatever you want, depending on your needs.
Power of the 3126 comes in at 2200rpm which is just short of the redline at 2500rpm. Check the valve cover tag for the engine number and call the CAT dealer and ask them what the advertised power for that model is. Chances are it hasn't been tampered with.
The fuel system is basically the same as a Ford/International 7.3l V8. Uses HEUI injection controlled by various sensors and electrical system. The injectors are hydraulicly powered and electrically activated. It uses a high pressure oil pump to fire the injector. To keep it running smoothly and efficiently, religious oil changes are a MUST. That being said, I found my truck started easier and the power was smoother when I used synthetic oil. If you want more power, contact a diesel shop proficient in the older 7.3l powerstroke injectors, and they can build your injectors to spec.
The turbo on the 3126 is also smaller than it should be. CAT designed these to last forever in extreme conditions, and the turbo was very conservative in design. Any upgrade to the turbo will be expensive, but not extremely so. My truck had 400k+ miles on the original housings and I rebuilt it only once to prevent problems @ 250k miles. My max boost on several gauges was only 27psi in stock form. Best thing I ever did was to take the turbo apart and hand polish the inside of the compressor housing, aluminum usually on the later 3126. I got no increase in boost, but the nasty needle vibration I saw in all the gauges I used went away, and it seemed to whistle louder. That told me that the air was now moving smoother into the turbo.
I also addressed my fuel supply system. Most 3126's use a mechanical fuel pump routed through 2 filters. Each filter needs to be primed when replaced and the whole system evacuated of air in order to restart properly. If you are lucky enough to have a priming pump on your engine, good for you, as mine did not have one and my starts cold be very hard after filter changes or sitting for longer periods of time. I used an inline diesel fuel pump after the splitters in my fuel lines, my truck had dual tanks, that generated about 5psi or more. All I needed to do was wire up a switch on the dash, and I was good to go.
This solves a coupe of issues with older mechanical fuel pumps, loss of prime and declining fuel pressure at higher RPM. My truck started better in the winter, and ran better at higher RPM because the supply of fuel was better.
After that......
I could go on and on about little things that could be done to improve it, but the 3126 wont be winning any races anytime soon. So long as you know that going into it, and are prepared to be RELIGIOUS in your maintenance, it can be a great reliable engine for 400k+ miles like mine.
