2003 f350 v10 sluggish at 40mph under load

Are you talking about a go-cart, or a motorcycle, or a tractor-trailer 18 wheeler? Then OF COURSE, you must IDENTIFY the MAKE, the MODEL, and the YEAR of whatever vehicle you are asking about, if you want any REAL help!

These trucks have a history of epg valve problems . Sure sign is thick black smoke , Also you are suppose to drain water from filter case with a 6mm Allen wrench . Its on frame rail under drive seat . That filter is often over looked to . If the water hasn't been drained every month it destroys the injectors . My bet is the fuel filters . One filter underneath an one on top of motor .

sounds like you have a misfire problem

may be a vacuum hose is rotted out that leads to a device only activated over that limit; or a vacuum device that just fails to maintain good vacuum when it hits that level of demand.

Hello

The engine and the automatic transmission in this vehicle are computer controlled and in most cases when a fault occurs a fault code is stored in the memory of the computer control module. There are exception to this, such as the Mass Airflow Meter and fuel pressure problems. What must be done is to have all the basic testing done such as a scan of the system for fault codes as well as a check with a live data scanner tool for engine functions that are not within range of normal, a trained technician is required to know what is correct and what isn't, then based on careful diagnostics done from a factory repair manual the correct part is replaced or the wiring repaired or the computer replaced (known as the PCM), which is very rarely the problem. Other basic checks must be done as well such as mechanical problems with the engine as well as engine state of tune and mileage on the engine. The transmission can have mechanical issues as well such as no gear engagement at all or a failed pump or other pressure related problems.

my 01 did that and it was the plug wires it bucked real bad
next time it happened it was just sluggish up hills and light would come on. it was the fuel filter it had 255,000 miles on it

1 Ignition system not operating properly
2 Dirtyor clogged fuel injector(s).
3 Low fuel pressure. Check for proper operation of the fuel pump and for restrictions in the fuel filter and lines.
4 Fuel injectionor engine control systems malfunction.
5 Vacuum leak at the intake manifold or vacuum hoses.
6 Restricted air filter.
7 Fuel pump or pressure regulator defective.

I would check for a broken springs, very common. As for the missing, is there a check engine light on?

The fuel pump is on top of gas tank inside, you have to lower the tank, unless is mounted on gas tank side, follow fuel lines, thy are two, one main and one return, the fuel filter is on the main under driver

Check the fuel pressure while the problem occures....needs to hold 35psi or so under load. You might also have ignition breakdown. Is the Check eng light on? You have coil on plug right? you really need a scanner to id the misfiring cylinder the try moving coils around to see if the problem move with it.