I just bought a 1999 F250 V10 from a relative for pretty cheap. The drive home was about 2 hours and the truck performed fine until I got into town. I had to merge onto the highway and found that the further I pressed the gas pedal, the slower the vehicle went. When I pressed it all the way down, I was pretty sure the truck was just going to roll to a stop. I backed off and gave it just a little gas and it slowly accelerated up to speed (70 mph). Any idea what could cause this?
No odd engine sounds. The truck just simply slows down like pressing the gas has the opposite effect. The more you press, the less power there is. There is a "sweet spot" you can hit with the pedal just barely down that will get acceleration, but it's painfully slow to get up to speed.
Also, with a code reader capable of monitoring the datastream, you should be able to monitor the TPS position data in real time to see if it seems to be working correctly or not.
The TPS is basically just a potentiometer or rheostat similar to the volume control on older radios, and can develop a wear groove or even become cracked, in which case the unit could work on one side of the crack but not the other. And it could be intermittent.
On the other hand, while I am normally dead set against just throwing parts at an issue without troubleshooting it first, the TPS is easy to replace and is a "relatively" low cost part as I recall. It is located on the side of the throttle body and is actuated by the throttle plate shaft. So simply replacing it to see if it cures the problem could be another option in this case.
I took the truck out today and everything seemed fine. The engine throttled properly when I revved it up and there were no power issues driving it down the street.
I had it serviced since I didn't know when that was last done and told them to take a look at the fuel filter. The service tech said that it was "blowing chunks of dirt" when they pulled it off and was in pretty bad shape. I'm thinking that could have been the problem. Would such behavior be consistent with a horribly clogged fuel filter?
Sure, a fuel pressure problem caused by a clogged filter or bad pump can make a "loss of power when romping on the gas" symptom. The fuel flow can't keep up with the demand but, on my truck, that generally was accompanied by some sputtering and chugging and sometimes even a "transmission slipping" code. But when I had that issue, it was intermittent much like you said. It would generally start out ok then develop issues after driving for awhile. Then I would stop on the side of the road and let it set for a few minutes and it would run ok for awhile longer. I presumed that with the engine off, the fuel would backwash some of the restriction out of the filter and pump socks.
In my case though, it wasn't quite like you described where it was like it was smooth and "the truck just slows down like pressing on the gas has the opposite effect".
If you suspect fuel supply issues now, the thing to do is check the fuel pressure at the injector rail both idling and under load. The normal pressure should be 30 to 40 psi and rise slightly under load. It should not drop off under load. A fuel pressure gauge is needed to do this. I use an Actron kit, it includes a gauge with a long hose and some adapters for different connections, including one for the schrader valve on the injector rail.