2005 Ford F-250 Engine Hesitation Starting around 25mph

Engine Mechanical problem
2005 Ford F250 V8 Two Wheel Drive Automatic

Hi, I'm asking this question for a friend about his truck so I dont know the year/mileage that well.

The truck will start just fine and it will idle just fine and it can be driven at low speeds (< 20mph) without trouble. But when you drive the truck at higher speeds like 45, 50, even 30s there is what feels like an occasional misfire.

If we drive at 50mph and listen carefully there are occasional misfires like about once every 10 seconds. It feels like a momentary loss of power and there is a small vibration coming through the chassis at the same time.

The engine has an uncertain feel to it.

I tried to get information on the vehicle's service history but I know very little. I was told the fuel pump was replaced with a used one. I think the fuel filter may also have been replaced. I can also say that the cooling system could have a very small leak. I put 14 pounds of pressure on it and it dropped to 6 in about an hour I think. I was not able to locate any external leaks in the cooling system when I checked the connections.


So I think it could be caused by faults in the ignition wiring but I would expect ignition problems to be consistent across different engine loads. This truck has an electrical harness with both the fuel injectors and spark plug wiring.

I think it's more likely a fuel related issue. I'm guessing that the engine isnt receiving the necessary amount of fuel for some reason. I think perhaps the filter on the fuel pump is restricting flow, or possibly the filter, or the pump itself is bad. I believe the injectors are all functioning because I've listened to them with a stethoscope and they click steadily.

The truck is not that old so I don't think there is a compression related problem although I plan to do compression tests later.

Right now what I want to know is what the most likely cause of the problem is and what are the best tests to identify it.

Should I do fuel pressure diagnostics?

P.S. The spark plugs were all in good shape except one that had a burn mark across the insulator. And changing the plugs made no difference in the engine's performance.

Thanks for your help and advice.
-Andrew