1. Vehicle: 1994 F250 HD 4x2 with 5.8L and C6 transmission, 3.55 final drive ratio. 84,000 miles.

2. Problem: After operating for at least 1 hour, I get bucking and jerking. At cruising speeds, if I hold the throttle steady (no cruise control) the engine bucks a few times, then cuts out. After about an 8 to 10 MPH drop in speed it instantly almost violently accelerates back to full power. At idle, the engine will run "slightly" rough. The engine NEVER dies. If I stop and let the vehicle sit for more than 1/2 hour, the problem is gone. This problem is hard to diagnose, because it only happens on long trips, however with the colder weather, I am able to duplicate the problem with more regularity. I have been trying to solve this problem since April of 2004. I get no EEC codes that would point me in any direction. The problem does not change when switching fuel tanks.

3. What has been done so far: At first I though it was bad fuel, so I changed the fuel filter which was less than a year old at the time. I cleaned the throttle body, because it was coked up pretty bad, and since I had it out, I replaced the throttle position sensor. I changed the EGR valve position sensor because the shaft was rusted out. I also changed the Idle speed control valve because it was sticking. This stopped a "Tip In" hesitation I had for the last couple of years. Changed HEGO sensor because I thought it could be getting lazy with age. Problem disappeared for 4 months. Came back again on a trip to Arizona. With no tools to diagnose the problem, I changed the TFI CCD module because I know total failure could leave me stranded. I also changed the coil on the outside chance the TFI module was going bad due to a defective coil. Problem came back on the return trip. Replace the PIP sensor on the distributor figuring it was cutting out when hot. Problem came back two weeks later. Replaced the fuel pressure regulator, due to strong fuel smell in the vacuum line. Problem still there. Replace MAP sensor, when the problem occured again because when I pulled the manifold tube off, the engine idled smoother and did not puff black smoke out the exhaust (not normal). Problem disappeared, and ran fantastic with a slight increase in fuel economy for 5 weeks. But..... it's baaaaak! As best I can tell, something is telling the ECM to cut fuel injector pulse. But what? Is the ECM bad. The wiring to the engine, and to the injectors "looks" ok. No pinching or binding at all. Also, the engine coolant temperature sensor was replaced 2 years ago at the same time the temperature gauge sender was changed because it went bad.

Anybody encountered this problem? What could it be? Help!

 

Apparently the word " f-u-e-l-f-i-l-t-e-r " is purged by your post editor.

 

I wonder if this is a kind of oddball problem, like a bad vent or something. The tank vents run to the canister up front and then run up to an electrically controlled valve that lets them run to the manifold. If this system were partially blocked/clogged it might not manifest itself until the truck had been using fuel at a good clip for an extended period of time, and then the reduced speed would slow fuel consumption enough that it could catch up enough to run fine again. Not saying this is the case, it just sounds like the type of situation you should be looking for. It would think that if it were the drivers for the fuel injectors it would show up more often than it does, and if it were a sensor it should throw a code. Good luck with it.