I have a 1994 F350 7.5L Auto, when I tow with my gooseneck, somewhere around the 2-3 hour mark the truck is acting like the ECU is rapidly turning on and off till it finaly dies. Then it sits for about 5 minutes and I'm able to restart it and drive it about 20 minutes and it will do it again. I have swapped the ECU and fuel pump relay and checked the connections, and looked through the fuse box and it looks normal. I had the ECU rebuilt due to leaking caps and it still does it. I even tried another ECU I hve from a different truck and still does it. I believe it to be a break in a wire or something getting hot in wiring. Anyone else ever dealt with something like this. It never breaks down where I can work on it, and the one time it did, by the time I put a test light on it it started back up almost right away.

 

What are the symptoms you’re getting? What makes you think the PCM is cycling on and off repeatedly?

 

Based on your symptoms, I'm guessing your PIP sensor in the distributor is failing.

 

this Or TFI module is on its last leg. Have you pulled codes?

 

+1 for the above
Check codes
You'll have a whole rash of codes if the processor loses power
A 211 or 212 usually means TFI module, possible stator failure, or spout wire grounded

 

I didn't explain very well. I tow with O/D turned off. When it happens the O/D turns back on and the truck bucks like it's loosing spark or fuel, and the fuel pump turns off and on when it happens. This happens rapidly for about 20 seconds before the truck completely dies. Truck has been parked for a while and has dead battery so I would probaby have to drive it to re create the problem to pull codes, but I will give it a shot.

 

I would run wires from the ignition coil to the cab. Using a 12v test light hooked up to each of the wires, when the engine is running, the light will flicker. The lower the RPM, the slower the flicker. If/when the engine starts to act up, if the light shuts off or is no longer flickering, you know there is an ignition issue. If the flickering still is occurring as the engine is failing, then it's probably fuel related. It's not a certain doing this test, but it certainly is easy to do and does tell you a lot.

You don't mention if this problem happens with both fuel pumps or just one. I see fuel pumps wear out around 150k but that really depends on a lot of things. Basically the electrical brushes wear out and that is why the pumps stop working. Sometimes they are intermittent before they just stop completely.

If the engine does stop. Don't do anything. Check if the fuel rail has pressure in it. It's just a schrader valve and if you press the valve and fuel shoots out, you probably have pressure. If it dribbles out, you don't.

There are only a few tests you can do without tools so I would gather tools and put them in the truck. When it fails, don't do anything. Gather information before you have to tinker too much.

Good luck

 

Ignition switches are known to fail from age/wear and cause the ECU to cut out. Excessive amp draw through the switch causes the contacts to heat up and melt the plastic housing. If you have extra lights or audio equipment hooked up to your ignition switch, it can cause it to heat up and fail randomly. As it cools' it makes contact again and the truck will start again.

 

If the PCM is losing KAM(keep alive power) there will be a code set for it, also repeatedly losing an ignition pulse from the PIP or TFI will also set codes. Your symptoms are classic for a failing TFI module, but you can't rule out wiring issues on any of these old trucks, it is what puts most of them in the grave.

 

I will check the ignition switch. My does not have the TFI module. I am at the same thought about running wires into the cab to see where it fails. The main problem is it almost always does it after the 2 hour mark, and worried the next time it does it it won't restart.

 

Care to explain?

 

Could be this part of your ignition switch, under dash, at/on steering wheel.

 

All 8th and 9th gen gas ford trucks have PIP and TFI modules. That was their Ignition system solution from the mid 80s to the late 90s.

 

I picked up a new ignition switch. I'll install it once it stops raining. Didn't know the TIF was remote mounted on these, it's my first 460 truck. But if it was failing would it cause the the ecu to reset? The only reason I say that is it only happens when I am towing with O/D off, and the first thing I notice before the chugging and stumbling gets bad it the O/D kicks back on. Both my fuel pumps were replaced in the last 2 years and it does it no matter what tank is selected. I went for a 2 hour drive today with out the trailer, hitch isn't installed right now, I just put on a new bed. I pulled codes and it was only an EGR code (it's not connected, tube broke) and a converter clutch slipping code (not happy about that either).

 

I’m also jumping on the computer not losing power bandwagon. What part of the KOEOtest did you get those codes?

I had a bad connection at the computer. It would lose power and relearn the idle settings every time I would restart the truck after sitting for more than a few hours. The KAM memory code is tipped me off to what was happening.