1973 F100 390 stock

I feel like I'm going down a rabbit hole trying to chase problems so I need to step back and try to get more clarity here. Open to any suggestions.

Right now the main problem is stalling. Four times in the last two weeks. First time it was one the freeway doing 60. It stumbled, slowed down, I shifted into neutral and gave it some gas and it came back. A few days later it happened when I turned a corner in the neighborhood, again stumbled but didn't die. Both times the truck had been running for half an hour or so, so it was completely warmed up. Then it stalled a few days later, at freeway speeds, and did die. I pulled over, wouldn't start right away, but did after 3-4 minutes, and got me home without a problem (2-3 miles away). Then once more in the hood just kind of idling down the road it stumbled but came back without stalling.

The time it actually died on me I had only been driving about 10 minutes.

I had this problem a little last summer but only on really hot days, and only in traffic, so I thought it must be related to temperature. Over the winter I did a few things. I had the radiator serviced, acid boiled or whatever, didn't really do anything. I also installed an additional temperature gauge. I've got the stock gauge plugged into the thermostat housing and the new gauge in the manifold. But read fine all the time. I've got a 180° thermostat and it runs at 180 all the time. It will start to creep up when in gear and in traffic but it takes a while, and putting it in neutral gets the fan moving fast enough to cool it right back down. When I say creep up a little I'm only talking about going from holding steady at 180 to maybe 190 or 195, never gets hot hot.

I rebuilt the carburetor and I replaced the fuel tank a couple of months ago. Immediately it was like a new truck, running much smoother at all rmps. The old tank was full of rust and I had gotten used to changing the fuel filter every few weeks. But it stays clean now.

Plugs look fine, and all the plug wires are sparking.

I have a vacuum leak somewhere that I can't find but I've had that for a year and it's run ok in the past, though I have to keep the fuel mixture screws out about 6-7 turns, and I assume it's like that to compensate for the vacuum leak.

I don't really know where to go next. Fuel pump maybe? It's not an all the time problem but maybe things get squirrelly when it's hot? Maybe the coil is getting hot? I was planning on upgrading the fan, but...eh. Thought maybe I should replace plugs and wires just because, but...eh. I really don't know what to do so I'm trying to take a step back and see the bigger picture. Again, any ideas are welcome.

Thanks

 

I had a similar problem and it was caused by old rubber fuel line (maybe OE?). Anyhow, when wet with fuel+alcohol, the rubber got so soft the pump wold suck it shut. After a while it would open up and the truck would run again for a while. A few feet of rubber hose to replace all the old hose fixed it.

It could also be a fuel vent issue. Loosen or remove the cap to test.

Could be other things, but I'd start with the above.

 

Sounds like fuel flow problems to me. If it was the coil, It wouldn't just hesitate and continue running. Maybe something in the tank blocking the pickup tube?

 

I just replaced the tank and the sending unit. There should be nothing in the tank but clean gas. However, and please tell me this isn't the problem, when the new sending unit came it was a 5/16" whereas the original was 3/8". The gas line from the fuel pump to the carb is 5/16" so it's not like the sending unit size is causing a restriction. Plus it's delivering gas great at least for the first 10-15 minutes. I replaced the rubber fuel line at the sending unit when I changed it. I think the only other rubber section is right before the switch (dual tanks), which I replaced last year.

 

I would try this - venting issue is a good possibility and easy to test.

 

If it isn't the venting, look into a blockage possibly at the dual tank switch.

 

Do you have the proper gas cap for the tank? Is the tank vent clear? If there isn't a tank vent you need a vented cap.

I don't see where you say you replaced the fuel filter(s). A clean filter never hurt.

 

There is usually another rubber section on the suction side of the fuel pump. The suction side of the pump is very sensitive to any form of blockage/collapsed lines ect.

 

I've replaced the fuel filter several times. I have spares. B&W, yes there is another section before the pump, just went out an looked at it this morning. The other sections I replaced were in terrible condition so I'm sure this one is too. I'll replace that this afternoon if the rain lets up.

 

*UPDATE*

I finally got around to replacing the last section of rubber fuel line, just before the fuel pump, this morning. Took it out for a spin, barely even warmed up, and it died on me about a mile from the house. I thought maybe it was the air in the line from replacing that fuel line which just needed to burp itself out, but after getting it started again it died twice more on the way home.

So now every piece of rubber fuel line is new. None leaking. New tank and sending unit. Not leaking. Less than 100 miles on rebuilt carb. Also, I bypassed the tank switch. I looped the aux tank back on itself through the switch and the main tank runs straight to the fuel pump.

When it dies now it acts fuel starved. It'll turn and turn but only start after I've pumped the gas pedal 4-5 times. The truck runs great in the driveway, BTW. I can let it idle in gear for 15-20 minutes, or rev it up and down, or hold it at high RPMs, and it sounds great through all that, as long as it stays in the driveway.

Do fuel pumps have intermittent, mainly affected by movement, problems? Maybe I should re-rebuild the carb? That rebuild was done before installing the new tank, so I suppose there could be some debris there. I just feels like doing that would just be an exercise in futility at this point, and if futility were a muscle I'd look like a body builder.

 

Please see above comments. I had the same issue, but not as bad. My engine would seriously bog down on heavy demand, but not stall out. Being the genius that I am (NOT), I started with the carb first and worked my way to the fuel pump. I installed a nice Edelbrock unit that only puts out 5 psi so I wouldn't have to regulate the pressure to the Holley. That did the trick. Good luck to you.

 

It's certainly not a fuel filter problem. I was going through those like a crackhead goes through pipes with the old tank, but since I've replaced the tank I've checked the filter several times and its always completely clear. As to an earlier comment regarding proper ventilation, I have a vented cap, even though its an EEC tank which shouldn't need one. The cap came with the truck. Purge cannister isnt hooked up so I assume PO put on a vented cap when he disconnected it. In any case theres more than adequate ventilation. The fuel pump appears to be the original. Even though it seems fine in the driveway, it is 42 years old. Replacing it seems to be a good idea. It also seems pretty easy to do. Anything tricky with reaplcing the pump that I should be aware of?

 

I've got to do the same on my 77 with 400, pretty much a simple job, light silicone to keep gasket in place, 2 bolts, make sure you wear goggles/glasses when underneath, gas in the eyes sucks!

 

If you have power steering the bracket makes getting a wrench on the 2 fuel pump bolts a bug bear job.

 

Hope the fuel pump fixes it. Good next step. The 5/16" pickup may not be helping. I just changed to a 38 gallon tank. I used a 5/16" pickup since that is what was available. The truck did not like it. Cruise on the highway, get it warm then hit it and she stumbled.

I rigged a temporary fuel pressure gauge "T'ed" in front of the carb and taped to the outside of the windshiled. At highway speeds, fuel pressure was dropping from normal ~8 psi down to 2 or 3. Fix was a small electric fuel pump as booster added on the frame rail just in front of the rear axle.

I had really wanted to do that for a while as I had borderline vapor lock in really hot weather with the 3/8" pickup.