Hello, I have a 1977 F150 4x4 automatic with 351M. Stock with no modifications whatsoever.
The problem I'm having is the truck starts up fine and runs for about 30 minutes then will just shut off like you've turned the key off. Then when I try to restart it is very slow to turn over and will not start. I decided to focus first on the slow cranking when hot as I thought those two conditions must not be related. For that I replaced the starter, starter solenoid, battery, checked all grounds and power wires. This didn't fix it. I thought perhaps an ignition problem could be causing it to turn over slow when hot, like timing too advanced. So I got it hot then tried to turn it over with the coil wire unplugged just to see if it would spin over faster. It did not. So I'm starting to wonder if the dying problem and the slow cranking could be related. As in a bad ignition switch. Where the contacts in the switch are bad. Another thing that happened yesterday that made me start thinking about the ignition switch is that I went to turn the truck off and it kept running with the key off. Anyone have any input on the switch on these truck failing or something that I've missed? I know there are lots of things that could be going on here like an ignition module or pickup in the distributor but I'm really trying to connect both conditions to one thing and what I come up with is the switch.

 

Sounds like the problem is heat-related and getting worse.

The "continuing to run" with the key off sounds like the ignition switch. I'd start by pulling it and looking (and smelling) for problems. It sounds fried.

 

Put A timing light on it , and see if its high in time, stretched chain can cause an issue of slow starting when warmed up

 

Thanks I will check these things.

 

Sounds a little like what my truck did when it was missing its engine-to-cab ground. Usually it would not stop cranking the starter after I released the key, but one time it did the engine run-on thing, and I had to stall it with the clutch.
The relay/solenoid needs to be grounded too, so you don't want anything like fresh paint under its mounting/mating surface.
If your battery negative cable goes to cab and there is no large cable to the engine block, then that would explain the slow cranking. (Good idear to try it with ignition disabled!)

Mine went/still goes to the engine, but there was no ground between the bellhousing and the firewall, so it always cranked strong.

I don't think the ignition switch can affect cranking speed because of the relay splitting the key switch, and starter motor, circuits. Either the relay is energized and hence the secondary circuit is closed, or it is not. A poorly grounded solenoid, engine assembly, and/or perhaps even the ignition switch, might affect cranking speed though. Easy to check and cheap to correct!

 

Do you happen to have a tachometer? If so, next time it dies watch the tack.
If the needle goes instantly to zero, then it's indeed an electrical/ignition issue. If it hovers at some rpm while the truck slows down, then it's a fuel issue.
Of course, it's rarely that simple, but if you don't have one maybe you can hook up a temporary tach to help diagnose.
Of course, if you're dead certain (no pun intended) that it's dying instantly, you're likely correct already in hunting down an electrical issue.

At this point, since you've already thrown some money at it, I would not just "check" the battery cables. They do go bad, in case you've never experienced that before. So I would replace them.
And go larger if you are still sporting stock sized units. It's only a slight increase in cost to go big.

Only a few things electrical can kill a running engine. Something with the ignition, or the negative battery cable. The positive cable could do it in theory, but if the alternator is functioning properly it will keep the system powered up even without a positive battery cable. As long as the ground cable is still good.

Good call. That's a main clue, but unfortunately once again it wasn't the quick-n-easy answer. I would have thought you nailed it until you said there was no change.

I'd have to call that a battery cable issue again. I can't see how an ignition switch could cause the trouble unless it was somehow fooling some other system. It can't effect the starter circuit like this directly however. If there's enough juice from the switch to close the starter relay/solenoid and crank the starter, then it will CRANK the starter.
If the starter is slow from lack of power, that can only be from the battery to the starter. Nowhere else. Of course that means both power and ground, but not ignition power.
If the relay closes, the starter should spin unless other things (like too much timing advance as you were checking) were at fault.

There are such things as bad relays though. I have learned to suspect even brand new starter relays because most of the stuff out there now is crap. Lots of owners have bought brand new only to have them fail within a week to a month. Or right out of the box after just one start! Crap I tell 'ya...

Well, this could also be the new relay staying engaged or at least closing the circuit to the Brown "I" wire on small post at the starter side of the relay.
This actually happens quite often unfortunately.

But you're not out of line to suspect the ignition switch either. And no matter that I say it can't cause the slow starting, I've seen stranger things and learned to never expect the obvious to always be the case. Maybe there is something in the switch causing the problem.
And after all these years of good service, hey, it might not hurt to replace it. I just hate to see you keep throwing parts at something, only to not have it fixed.

And while I'm thinking about it, though fuel won't keep it running without ignition, too much fuel dumping into the cylinders can cause the slow start (almost like it's jamming up) symptoms.
And can kill a running engine too. It's just that fuel usually lets itself be known by more sputtering rather than just out and out dying instantly.
Still, can't ignore the possibility.
Next time it cranks slow then, peek down the carb and see if it's flooding.
Hey, it's free at least!

Not bad to think of the module though. What color "grommet" (wire strain relief) is yours? Some had a start-retard function (which would not cause this) but some also had advance that could be added. I think the Yellow grommet (maybe?) for high-altitude running had that feature.
Hell, if it's retarded enough, it could act like it's advanced! But that's pretty extreme.

Good luck. Can't think of anything you really haven't touched on yet. But I've seen bad battery cables (and had them myself) cause a lot of grief for owners thinking no way it could be that.
They usually let themselves be known during starting, but if it's bad enough, a bad one could kill the engine too.

Where are your battery grounds by the way? One to the engine block of course (closer to the starter motor is more efficient) and one to the body? Then another one between the back of the engine and the body at the firewall?

Paul

 

Hah! meangreen beat me to it. I never remember to hit refresh when I've been sitting on a page for a couple of hours while doing other stuff.

Paul

 

I like the ground problem idea. I'm going to go back over all the grounds. I didn't see that one going from the back of the engine to the cab before. It looks pretty bad.

 

I replaced the ground wire that goes from the firewall to the bolt in the rear of the intake manifold. So far so good. The truck ran for over an hour without dying and restarted at normal cranking speed as well. Thank you guys for the help

 

That'd definitely do it! Probably fixed your gauges, too

 

Some things I don't like is the solenoid has 5/16 terminals and cables have 3/8 rings. Also, 4 gauge wire from battery to solenoid, 2 gauge to starter, so I'll make an upgraded one with proper ring terminals. All in a quest to lower that resistance!