Hello, I have a 1978 F150 with the 400ci engine. I have noticed an intermittent problem where it seems to be running just fine, then while driving down the road I notice some pretty bad engine hesitation, however very intermittent (may happen once on a trip for a few seconds then goes away). Pressing the accelerator does not help, feels like the engine is just falling on it's face. That has happened 3 times and then it just goes away, seems to drive just fine. Today it did the same thing but actually stalled out this time.
This truck is new to me and previously sat for at least a year or more. I have replaced the carb, plug, wires, cap, rotor, fuel filter (I also dumped the tank and it has since been refilled with fresh gas) prior to getting it on the road. New electric fuel pump as well. I think theres a good chance it might be running slightly retarded on timing but a stuck distributer has been a slight issue but not sure that could be it since it seems to run fine 99% of the time with this one intermittent problem that suddenly goes away.
Could this be an issue with the ECU/ computer? The fuel filter looks clear with no obvious blockages. I thought initially I was losing a cylinder but if that was the case I don't think it would have totally quit on me.
Also when it did die on me today it did not start right up immediately, needed to give it a bit of pedal and about 15 seconds on the starter. Thanks for any help!

 

That could be a issue with the ECU/ computer.
Some will say no way but if you hang out in the 80+ truck areas that have DSII & TFI IGN systems you will see they some times do what you are having,
Sometimes they skip a beat and keep running, other times they shut off just like you turned the key off. Some times they will start right up and other times you have to wait for the box to cool (just opening the hood could be enough or wait 10 mins?) and yet other just stop working / no spark.

The no spark are the easy ones when they just stop and will not restart as you can then check why it stopped.
How do you check when it is running again?
Because the box is cheap I would pick up a spare and keep it in the truck for when yours stops working and will not restart.
Now on the spare box get top of the line box and ask for it as 99% of the places will only look up the cheap ones and they just dont last.

The other thing you can do when you mount it and may do it to this one before the spare is used is to space it off the inner fender so air can get around the box on the bottom.
If you do that to yours now, get air to cool it, and it stops the issues you are having you know it is the box and will fail in time.

The other thing it could be is the pick up coil inside the dist.
Being you cant get the dist. to turn to adjust timing it will be hard to pull dist. to swap out. Guess you could swap just the pick up coil when in the motor but is is a lot harder doing it that way.
But I have seen the box cause more issues than the pick up coil in the dist.
Dave ----

 

Could be alot of things

Electrical for sure

Some run out of fuel for a second too

You already did what I would do a tune up with fuel filters, rotors, plugs, etc and go from their

Easy things first and then harder things later as you rule things out

You could also have a mechanic ride along, that can save throwing parts at something

Alot of mechanics won't even mess with these trucks anymore tho

I read someone had some junk in the tank that would intermittently block the fuel intake

Could also be a shorted out wire

Good luck, just got to keep ruling things out and trying things until it goes away

Keep the ambition up, you'll figure it out

 

Thank you both, new computer will be here tomorrow and hopefully that will take care of it!

 

I had a similar issue recently and I found my distributor was loose, spinning freely, and sliding up and down. I re-did timing and tightened it down and it hasn't happened since. So from my novice but optimistic advice, check to make sure that dizzy is secure.

 

I hope you went with the top of the line box and not a cheap one.

Also note todays "new parts" can be bad out of the box.
I have also seen it posted where someone bought the part new and it did not look right (new) but installed it anyway only to find out it was worst than what he had.
Someone put their old part back in the box and returned it for a refund.
No one checked it and put it back on the shelf till he got it.
Pull the new part out of the box to make sure it is the right part and not a used part.

Good luck and keep us posted
Dave ----

 

Thanks for the help! I did order the Motorcraft one, there were a few cheaper options but didn't want to take the chance on it...

 

Update, I installed a Motorcraft EEC and the issue returned after driving for a short while. Power seems to surge, like its "on again off again" and sometimes stalls, sometimes not. The issue then clears up and it drives fine again. I checked the distributer and its secure so I don't think its moving around. I can't shake the feeling it's electronic but more than just a cylinder or 2 not firing all of a sudden. I don't see any shorted wires and the way ot acts it doesn't "feel" fuel related if that makes sense?

 

You might try after it gets dark if you have any RFI being created with you spark plug wires. Look for electrical current arcing, use a piece of wood like a paint stir stick to gently move your plug wires to see if there is any arcing. I'll be doing this Friday night with my 77 F-150, as I suspect I got RFI that is messing with the Holley Sniper EFI ECU and creating an idle surge.

 

At least in my experience, surging's usually traced back to a fuel issue, like dirt intermittently floating into the carb jets, or a vacuum leak that gets worse as something warms up.

 

Sorry to hear the ECM did not fix the issue but now you have a spare, tuck it behind the seat for when you do need it.

I cant see it being a fuel issue as it would not just "shut off" when driving but run out of gas type of stop.
What was the old gas in the tank like and why the carb replace over a rebuild?
Have you replaced ALL the old rubber fuel line from tank to carb?
Again I dont think this is it but just asking.

What it could be is a bad pick up coil in the dist.
I believe I had one do the same thing on a non-Ford car.
It had a tach and driving down the road it would just shut off and tach to 0
When try to restart if the tach did not move it would not start.
I say believe as I had to move the car and dropped in a ready to run dist I had on the self.
I did change the pick up coil but never put the dist. back in the motor as it was running so why do that? LOL

Dose this happen hot or cold or both? Just so I know.
When it happens most (lets say hot) move the wires going to the dist. all the way back to the ECM on the fender well.
Move them a little and a lot and see if that dose anything.

If nothing with motor off look the wires over really really good where they go into the dist. for cracking and wire showing.
Pop the cap and do the same thing just inside the dist where the wires come in.
Then check them up to the coil. Also check the coil the best you can to see if there is any cracks in the unit.
I think someone had one that cracked on the bottom that was causing issues like yours.
There may also be a ground wire in there, black wire coming in from the outside, is it screwed down tight?

It is hard to fix something that is not broken all the time like yours
Good luck
Dave ----

 

Thanks again for the help. I took a look at the wires and made sure they were as clear as possible (added some wire spacers since they were missing), and replaced the coil this morning, but the issue returned again.
It doesn't just shut off so much as a severe drop in performance, and does not always stall out. I was trying to see what it was doing today and when it was acting up, rather than try to keep it running I let off the gas for a few seconds and it "seemed" to clear it up. I have an electric fuel pump that puts out about 12psi. Could it be picking up something in the tank then dropping it when I let off the gas? I wasn't sure if that would do it or not since it should (I imagine) always be pulling about the same amount of fuel from the tank no matter what?
There was old gas in it to begin with and I initially planned to rebuild the carb but the jets were not coming out without getting drilled out and I took the easier (and pricier) way out and bought a new one instead.
The only "old" fuel line would be the one that goes from the tank itself to the metal supply tube near the back of the truck. Everything else is new fuel lines.
I did have to get a bit forceful to get the distributer to turn, it was seized in the block when I got the truck. I used plenty of pen oil, torch, brass drift, anything to coax it to move. I got it to where I can turn it but it don't want to come out without some serious persuasion (was afraid I would break it off in the block so I let it be where it was, moved enough to set timeing at least close).
I thought it was only when it was hot but it did it this morning after only about 5 minutes of driving. Once I let off the gas a bit and let it calm down it kept running and problem went away again. It drf feels like its surging if I try to keep it running.
I rerouted the wires coming from the distributer as they seemed awful close to the water neck and down near the engine so there is enough clearance now i think. I will look at hose wires a bit closer for damage. Wires on the cap seem good, I thought they were a bit loose so I crimped them doen a bit before I put them back on the new coil.
Thank you again for the advice, it's actually been a little fun trying to troubleshoot it. A little lol.

 

WHAT 12 PSI!
I hope that is a miss type as you should not be over about 6 PSI but 4 to 5 PSI is what you want.
Why you running an electric pump and not the one mounted to the motor?
Is this electric pump wired with a relay or just power off the IGN switch?

Could there be something getting sucked up covering the tank pick up? Could be but you posted something else that I would fix ASAP.
That fix is the rubber hose from the tank sender to the metal line that runs down the frame to the motor.
It may look good on the out side but in could be falling apart inside and blocking the fuel flow.

When you checked the wires I meant wires to the pick up coil inside the dist. not the coil outside that the spark plug wire is in.
If you pop the cap off the dist. and remove the rotor you will see the wires I am talking of and a small coil.
That is what you need to check the wires going to it and the magnet of that small coil for cracks.

But now with this last bit of letting up on the throttle I would look at replacing that rubber hose at the tank.
Dave ----

 

I think you were dead on my friend! The issue finally got bad enough to get me stuck on the side of the road. I pulled the line from the tank off the input side of the electric pump (yes, 10-14 psi according to my guy at O'Reiley) and found the in line filter just before the pump occluded. Knocked out quite a bit of trash, then blew thru the line into the tank (took me almost blowing my eyeballs out to realize I need to open the gas cap). I could tell it felt blocked because after blowing into it for a good solid breath it blw clear and was much easier to blow thru. Need less to say I will be dropping the tank and cleaning it all out, plus replacing that sending unit that includes the pickup tube and mesh filter. Thanks agin for your help on this, I think I got it beat.
Oh, the mechanical pump was shot when I got it so I was told the electric pump was a better bet to replace it with. It is wired to the ignition switch, not a relay. I think it may also be mounted too high on the fire wall so there will be some mods when I pull the tank next week. Will also replace that last length of older hose from the fuel tank to the steel line that runs up front.

 

Well good to hear you found the issue.
When it blew clear it may have blown the filter sock off the end of the pick up tube.
I would look at just replacing the tank because you will never get it clean.

On that electric pump there is all kinds of wrong with it,
There is no way it has that much PSI or the carb float could not close off the fuel and would flood.
If the parts guy told you that about the PSI and also that a electric pump is better than the motor mounted one find a different parts guy.
Filter should not be before a pump as it makes the pump work harder and could also cause vapor locks easier.
Electric pumps do not pull fuel they push it and need to be mounted as close to the tank and below the fuel level keeping the pump wet all the time.
Also with out a relay the pump may not be getting a full 12 volts so it may not be putting out the full amount.

And the BIG ONE for safety it should go through a shut off so when the motor is not running the pump dose not run.
If a relay was used it would be easy to do along with a oil PSI switch.
You also need a way to fill the bowl before or during the cranking because if no fuel in the carb bowl and the pump will not run till the motor is running it will never start.

When you install that new tank replace that electric pump with a new one on the motor like the factory had and you should be good for a long time.
Dave ----