Ok i thought i could be smart and diagnose the truck myself from all i have read on here of people with similar problems. It's a 1991 f150 with the 4.9 and 5 spd m5r2. Here's the problem, after the truck gets warm, it will die, right around 20 mins of run time. It will take longer when it is cooler out to die, and every time it will not restart for 20-60 mins. Sooo i figure heat related electrical issue, and when it dies i have no power to fuel pumps, fuel pump relay, eec relay. I tested the tank selector switch, iac, egr, a lot of things i didnt think were the problem but wanted to rule out.

This truck has been breaking down on me every day for 2 weeks and i'm tired of getting stuck at lights and having to push my truck to the shoulder. Most of the time it will die under idle, but it has died under load. Today i finally went and got a PCM because i figured that HAD to be (wishful thinking i know) the problem. The truck ran fine for 20 mins with new (used) pcm and died, and i'm back to square one. I am beyond frustrated with it and am asking for any help, and it is very much appreciated. I opened the PCM that came with my truck and it looked pretty good, except for 2 of the blue capacitors looked a little burned/corroded. Also i should mention sometimes there is an rpm spike or drop right before the truck dies, but most of the time it cuts out like i turned the key off.

The junkyard PCM is in my truck now, i'll take it out and see how it compares to my original one. Again thank you all for the help, and sorry for the wall of text, but i know you folks love details

 

Damn i keep forgetting those details, i should mention i have checked for codes more than a few times, Key on Engine Running and Key on Engine off. The only code was a high idle code, and this truck has had a slightly high idle(800-900) since i bought it. Also checked fuel pressure regulator, that and some of the other components were tested before i realized i couldn't hear the fuel pump run. That's when i started testing with a multimeter and found no power at the relays

 

I had a similar issue with my old truck try putting a new pick up in the distributor or throw a new distributor in it. My old truck was 97 f350 460 but almost same identical problem .

 

Yeah but why would the EEC relay be cutting out if it was a distributor/PIP problem?

I can see the fuel pump relay cutting out, it's normal for the EEC to cut out the fuel pumps when the engine ain't running, but as far as I know the EEC relay provides power to the EEC so it should remain powered as long as the key is on. Somebody correct me if I'm wrong about that. I would try swapping the relay out. If that don't help any then we can try another test I have in mind.

OP, yes I'd take a look inside your replacement EEC as well, they ain't exactly getting any newer.

 

I was thinking about it last night and it makes no sense for the EEC relay to not have power, ill double check that today after work.
Could the pip fail to give proper information and still provide spark?
Thanks for the replies and i will open the junkyard pcm today as well. Is the PIP just tested for +/- 12v?

 

The EEC Relay is controled by the Ignition Switch and has nothing to do with the Computer other than supply power to it.

The PIP will fail in a number of way. Most the time when it is bad it will work OK untill it get warm and then kill the engine like you turned off the key.
But it will not stop the EEC Relay from getting power. The power for the PIP sensor also comes from the Ignition switch.

I use a 9VDC battery, a distributor plug and a LED to test the PIP sensor.





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I have changed the fuel pump relay as well as the EEC relay, and i was wrong about the EEC not having power, sorry about that haha
When the truck dies the eec relay is pretty hot though.
Thank you subford for that info, i've just tested the ICM through with this link's help
Part 3 -Ford Ignition Module Test. Distributor Mounted Module (ICM).
The ICM has checked out so far, and that guy has a very similar homemade test light method, but testing it through the ICM
So i'm going to radioshack to get parts for the test light, and then i'll check the PIP signal

 

So after letting the truck idle for 20 mins and die for probably the 15th time ,(Can't test anything if it will fire up) I checked the PIP signal coming into the ICM and it was not present. The junkyard PCM looks perfect on the inside, and its a bit cleaner than mine so i will keep that one in the truck. Tonight and hopefully only tomorrow i'll throw a new distributor in it and pray there are no more gremlins

 

So thanks Crewcabber! for being the first one to point me toward the PIP, i guess we did have identical symptoms

 

You need to check for the PIP with the computer unplugged as the PCM can kill the PIP.
If the distributor was unplugged from everything on the tester you made and you got no PIP then yes the stator in the distributor is bad.

 

Those capacitors can be changed on the PCM using old computers boards for parts as long as the voltage rating is equal or better. Also can be ordered from Mouser Electronics. Mine caused the fuel pump to stay on and it would never stop. I changed the pump but to no avail and that's when I came across a post that pointed me the the computer. I changed all the caps and fixed a corroded track. If you can solder it should be no problem. Then you will have a spare.

 

Subford- Every time i have checked something on the ICM plug i have checked it both plugged into the dizzy and unplugged, so i have that covered. I'm glad to have someone definitively back me up on my diagnosis, and i know you are very knowledgeable of these trucks, i sincerely appreciate the help, thank you sir! May i ask how old you are? I'm only 21 but i have a feeling you've had a few more years of breakdowns under your belt.
Byron- thanks for the tip, though my original PCM was working fine, i'm only keeping the junkyard one because it is a lot cleaner. I'll save my original as a backup

 

Ok so now i'm on to timing the new distributor, I can set the time at 10 degrees BTDC and then the mark will jump all over the place, with the SPOUT disconnected, is this the computer trying to correct time? Or a faulty distributor? It is a new Richporter distributor, not reman Richporter Technology FD10 - Distributor | O'Reilly Auto Parts
I can get the truck to run, but right now it sounds pretty diesely and the timing mark is way off where i had it set

 

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Subford i guess i only have until 2064 to become a guru like you haha. Anyways the truck sounds to me like it is running on 5 cylinders, would the TFI do this or the distributor is bad? Because i can throw my the old TFI on it and try it, i guess thats what i'll try now. Otherwise i figure i can diesel it over to ford and i know they will time it right.