I've read dozens (hundreds?) of threads on intermittent start problems all to no avail.
Symptoms/description:
1. Starts most of the time. Typically doesn't start 1 out 20 times. I can go 3 weeks with it starting every day, then fail starting every day for a while. After starting, engine runs strong with no hesitation.
2. When it doesn't start, it cranks but does not fire up. (batt & starter good)
3. Check engine light comes on normally when tuning key to RUN. When turning to START, the light is supposed to go out, but it does not in this no-start condition. Note that I do not have any error codes. If I did, the check engine light would stay on all the time & I wouldn't have this check engine light not extinguishing at START clue.
4. After waiting a few minutes it starts normally. Rarely it takes 10 or more minutes.
5. When it doesn't start, if I leave the key on RUN for a few minutes, I'll here a slight buzz from near (maybe in?) the relay box, driver side eng compt, I think. Sounds like a relay.
6. I always hear the fuel pumps run when in START, whether it starts or not.
7. Once it starts, it never stalls while running.
8. If engine is warm (after running) it always starts.
9. In the morning, it almost always starts, unless it's unusually cold (below 50-degrees). When cold, it occasionally doesn't start.
10. Most of the time it doesn't start is when it's been sitting in a parking lot at work all day. (I know the last three don't make sense.)

What I've done, with no change to the symptoms:
1. Replaced both the PCM and fuel pump relays.
2. Replaced computer. No change.
3. Replaced all main grounds with welded ends on cables.
4. Checked voltage across the terminals in the relay box for PCM and fuel pump relays. Have correct coil and switch voltages for both relays in both START and RUN positions.
5. Verified pressure in fuel rail when not starting. (Been awhile since I've done this...will re-confirm.)
6. No spark when in no-start condition. When the check engine light goes out properly in START, the spark comes back. (Been awhile since I've done this one as well.)
7. Always starts within 15 minutes of running for a while. I think this means it's not the pickup in the distributor. Would expect pickup problem if hot engine.
8. Checked for proper continuity on ignition switch terminals.
9. Left it with a pro technician for 10 weeks to no avail. He thinks it might have something to do with a connector somewhere. If it was a connector, it would fail more often than or more erratically than what I'm seeing.

Recap:
1. Turn key to RUN
2. Fuel pump runs
3. Turn key START
4. If check engine light goes out while in START, then it always starts.
3. If check engine light does not go out, it cranks, but does not fire up:
a. Leave it in start until I hear a slight buzz (half the intensity of a fuel pump sound...about same as a relay.)
b. After buzz, the check eng light will always go out when turning to START.

Questions:
1. What is keeping the check engine light from going out in the START position?
2. What is the buzz sound? I'm getting voltage at the computer and fuel pump relays. Relays are new & tested & switched around with relays on working circuits. Fuel pump is running, so the computer and pump relays must be switching correctly.
3. I hear the fuel pump, then I hear the buzz. The buzz cannot be the computer or fuel pump relay or the fuel pump would not turn on before hearing the buzz.

Finally, a few updates:
Measured fuel rail pressure. Rear tank: 35-psi in run, 42-psi after starting. Front tank: 20-psi in run, 30-psi after starting. Pressure from front tank is low, but today it started and ran strong from both tanks. YESTERDAY MORNING IT WOULD NOT START (CRANKED FINE, BUT WOULD NOT TURN OVER), SO I GAVE UP AND WALKED AWAY AFTER 5 MINUTES. TODAY, IT STARTED IMMEDIATELY.

Murphy is involved here. Every time I'm ready to check voltage & fuel pressure, it works perfectly. Only when I'm in a rush and not able to test does it not seem to work.

I still think it has something to do with the check engine light not going out when turning the key from the run position to the start position. EVERY time the light goes out, it starts. EVERY time the light does not go out, it does not start. What might cause this behavior?

Log from 5 days ago:
7:45. No start for 10 minutes.
9:45 Cranked right up.
3:30 No start for 10 minutes
3:40 Stalled while driving. This is very rare. typically it just doesn't start. Started after 3 minutes.
3:50 Stalled while driving again. Started after 2 minutes

Once pressurized, the fuel rail maintains pressure okay.

Of course... any clues you might be able to provide would be greatly appreciated.

 

just a thought, pull the cap off the distributor and see if its clean in there. the contacts on mine was so corroded that it quit working.

 

I'm guessing one of the INFAMOUS big 3 1. ignition coil 2. ignition module 3. the pickup inside the distributor.

Forget trying to test those components. If you test them while they are in a live state, they will probably test good even though they may be causing your intermittent problem. Just have to replace trial and error.

The 1996 also has a crankshaft position sensor, which i don't understand with it having a distributor. That may be something to keep in mind that could be causing your problem.

 

You stated "Replaced all main grounds with welded ends on cables." Does that include grounds G100 and G100? G101 is the main PCM ground located near the battery on the passenger side fender. G100 is located on the radiator support near the driver side headlight.

The issue being tied to the Start lamp going off or staying on leads one to think the PCM is at fault, but you already replaced it. Poor grounding will cause all sorts of weird symptoms.

 

In this vintage truck that sensor is referred to as a "Misfire Sensor". It is not used as an active component of the ignition circuit like a distributor-less ignition equipped vehicle does.

 

Sounds like bad starter cable wiring near the starter. The shorting wires in the starter cable wiring are for the HO2 sensor.
What happens is these wire short from the heat of the exhaust manifold.

Why it dose it when cold I do not know as I have not pulled the wires out of mine to find out why or what they are doing for sure.
All I know is mine was doing the same thing and after replacing three PCM computers, two distributors and two ICM's I bypassed the wiring to the O2 sensor and ran it down the fender instead of the engine and the problem has been gone for a year now.

I saw others on forums have had the same problem and fixed it by re-running the O2 wiring. That is were I got the idea to fix mine.

The wiring I used, the three PCM computers, two distributors and two ICM's were from my tested good spare parts that I have taken from other trucks that I have scraped. So the whole problem did not cost me anything.

 

I remember you went through quite a bit to find the root cause of your very similar issue, Bill. I was going to look for your thread if the ground check did not turn up anything. Thanks for jumping in!

 

That was going to be my suggestion as well. In fact, I always carry spares of all three. Sometimes trial and error swapping is a viable diagnostic procedure, especially with intermittent problems.

The only other thing I'd ask is whether the replacement PCMs were new/rebuilt or just junkyard pulled? So far, every one of the early-90's F150 junkyard-pulled PCM's I've pulled (now up to 4) has had leaky capacitors. It really is a chronic problem. That said, they often work any way, and the fact that your symptoms haven't changed with three different units leads me to believe the problem is elsewhere.

 

Thanks! PCM we rebuilt from Advance and there was not change once I installed. I'm putting in a new ICM tonight. If that doesn't work, I'll try the pip.

 

Completely new potential solution to me. Much thanks for your input. I'll give it a shot.

 

Thanks for the reinforcement. Gives me the confidence to try the crazy ideas.

 

Thanks. My technician replaced the grounds so I'm not sure. I will check both of these ASAP.

 

Yes, did check this out & it looked really clean. Much thanks for the input.

 

Ignition module going in tonight. Pip is next. Then my AK47. Much thanks for the input.

 

Did you read my post #6 above?