Needing any advice available to help diagnose and get to the source of this problem:
I have a 99 F150 4.6L Long-bed work truck w/tommy gate lift-gate that I purchased about 2 years ago,
Truck has over 200k miles, but was purchased with smooth running recently rebuilt engine, and has seemed to be a well maintained fleet vehicle and has only required a new battery, new front brakes and rotors, oil changes, and just minor repairs.

Here is everything that has happened so far:
-About 3 weeks ago I parked the truck at a job site with engine off for a few minutes to load equipment, (engine was still hot) and then engine would not start. kept trying every few minutes for around 30 minutes and then it finally started.

-Battery was tested as bad, and was replaced under warranty.

-A week later, I drove truck for about an hour with a/c running, turned engine off, and would not restart, waited a couple hours and it started, battery was tested, and again tested bad, and battery was replaced again under warranty.

-Next day, drove the truck about an hour with A/C running, turned engine off and would not start. Again waited about 30-45 minutes then it started.

-had battery tested again, and tested "good" this time.

-had alternator tested at 2 different shops and tested good at both.

-replaced starter

-changed oil

-seemed to be working after starter was replaced and oil changed, but yesterday I drove it again for about an hour with a/c running, turned engine off for about 5 minutes, then it started right up, then I drove around the corner, turned engine off and ran into a store for 5 minutes, and then when I came back out the engine wouldn't start again. Tried to start it for about 15 minutes, but would not start. Then I walked home and grabbed my Ford Mustang and jumper cables, but would still not start even with jumper cables 20-30 minutes later. Then I left the truck for about 2 hours to cool down, and then it started right up.

Notes:
*I am based in Las Vegas and it is HOT here right now! (takes a while for engine to cool, even at night is close to 100 degrees outside)

*When it will not start, it seems like there is plenty of electrical power, (lights/radio/etc all work) but the starter does not even attempt to crank, ...turn the key and nothing)

*a mechanic friend was with me one time when it would not start, and he took a look under the hood and said the engine seemed like it felt hotter than it should be, and he suggested that it might be a heat issue, and told me I should replace the thermostat.

*I will be replacing thermostat tomorrow, but instrument gauge has not show the engine is overheating, and coolant levels seem fine.

*thought it might be ignition issue, and the truck includes a somewhat unusual factory-installed "fleet-vehicle kill-switch" that is built into the wiper/turn signal assembly, that requires that you pull back on the turn signal/wiper arm while key is turned in the ignition, or the engine will not start, (thought it might be the ignition/kill-switch causing the problem, but this makes no sense because whatever is causing the issue seems to be directly linked to engine heat.) Also we took apart the steering wheel column to check the wiper/turn-signal assembly, and everything was fully intact with no loose wires and everything looked clean and was factory installed. Again, thinking at this point the ignition & kill-switch is not the issue because it aways seems to start when engine is cold. Also, the turn signals, high beams, etc all work fine so I do not think it is an ignition/kill-switch issue,
(but please advise if this is incorrect?)

*seems that every time truck will not start, engine is hot, & a/c has been running, (not running "max a/c", just regular a/c with fan set to medium)

-Is it possible that the thermostat could be just bad enough to not totally overheat, but make the engine too hot to start after a/c has been used?

-Is it possible that something intermittent in the a/c system is shorting out, or making something in the engine overheat? (a/c seems to be running fine, very cold air, and alternator/electrical system was just tested by my local Midas with "max a/c" running, and with fan on high, and alternator and electrical system tested as ok)

I depend on this truck for my business, and really appreciate any ideas, experience, or advise anyone out there can share,
Thanks in advance!

 

Just for grins, replace the starter relay on the firewall. It's cheap.

 

Thanks so much for your advice,
I will replace the starter relay and the thermostat today,

Can you think of anything else in the chain between the ignition and starter that could be failing due to heat?

Thanks again!

 

wow this one has me stumped. I have an idea that will only cost you time, and won't hurt anything if the problem lies elsewhere.

Suggest checking all your ground cables for corrosion - battery to cab, battery to frame, engine to frame. Maybe unfasten them, check for corrosion, clean contact areas, apply dielectric grease, and re-install.

Same deal with the wires to the starter and alternator.

Good luck, and keep us posted!

 

Thanks again for your advice,
I ended up inspecting all wires and cables and everything looks fully intact and in great condition,

I bought a thermostat but had several people tell me the thermostat was not the problem, so I am just saving it as a spare,

So I ended up replacing the starter "remote solenoid" (found it for around $28 at Napa, also called our local Ford Dealer and part was $65+, and local Auto Zone said it was unavailable, so Napa seems to be the best retail source for these)

Truck hasn't acted up since replacing the remote solenoid, but will give it a few more days to be sure! After going over all the symptoms, I think this was the problem, but the only way to be sure would have been to wait for it to not start again, and cross the contacts on the solenoid with a screwdriver and if it starts, then the part is bad.

One note, the first remote solenoid I picked up was correct for my model year, but did not fit my existing factory wire harness or bolt holes, then found out Napa also had an alternate style that almost exactly matched my original, (to save time, I would recommend either bringing in your old part, or a photo of your old part before picking up a replacement)

Hopefully this is the end of this thread, but will let you know if it happens again!
Best always!

 

Great news. Thanks for the update. I'm glad you found a simple solution!

 

After more then a few unnecessary starter and other parts thrown at fords I've owned when ever I have a will not crank issue I always replace the firewall relay/solenoid first thing. Cheap part that seems to be the weak link..