This is 1987 240, automatic, 188k miles. It runs beautifully in cool weather and now in summer but at night when it's cooler. In 90f temps and higher it will die off after 10 or so miles depending on heat, less miles the hotter it is outside. It does not die abruptly, it fades away and you can judge a pull off place within 30 seconds or so before it shuts down. I have battled this last summer and now this summer again. It ran ALL winter flawlessly.

When it dies, it acts as if you are running out of gas. Electrically, the dash lights come on normally, all lights and the radio are fine.. just either no spark or no fuel.. Cranks fast but no start. After it dies you can recover by opening hood for faster cooling. More than any other action, opening the hood gets you back on road faster than anything else. It really seems to be something in engine bay that is heat related.

Before everyone jumps on me, I have read all I can find on the subject here and tried many , many , many things. I have another runner that I have swapped many parts from with no luck and no changes in the symptoms whatsoever.

PARTS CHANGED:
New in tank fuel pump.
THREE different main pumps tried, all same useless effect.
New fuel filter.
Swapped FI computer in passenger kick panel.
Soldered, swapped, bypassed you name it, Fuel Pump Relay.
Swapped ignition coil.
Swapped Ignition Computer on passenger side fender well.
Swapped Distributor with hall sensor inside.
Fixed bad wire few inches from infamous 25 amp fuse.
Cleaned infamous 25 amp fuse connector pins.
Swapped engine temperature sensor, the one feeding the Fuel Computer.
Swapped fuel rail pressure regulator.
Swapped o2 sensor, found original plugged solid with carbon soot.
Vehicle ran thru emissions two days ago, passed idle perfect, loaded had high CO but it passed test completely. Now with o2 sensor changed, I believe that the CO is under better control.
Disconnected Idle Air Motor, no effect.
Unplug AMM, no effect, still will not start until the cool off cycle ends.

I have run with voltmeters connected to too many to mention points such as fuel pump relay output, igntion computer power wire (blue), orange and gray wires from Fuel Computer out to the Ignition computer (pulses on these), temperature sensor line... and others I forget what all.. nothing unusual noted, no changes in these voltages prior to engine die out.. I have not tapped all the wires, but the most obvious ones I could find.

All I can do now is yank the wiring harness where it runs under the front of the motor and inspect it, etc. but I have nothing to go on especially where this problem has NOT CHANGED in two summers! I think wiring would gradually worsen and can't believe it would restart after cooling again and again and again like clockwork.

I have used the pattern to troubleshoot by driving it short distances from home with my tools and can almost make it fail on cue... pull over take readings, etc.. get in and go another 5 or so minutes, pull over again and try again to find the problem. Once the sun goes down, it will not fail again because it isn't quite hot enough outside.

Using the AC to help heat up things up under the hood also works well to bring on the failure just when you want it to.

Can anyone help with more ideas and also tell me about possibility of wiring harness doing this by being so RELIABLY FAILING and recoving by cooling off?
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Genesis 7:11