Helping out a friend with an old E350 based El Dorado Bus. When its very hot out ~90F, the bus will stall until it cools down, then will run fine until it gets hot again.

Looking for tips on possible areas to focus on, the only troubleshooting he performed is replacing the gas cap (actually didnt have one).

I am thinking electrical - possibly the ECU, ignition module, etc.

 

You might have the ignition control module that is on the distributor, those would overheat and fail and the van would stall when hot. Ford moved it to the drivers side fender later.

It looks like this:

Part 1 -How to Test the Ford Ignition Control Module (Fender Mounted)

 

I Second that the ICM is mounted on the distributor, and can/will over heat and shut down. The picture in that link is the ICM set in the heat-sink, that is mounted on the drivers side fender, almost under the front cowl.

Other's have had this problem, and re-wired the (I think) 3 wires out of the distributor, and relocated the ICM away from the engine, or at least up above the engine.

Another cause of "heat shut down" is the location of the coil, earlier models, the coil was mounted just above the engine intake, center, couldn't take the heat, later models, and an DIY repair, was to get the coil away, above the engine, out of the heat per say.