I'd bet it's the transfer (in-tank) pump (archive)

Remove the trunk mat and the metal plate on the right side of the trunk and listen for humming from the top of the tank. If you don't hear anything, the transfer pump isn't working. Check the voltage at the connector, and if that's good, replace the pump. If this pump isn't working, vapor lock can occur easily; hence the problem isn't there in cool weather and after you let the car cool down (although the windows & trunk ventilation probably have nothing to do with it).

-Shawn

I?m really more at home on the E21 board, but my old man needs some help diagnosing a warm start problem with his 87 528e. The problem is this, the car starts fine in cool weather, but whenever the temp gets up into the high 70s, the car just cranks. It doesn?t even think about turning over. It has worked in a few times to open the windows and the trunk and let the car cool down for a few minutes. Does anybody have any thoughts on this perplexing problem?

Cheers,

Ben
82 320i

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