Hot start problem 88 5kt automatic

[Doyt W. Echelberger]

Friend's 1988 5000 turbo (fwd automatic) has developed a tendency to not 
restart after being shut down hot. In desperation he asked for suggestions 
yesterday.

He drives for 30 minutes, parks in driveway, turns off engine, trys restart 
and all it does is crank. Won't catch.

MUST cool down before it will restart when this happens.....20-30 minutes. 
And it doesn't happen all the time. No apparent pattern.

Runs_ great _when started cold and stays that way as long as it is driven 
without being turned off.....sometimes. He already replaced the oxygen 
sensor, the plugs, the wires, the cap and rotor.

Good-running spare parts type 44 of same year is sitting there waiting to 
swap anything and everything to see what will make the problem go away.

Here is what I found in the archives:

This problem is rampant among UrQ's and 5000 models. Apparently is a 
vapor-lock thing. It is the reason Audi put the jury-rig blower in there to 
cool down the injectors and reduce under-hood temps.  Also one of the 
reasons for coming out with the next generation of inline 5's.

This vapor-lock explanation makes sense to me because we have been having 
unusually warm winter weather (50 degrees F) here in Ohio, and the gas 
station fuel mix is still being supplied to our area is set for normal winters.

Also could be aging of a relay that controls the fuel pump or the one that 
causes hot-start pulsing of the cold-start valve. Or maybe a poor ground 
for any of those relays. Or maybe a temperature sensor is telling the ECU 
that the engine is cold. and needs the cold-start valve open, and the 
mixture is so rich it won't run. Or maybe the RPM sensor resistance is out 
of its range and needs replacing.

Anyhow, before I get back to my friend, are there any other ideas?

Doyt Echelberger
87 5ktq that runs just fine (!) (today)