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99 Mercury Mystic stops in hot weather

We bought our daughter this car to get to school and work. In hot weather it stops but it will start up in a few minutes. It is not overheating. we have replaced fuel pump, fuel pump relay, fuel pressure regulator. We are now looking for another car to buy her because no one can figure out what is wrong with this one. The diagnostic computers has a bad plug so no diagnostic had been run. No one we talk to has seen this problem being caused by a bad diagnostic computer.

What exactly do you mean by “it stops” in hot weather. Does it stall on the highway, or at idle? Is the check engine light on? If so, I would try and find a shop that will cut off the old OBD connector and install a new one, so the codes can be read. (The fact that the plug is broken does not affect the ability of the on-board computer to control the car, it only prevents you from getting trouble codes from the computer if there are any.)