My 88 wagon has developed an overfueling problem that has made it undrivable.

The car starts and idles fine. As the engine starts getting to normal operating temp, it starts sputtering, vibrating, and emitting black smoke. Letting the engine cool off makes the problem go away until it heats up again. I exhausted my knowledge and the mechanic has had it for 3 months and can't figure it out. I have replaced or tested the: coolant temp sensor, main fuel pump, fuel filter, fuel pressure regulator, coil, timing, distributor, plugs, cap, rotor, ht leads, MAF sensor, ECU.

The engine does not overheat. The plugs are black and wet when this happens. There is no cold start injector and all the wiring associated with the fuel injection has been visually inspected.

I'm living in Laos but this is a US spec car. There are no Volvo dealers and most mechanics have no formal training.

What could be causing the fi to constatly pump excessive fuel? Are there any ideas on where to check next?