1990 928 GT - Failure to start following rain...

Are you getting a wet passenger floor board after a rain???

Have you sniffed its butt?

[sorry, I'm a pervert, and proud~]

Seriously, how do you know that it is getting too much fuel? When it won't start, have you sniffed the exhaust pipes? If it is getting too much, then it should be obvious.

For some reason, I suspect that your fuel pump is NOT working. You crank it and residual fuel makes it work for a few seconds, but then it shuts down. If your fuel pump is working, right after it refuses to start you should sniff your exhaust pipe and CLEARLY smell gasoline.

How do you know your fuel pump is working?

N!

From what I've read here, we don't know that it has fuel or spark yet, do we?

The only way to know for sure about the fuel pump is to jumper the relay.

Then, we have to know if the injectors are firing

And we need to know about the spark and if it's happening.

My bet is the spark is missing due to the water/moisture somewhere. A likely candidate, as posted previously by Brad, would be the Crank Position Sensor connector.

I'm in the camp with the moisture getting in somewhere. Even tho your mechanic replaced them, could be a bad set of plug wires or a crack in the distributor cap that developed afterwards, aloowing moisture in. Something has been breached. Test and very carefully inspect the new things that were replaced. If you have the old parts (cap, plug wires) put them back on and see what happens.

Verify fuel & verify spark

But

Also make sure the battery is not week!

If I have a week battery, even with booster cables from the truck on, the car will not always start or even fire (try to start)

Based on your symptoms - it seem to me it must be lack of spark/adequate spark.

The most likely thing to be affected by mositure (and recover over time as it dries) is the high voltage side of the ignition.

If you have any degradation in insulation then moisture can create a bypass path that for 12v would be totally irrelevant BUT that for ignition spark voltages looks much like a low impedance short.

So this narrows it down to Coil, Coil to Dist, Dist, Dist Rotor, Plug Wires, Plugs.

What on this list hasn't been replaced? Seem you have said all - but are you really sure? - double check?

Did you replace the distributor body/cap - this is the most suspect to me since its a single point failure affecting many cylinders. BTW I think the rotor is the least suspect since its protected. Failure mechanism = hairline crack often quite invisible.