Will not start after rain

[keithmcd]

Starting Problem after rain Mk3 Golf GTI 8v

The car will not start if it rains. If it rains over night it will not start in the morning. Once it has dried out and had some sun starts fine, first turn of ignition in the afternoon. Vice versa is true. The car WILL start in the morning if no rain, but then WILL NOT start in afternoon or evening if it rains.

I have replaced the following with no joy:

Distributor Cap
Rotor ARM
HT Leads
Spark Plugs
Battery
Mass Air Flow sensor

ARRG

[Crasher]

Two common possibilities, ignition coil and crank speed sensor.

[keithmcd]

Thanks for your help I will give them a go

[Perkele]

Same problem.
I changed the distributor arm & cap and put in new sparks plugs. That cured the issue for me.