'87 Vanagon was experiencing a rough idle when cold. Replacing the temp II sender fixed the problem but, as always, the process meandered its way to completion.
You said that it started when driving in a heavy rain storm. There may be an additional solution to this besides the temp II, plug wires having small cracks in them and hold moisture/water. WD40 is your friend, spray the engine wires down really good and see if it cures a rough idle. BTW, 87 Westy's are the best year!
