water in ECU

[Nicholas Lawrence]

    Car is a '90 200 TQA.  Tuesday it wouldn't start during a rain shower.
Didn't connect at the time because it has had difficulty starting before.
Later in the day it started right away and I began driving. I noticed the
digital guage did not match the mechanical boost guage and it began to read
higher and higher.
    It stopped running and while I was waiting on rescue I removed trim
cover to see ECU.  It had a plastic bag around the wires and a large amount
of water sitting on the connector. I dried it off and drove home, digital
guage still climbing.
    I removed the ECU and opened it up to find water and small amount of
corrosion  one chip's pins. I'm cleaning it up with contact cleaner and a
brush. Haven't reinstalled yet.
    So... any experience with this problem?  First I'll attempt to track
down source of water leak.
    What is the likely hood of permanent damage to the ECU?
TIA
Nick