air leak in intake temporarily plugged....?
water shorted something out?

 

I'm thinking it's something 02 sensor related. Under wash cooled exhaust enough that a normal lean condition that has been causing rough idle, was altered temporarily, enough to cause max fueling for a few minutes, then exhaust temps start normalizing and lean fueling condition returned. Do you have a scan tool that reads live data? This could possibly give you an indicator as to what is happening. Watch short term fuel trim monitoring. The issue may be minimal enough that it's not setting a hard code.

 

I have an Ultraguage. Would that work as you suggested? If so, could you tell me, more precisely, what to look for? Thank you in advance!!

 

I would tackle the existing code. If the ECU is not getting correct information regarding engine temperature, that could easily effect idle.

 

Agree with Deputy.
the cold/cool water on the 02 sensors.
after a spring ride in muddy conditions- I will bring to self spray and wash - and the heat from the hot water pressure washer will send O2 readings temporarily in a funk pulling away.