Update: After sitting all night, I fired the 1100 up and a nozzle check showed PINK where the Yellow should be. I printed a purge page (that the recently and strangely missing(?) Gilligan informed me of a while back) and each stripe of CMYK color printed just fine, EXCEPT the Yellow which printed a uniformly pukey PINK from the first row to the last.
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After calming down, I called Richard at Cobra and he said to pull out the Yellow cart and look for Magenta ink that had cross contaminated from the capping station, and sure enough, there it was. Just at the lower opening, but it was clearly Magenta ink.
Huge swings in barometric pressure can do that, and I know it. But I doubt that in this case.
I used a LOT of Greased Lightening from Ace Hardware to clear the capping station (and the starboard side 'puke pit') and I sucked it out of the capping station repeatedly with a syringe--right through the pump and exit hose. I'm wondering if enough of that solution in the ink management pump could have caused this.
But, at any rate, I did a couple a head cleaning and a couple of purge sheets (thanks Kevin!), and all is well now.