Thanks Curly, but it didn't seem to help. I went out to the cold car this
morning and opened the radiator cap. It appeared to be full to the top. The
reservoir was about 1/2 inch below the min mark so I filled it to 1 inch
above with RO water. Started up and it wasn't surging but the check light
was still on. After driving a block it started surging again. Sitting still
idling in neutral with the engine surging after a minute causes the TCS
light to come on. When this happens the button has no control on the TCS
light. Turne off and restarting engine restores function to the TCS button.

After reading the manual it said the check light could mean a loose gas cap,
I removed and reinstalled the cap.

After another test drive, the engine is not surging anymore. It seems to
sometimes idle very slow, about 200 RPM in neutral but is still smooth. I
feel that some sensor or control valve was stuck and has not unstuck itself
but it will probably happen again. Any idea what could it be?

Anon