2001 Accord EX V6 surging

At idle in neutral, the engine surges between 1100 and 1700 RPM. After 5 miles of driving the check light came on.
Any ideas what I should check or is it better to go to the dealer.
TIA Anon
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Anon wrote:

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Be sure the rad is full and the reservoir is well above the -MIN- mark. If it's low it will improve the second time you run it since the system will displace the air from the cooling system and sensors in the rad / block will start working correctly.
'Curly'
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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
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