94 Del Sol Strange Revving Up & Down

Hello all:
I have a 1994 Honda Civic Del Sol. Recently the car has been revving up and down and up and down during idle. In addition, when the car is doing this it
begins to overheat immediately, even during frigid temperatures and the heater has stopped getting warm. The engine is a SOHC VTEC.
I have never experienced a car doing this. Any advice?
TIA.
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The Green Potato wrote:

check the coolant level - inside the radiator, not the expansion bottle. should be full to the top. check only when cold.
when refilling, use a good quality long life anti-freeze diluted with distilled water.
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The Green Potato wrote:

-------------------------
You've allowed AIR into the cooling system by forgetting to keep the system full enough. The air will displace itself in about three 'trips' as long as there's some reserve in (of all places) the reservoir. Like Jim said use good coolant, preferably HONDA premix and do it _according to the owner's manual_. Fill the coolant reservoir to MAX and add as necessary for three days. It will appear to 'sip' coolant so you have to add some each morning.
Don't overheat a HONDA ! !
'Curly'
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That's gotta be it! *:-.,_,.-:*'~'*:-*:-.,_,.-:*' Thanks so much.'*:*:-.,_,.-:*'~'*:*:-.,_,.-:*'~'*:
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The Green Potato wrote:

If it's not low coolant, it's a bad temp sensor.
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