Overheat Indication

91 GMC pickup with a 350 engine. Last night I was driving when the "Check Gauges" light popped on. My engine temperature gauge was pegged at the top. Yet, the engine
was running fine and did not appear to be overheated. I pulled off the road and saw no steam or leaks, no boiling sound. I got back in and drove away. That gauge was still pegged. All of a sudden it dropped back to normal (about 200 deg). I drove home and had no more problems. The gauge stayed at normal.
What the heck? Why would this gauge peg then go back to normal? I suspect faulty wiring to the gauge, rather than an actual overheating. I checked the radiator after the engine cooled. It needed about a quart, and I put more in the resivore. But that's not really all that low.
The outdoor temperature was around zero anyhow. The only other thing I can think is that the thermostat was momentarly stuck, but I had just driven 30 miles with no problems.
Anyone have any suggestiond?
Thanks
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On Fri, 30 Jan 2009 06:45:09 -0600, snipped-for-privacy@invalid.com cast forth these pearls of wisdom...:

Sender unit possibly.
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-Mike-
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My guess is the sending unit. Had a similar thing with my '91 Regal. Temperature gauge would peg once in a while, come back and behave for months. Finally though, it would peg and return to zero all the time, but I drove the car for three more years like that.
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wrote:

Thanks to all who replied. That makes more sense than anything else I thought of. It was fine today. I'll just watch it for now, if it keeps doing it, in better weather I'll replace the sender. That should be easy enough to do.
LM
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snipped-for-privacy@invalid.com wrote:

Before you replace the sender unplug it when gauge is pegged and see if gauge goes to zero. You maybe able to find ohm specs for that sender on the web, too.
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That is great advice... I'd also replace the thermostat... it's awfully cheap insurance... HTH, Ben
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On Mon, 2 Feb 2009 15:48:57 -0800 (PST), snipped-for-privacy@gmail.com cast forth these pearls of wisdom...:

Changing the thermostat is indeed cheap insurance, but that is not consistant with the symptoms. He first needs to figure out his problem. Changing the thermostat does not do anything to contribute to that effort. Though... it is a good measure after the face.
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Intermittent sticking Tstat *can* cause those exact symptoms. HTH Ben
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What's your anti freeze protection level at? If it's weak you could have froze the radiator at zero temps and lost circulation. I've done it years ago in my youth. Heater was cooling the motor and partial froze the radiator. Stopping to check it thawed the radiator. Just another thing to check.

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