fluctuating temperature gauge in 97 bonneville - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


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Old 03-14-2004, 06:56 PM   #1
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Default fluctuating temperature gauge in 97 bonneville

97 Bonneville SLE with about 75,000 miles (not the supercharged engine)
It used to always run at a pretty constant temperature right below 200 unless I was going up a big hill. Lately it started fluctuating from below 200 to about 210 or so and keeps doing that, back and forth during highway driving. Coolant level hasn't changed at all. I keep it a little above the hot level when the engine is cold due to all I have read out here about the need to keep the coolant level up. I had the dex-cool coolant flushed and replaced about 25,000 miles and 3 years ago. Anyone have any ideas about what might be going on?
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Old 03-14-2004, 07:09 PM   #2
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How quickly does it fluctuate? From 200 to 210 and back in a few minutes or a few seconds?

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Old 03-14-2004, 07:16 PM   #3
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Default re: how wuickly does it fluctuate

About a minute or two. Once I get up to highway speed it'll spend a minute or two above 200 and then it goes back down for a couple minutes, then goes back up for a couple minutes and so on...
If it was waving all over I'd probably figure the gauge was broken or something, but it is pretty steady. Moves up to 205-210, then after a minute or two moves back to 195 or so....
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Old 03-14-2004, 07:44 PM   #4
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Default fluctuation

My 90 SSE does that when it is extremely cold out.
The cold radiator water coming in shocks the stat and keeps it cold and closed
longer while the temp sensor is in the warm block shoots up above range.
It should go away when it gets warmer out.
If not the stat may be sticking, Try some water pump lube or change it.
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Old 03-14-2004, 08:10 PM   #5
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Thermostat action would be my guess too, since it happens somewhat gradually. Ideally, once the engine is warmed up, the thermostat should stay open, but I've seen some that will cycle like a furnace thermostat, depending on driving and climate conditions. Generally, this is a Bad Thing, and you might want to consider replacing the thermostat just to be on the safe side. Also do a forum search on drilled thermostats -- the idea behind them is that they allow some leakage flow through the thermostat from the engine block to the radiator even while the radiator is closed, to keep the chances of shocking the thermostat closed again immediately after it opens. If you do much cooking, it'* kinda like tempering an egg when you're making custard -- you add a spoonful of the hot milk to the egg to get it warm first, so that when you pour the egg in it doesn't get scrambled

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Old 03-14-2004, 09:57 PM   #6
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When I had that problem, a new thermostat fixed it. Not always the cause, but it is a cheap solution.
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