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Last night I was driving my 93 sidekick (w/96 motor) and the temp gauge started going up and down quite quickly. It didn't go higher than half way but went all the way to C and then back up and down etc. I noticed a bit of a hot engine smell as well. Haven't had time to to a thorough check under the hood but I checked the coolant and its full.
Could it be the thermostat maybe?
cheers
 

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How quickly is quite quickly? I'm sure you've boiled water to make tea/coffee and you know that even for a small quantity of water it takes several minutes to go from cold to hot.

Water/coolant takes time to warm up/cool down, so it is, at least to my mind, unlikely that rapid variations on the gauge are representative of actual temperature variations.

Also - the thermostat is "downstream" from the gauge sender, or to put it another way, the gauge sender is on the cylinder head side of the thermostat, so a closed thermostat allows hot water to surround the sender, and opening the thermostat allows the hot water to flow through the thermostat to the radiator, cooler water at this time will be entering the block where it will be warmed before it reaches the gauge sender, so once that motor has been running long enough to get warm, the chances of cold water being anywhere near the sender are slim to nil.

Having mentioned all of the above - please be aware that the gauge sender will only indicate coolant temperature - if the coolant is too low for it to surround the gauge sender, the sender will indicate cold, and if the engine is revved, it is possible for hot coolant to splash onto the sender causing it to indicate a sudden and possibly rapid change in temperature - BUT - you say you've checked the level and it's full.

Was the system recently worked on and refilled? Is it possible that there is air trapped in it? Are you checking the level in the radiator or in the expansion tank?

One last question - are you running the stock temperature gauge?
 

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I would check the temperature sensor for being snug or for the sensor wire (mine is a yellow/white striped wire and a rubber boot) being loose or corroded. The temp sensor on my 92 is located just underneath the thermostat housing. A bad gauge would be my last guess - gauges seldom fail. If wiring OK, likely a failing temp sensor or intermittent electrical problem. Temp sensors are available at most auto part stores, $20-30. If new temp sensor still shows bouncing temp, I would next check the connectors plugging into your gauge cluster being dirty or loose. However, a failing temperature sensor is the most likely. Ensuring your engine is operating in the safe temperature range is very important for the long term health of your engine. Good luck and let us know what you find.
 

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And aside from the good info above, welcome to the forum!
Other than air pockets, etc., if your gauge is going up and down quickly (and air pockets, I believe, would cause a slower response in the gauge) I’d check that yellow/white wire coming from the temp sensor to the dash and it’s connection to the thermostat housing.
 

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air pockets, or water slosh will make the gauge react rather quickly. As Fordem has stated, low water usually results in a low temp reading. Faulty thermostat can give the same results, they do fail and can snap open and shut if they get old and cranky.

Diagnose the issue. Check the wiring first, confirm the gauge is ok, check resistance of the sender cold and hot, if all that checks out, pull the thermostat out and check it using hot water to confirm its operating correctly. It would be unusual for the sender to be faulty but as always, work the diagnostic process and then spend money when you know what the cause is.

warm it up with the radiator cap off, do you see flow appear at the filler when the thermostat opens? is the water pump actually working?
 

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It would be unusual for the sender to be faulty.
Just my experience, but it's not that rare, and it might relate to the source - last year I went through three or four sensors on one vehicle, the standard "single pin" gauge sender, not the one that feeds the ECU.

I got fed up and found a store that had parts for the older british vehicles and found one with the correct thread and haven't had a problem since - amongst the failed ones was an OEM unit.
 

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My '96 Tracker did exactly this, gauge would drop then pop back up & back & forth. I had my mechanic help diagnose it, he had me watch the gauge while he grounded the connector... told me to watch if it maxed out... it did. He then trimmed 1/2" off the wire & soldered on a new end: Done. Corrosion is a beast in the rustbelt. :(
 
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