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2005 i5 Colorado, 2WD.
My Colorado coolant temperature fluctuates regularly between 190-220 (measured on the gauge and my scanner) I know it isn’t “technically” overheating since it is below 220. But it doesn’t seem right that it fluctuates so much, daily based on my driving conditions.
I’ve Been testing all summer to see it’s trends and It only sits at 190 during highway speeds when I have the heater on (this is during summer time). Highway speeds with the AC on it runs about 200-205. Then idling for a long time (have tested it up to 15 minutes idling in the summer sun) with or without AC on it runs 210-220.

Radiator and coolant overflow tank are full.

Radiator has been flushed, chemically cleaned, and refilled.

Thermostat has been changed (I know this can raise operating temp but doesn’t explain fluctuation).

Coolant temperature sensor has been changed.

No leaks from water pump and heater runs hot (was told this eliminates water pump circulation problem?)

And radiator fan runs well, speed increases with RPM’s. Clutch seems to engage as the fan does not spin freely when I try to spin it.

What can be causing the regular 30 degree fluctuation? This is all happening on regular, level, highway driving conditions. Am planning on taking it to mountains with a load and am concerned that those conditions would push it over board.
 

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The first thought - of course - would be a cooling system problem. Sounds like you've managed that pretty well.

My second thought would be a head gasket problem. You can buy a coolant test kit at HF Combustion Leak Detector (harborfreight.com) Or you can borrow it from Autozone. AutoZone Loan-A-Tool Program - Tool Rental Near Me

Also, borrow a coolant system test kit from Autozone (or wherever). Follow the instructions and perform a full pressure test of the coolant system.
 
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