The thermostat is a good and inexpensive place to start.
Fan working?
Other considerations might be old, wrong mix of coolant,
clogged bent radiator fins,
Twelve year old radiator scaled up inside, possibly verify it is cooling ability with an infrared temperature gun, Some are as cheap as $30.00, see if a shop will check it in the parking lot. (flush, if possible or replace the radiator)
Old squishy radiator hoses sucking flat.
I looked at a auto store site, the water pump they show has a plastic impeller (first picture below) Another brand may be metal?
Here is more information from "repair guides", top right of our pages.
https://www.2carpros.com/articles/symptoms-of-an-overheating-engine
Not trying to scare you, but plastic and water pump does not seem to fit together in my book. First picture below is the one I got at the auto store site. The rest are failures from other vehicles, some are missing a fin, slipping on the shaft, others are total fails. Unfortunately, this would require removal to inspect it.
Site unseen, my Lottery guess is the radiator internally.
Sometimes it might just be a defective temperature sending unit.
Let us know how your investigation goes, check it out well before you start slinging in unneeded parts!
The Medic
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Sunday, April 29th, 2018 AT 8:27 PM