Z28 1994, I Have noticed recently ( week from now) that some times when I start the car the temperature rise slowly to reach above half and then it drop at once below the half! and never gets over half again!
is that symptom of failure thermostat in the water pump even I replaced the WP last year!?

 

What is the thermostat operation?
it opens once in car drive or every time the pressure increase?

 

a thermostat has a spring in it that when the hot water is hitting it is slightly gets weaker and opens up to allow the coolant pass and circulate then when the car is off and is cooling the spring get stronger again and closes.

it sounds like maybe the gauge is bad or sender, a way to test the thermostat is to pull it out and stick it in a pot of water on the stove and heat it up. and see if it opens up at the temp its supposed to

 

^^^ agree, test the thermostat. sounds like its sticking the first time of the day

 

Sounds like its starting to fail. They are like 5 bucks. I treat em like maintenance with every like 10k miles ok so. It might sound excessive but most overheating issues are because a thermostat. I can't tell you how many headgaskets I've seen go out because a thermostat.

 

And if your thermostat is stuck, it wouldn't be stuck closed, it would be stuck open.
Maybe someone put in a fail-safe stat once upon a time. If the engine ever gets too too hot, the stat will remain stuck open.

 

2day after cold start the temperature reached 100 and I heard the dashboard a sound of boiling water! and didn't goes down as It had to do ( usually it used to go down directly below half and never go upper any more). started it after few hours and normal never goes more than half.

 

when I usually start the car it takes 5 minutes for the temperature gauge to start moving ( normal healthy case when the gauge never goes more than half).

but when this issue happen ( it gets hot once for every cold start) I noticed that temperature gauge start to move after minutes to reach the first white line.

any clue?
thx

 

I bought 180 thermostat but the man didn't agree to change it. cause he said the symptom which " the car gets hot every cold start and until u accelerate the temperature goes down and never goes up again is maybe cause of air in the radiator or bad radiator cap even could be bad water pump needs a lot of power to push the water!!!!"
what d u think guys?

 

and y do I hear boiling water under the hood after driving the car normally and parking it even the temperature was under half all the trip?