1995 Toyota Avalon started overheating so I had the thermostat

check the installation of the thermostat
the body of the thermostat has to face the head or block as it requires hot water to make it open
installed incorrectly will cause over heating

Sounds like your thermostat isn't opening properly

May be its low on coolant or radiator hoses clogged or its thermostat issue,its remaining closed or it can be issue with coolant leaking from somewhere.Or cooling fan not running when when engine gets warm.
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There are many other possibilities causing these issues.:----
To troubleshoot the issue yourself,i provide you few very helpful links with solutions:-----
Click the link below:----

Dodge Dakota stalls, stumbles at idle. Has low fuel flow and runs hot?

http://schematicsdiagram.blogspot.in/2012/03/dodge-dakota-stallsstumbles-at-idlehas.html

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1991 Toyota Trecel: Engine Starts fine but dies when gets Warm?

http://whatisbyme.blogspot.in/2012/05/1991-toyota-trecel-engine-starts-fine.html

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1995 Lexus LS400: Running Hot?

http://whatisbyme.blogspot.in/2012/05/1995-lexus-ls400-running-hot.html

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Multiple problems on car: Car runs hot, A/C blows hot and car starts surging?
http://schematicsdiagram.blogspot.in/2012/04/multiple-problems-on-car-car-runs-hotac.html


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There are many possibilities for car overheating.

Faulty thermostat clogged radiator/plugged radiator, radiator hose worn out. Coolant running low. Low coolant pressure. Faulty temperate sensor.

I suggest you to go through the troubleshooting details mentioned in the link below:-----

All types of car overheating problems and troubleshooting?

http://technoanswers.blogspot.com/2011/11/all-types-of-car-overheating-problems.html

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These will help.

Thanks.

disconnect the heat hoses going into fire wall , put hose into one with pressure and flush as it sounds like it could be bloked in the heater core,the other thing that could be the problem be the hot ,cold switch as it restricts the flow to makes the heater hoter or colder , it might need tlc or replacing as it is manaul not elec

Could be your fuel pump. When it runs see if you can hear your fuel pump running, should be a low humming noise towards middle rear of vehicle. When the vehicle goes dead, keep the key in the on postion see if you still hear the fuel pump running. Also while it is running check to see if the temperature gause goes way up. It may be overheating, because of low coolant, or have a bad thermostat sensor and think its overheating and shutting its self off. If these check out it could also be a clogged fuel filter. Without being there its a tough call.

clean or replace IAC valve,i had the same problem. There is alot of info on the web on how to do.

I do not see a good connection between the serpentine belt and the engine overheating suddenly.

Perhaps you have a lot of burnt belt rubber on the water pump pulley?

Is the upper radiator hose hot? If so the stat is opening. If you start the car cold with the radiator cap off you can watch for coolant starting to flow and make sure the stat works...if the flow is low you have either a blocked stat or poor belt grip at the waterpump. Have you flushed the radiator recently? After 15+ years your car could be ready for a cleaning.

Seems like the thermostat is OK if you are now not experiencing overheating.

I too had similar problem when replacing the thermostat on my Z3. My advice is top up with more coolant and check the level.

the thermostat that you replaced it with may be too hot of a temperature thermostat ask the people at your local parts store what it calls for