Cooling system's Thermostat removal should be temporary, if necessary.
Many people have asked whether it is good to remove the thermostat in their cars.
For fuel economy and engine longevity concerns, the best is the have the engine's Thermostat in place. This can make the rings still be in good shape after many miles.
Where proper maintenance of using coolant solution and replacing the coolant (causing severe rust and/or scale formation) and Thermostat periodically are not followed, as in the case of many users in Nigeria, REMOVE THE THERMOSTAT to protect the engine from catastrophic failure as a result of overheat. Thermostat removal should normally be a temporary measure till you can get the cooling system clean, get quality coolant and quality Thermostat. Thermostat removal can be done for the purpose of troubleshooting too and then returned back to the engine.
Here in Nigeria, many thermostats on sale are substandard and fail easily thereby endangering the engine. Many car users in Nigeria out of ignorance use water or substandard coolants in the cooling system. In any of these situations, it is better to have the thermostat removed than have it get stuck sooner or later. This is not to say that Thermostat removal is encouraged but this is to save ignorant users from major expense that they could incur due to engine failure.
The negative effect is, it will take more time to reach operating temperature from cold start. The engine will also be over-cooled during highway driving in a cold weather even in Nigeria as the natural cold air passing through the radiator will be over sufficient for the cooling and there won't be Thermostat to help in flow control. The practice of Thermostat removal also reduces engine longevity due to increased wear as a result of the engine not reaching optimal temperature in time.
Removal of thermostat is not a standard practice but it is better that destoying the engine through severe or multiple overheat.
Note that some vehicles use electronically controlled Thermostat to control flow under varying driving conditions. I advise that you must leave Thermostat in such vehicles and use coolant, though it will still work if you remove them but you will get a fault code.