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What actually happens when there is no thermostat (bypass thermostat) in coolant system?

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The engine, if the rest of the cooling system is in good order, will run too cold.

This will affect fuel consumption, emissions and perhaps power delivery.

While this is often done for short periods, ie to deal with a faulty 'stat, it is not advised for long periods. Best thing is to get the thermostat replaced.

  • Absent of Thermostat causing any engine head gasket damage or something? – Maheshwaran ramasamy May 2 at 7:43
  • Overheating can damage the head gasket, which a faulty 'stat could cause. But not running without one. – Solar Mike May 2 at 7:45
  • Please express thanks by upvoting the answer. Also, marking the answer as accepted using the check mark on the left effectively closes the question for the system. – Solar Mike May 2 at 7:47
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    Under normal conditions running without a thermostat will only slow down the rate that the engine will reach running temperature. It wont permanently run cold. – HandyHowie May 2 at 10:20
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    I have a Golf GTI on the fleet at the moment which doesn't have a working thermostat and it never reaches optimum operating temperature during normal road use. It is possible to replace the thermostat with a restrictor ring which provides an obstruction in the cooling system of similar dimensions to a fully open thermostat but this is usually only done in competition vehicles, in a road car a functioning thermostat is usually a must. Another side affect of no thermostat is you may find it hard to bring the cabin upto a comfortable temperature as the heater core may never get truly "hot". – Steve Matthews May 2 at 10:40

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