I have recently seen a car of a friend that has been complaining the temperature of the coolant never reached 90 C. He bought a new thermostat kit (housing and O-ring inclusive) from a local dealer. The thermostat is rated 88 C.

The rated temperature is the temperature at which the thermostat starts to open. We can find the temperature at which the thermostat is fully open by adding 10 C. So for Skoda Felicia, the thermostat is working in 88 C  98 C range.

My friend installed the new thermostat kit but to his surprise there was no improvement. Further more, the old thermostat was not stuck open.. So he came to me asking for help. Here is what I did.

 

First I let the engine cool down, then started the engine and let it run at idle. From time to time I touched the thick upper radiator hose to see if it is warm while watching the temperature gauge.

 

Note: Touch the hose nearest to the radiator. Touching it near the thermostat is not a good practice because even with the thermostat still closed, heat doesn't drop abruptly in that area. It is normal to feel some heat for 5 cm.

 

The hose started to heat up on its entire length before temperature reached 70 C. That was not good.

 

Note: If you want to see exactly (just for a test!) when the coolant starts flowing through that hose, you can replace it with a transparent hose capable to withstand 110 C at 2 bar.

 

So I realized I had to check the new thermostat's rating. By the way, a +/- 4 C tollerance around 88 C is OK (84 C - 92 C) In the photo below you can see what you need to test it out of the car. The thermometer tip should be placed nearest to thermostat. A 0.4 mm feeler gauge is fitted between the valve and the body and the thermostat is inserted in water hanging by the feeler gauge. Heat is applied from a stove. When the thermostat starts to open. the thermostat will fall. Read the temperature.

The new thermostat bought by my friend started to open at 68 C! Way out of specs... So he returned the kit, added more money and bought another one (CALORSTAT by Vernet TH6733.88J)

This one was spot on.

Below you see what is the travel range of the valve.

Edited 21 January, 201511 yr by RicardoM

For those who might ask themselves "why should I care if my thermostat is correctly calibrated?" here are two basic scenarios:

• the thermostat opens at a higher temperature - the engine will overheat and all kind of costly damage may occur
• the thermostat opens at a lower temperature - the engine will never reach its normal operating temperature and the fuel economy will be horrible

My advice is to invest a little more money in this often overlooked part. Buy it from a trusted manufacturer that has high quality standards. Cheap thermostats use low quality materials, they rust, they decalibrate in time, etc.

 

Note: If you are not having a glass thermometer, you can use any digital multimeter with temperature measurement capabilities

I have made a recent discovery about Felicia thermostats that are fixed in place using a big metallic circlip.

 

The metallic shell of the thermostat is not sealing properly when seated in the plastic housing. Coolant can flow around thermostat and the engine warms up too slow. The solution is to use a high temperature liquid gasket sealant to create a gasket on the sitting lip of the plastic housing, then seat the thermostat, then put the circlip.

 

This little modification shorted the warming up time of the engine by 5 minutes! (from 12 minutes to 7).