How to change your thermostat.

So what does the thermostat do in the first place ?
It is basically a valve that sits in the water plumbing in the engine and senses the temperature of the cooling fluid. When the cooling fluid reaches a predetermined temperature level the thermostat will open up and allow the fluid to circulate through the radiator to cool off. When the temperature goes down the thermostat will close again. The thermostat does open gradually so that it won't have to open and then close again after a short while because the cooling fluid has gotten too cold. On the Escort RS Cosworth the opening temperature is 82° celcius (179.6° F). That is the temperature at which the thermostat starts to open. The fully open temperature in above the boiling point of water at normal pressure; 102° celcius (215.6° F).
The harder you drive and the hotter the weather the more the thermostat will open.

After your car has done enough miles the thermostat will eventually be worn out. Mine was shot by 80.000km. It will wear out in two ways: either the seals will leak or the spring that holds the water back will weaken and not hold the pressure. The result will be that the coolant leaks past the valve and gets cooled of before it's supposed to. This will be seen as the engine not reaching it's correct operating temperature; i.e. the water temperature gauge will not reach the middle of the scale, this is not a condition that will destroy your engine the first day, but the engine will not work at optimum temperature and because of that the performance will suffer, the engine will use more fuel and wear will increase. So change your thermostat when it's worn, it's a cheap fix that will save you money in the long run.

So this is how you do it (Don't do this with a hot engine):

  1. Collect the tools you will need: 9 mm socket wrench with ratchet, flat-blade screwdriver, WD-40 or it's like, thread locking compound.
  2. Open the cooling water expansion tank with a cloth over the cap to release any pressure. Drain the cooling system into a clean container by removing the lower water hose from the radiator. Or; just go ahead with the repair and let it all run out through the thermostat housing (you can't collect it then so you will need about 3-5 litres of new anti-freeze).
  3. To get easier access to the thermostat housing it is best to remove a few bits first. First open the hose clamps on the air-filter-box-to-turbo-airhose, then the hose clamps on the cooling water hose to the top of the turbo, then the clamps on the valve that's attached to the air-filter-box-to-turbo-airhose.
  4. Remove the air-filter-box-to-turbo-airhose, remove the valve connected to it. Remove the cooling water hose from the turbo.
  5. Bend the hoses away so you get easier access to the thermostat housing.
  6. Remove the three screws that holds the thermostat housing cover. Pull the cover and the hose attached to it away from the housing and remove the old thermostat and it's rubber gasket.
  7. Clean the seat of the housing and the thermostat housing cover of old rubber and thread locking compound so that the new gasket will fit nicely and not leak.
  8. Fit the new thermostat into the housing with the side marked "TOP" up. Be sure to align the gasket so that it won't get pinched or in other ways destroyed.
  9. Dry and clean the screws and the threaded holes in the thermostat housing. Apply a bit (not much) of thread locking compound to the screws and fasten the housing cover with the three screws. Tighten.
  10. Spray the air-filter-box-to-turbo-airhose with WD-40 on the inside, to make it slip on easier, and refit it between the turbo and airfilterbox. Install the valve connected to the airhose. Refit the cooling water hose to the turbo.
  11. Tighten all the hose clamps, going over all of them at least twice to make sure every one of them has been tightened, and tightened tight enough.
  12. Let your work sit for at least 45 minutes to let the thread locking compound harden.
  13. Refit the bottom waterhose to the radiator if you removed it. Refill the cooling system expansion tank with a mix of 50% pure antifreeze and 50% clean water.
  14. Start up the engine with the cap of the expansion tank and let the engine idle. As the waterlevel in the tank sinks continue to refill with a 50/50 mix of water and antifeeze. When the water no longer sinks, fill to the "max" mark and refit cap.
  15. Go for a drive and check the waterlevel in the tank when done, refill if necessary. It is also a good idea to check the waterlevel more often than usual in the first week after this as airbubbles can get trapped for a while.

You're done!

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