Golf MK4 Air Fillter Box Heated Air Flap Wax Thermostate Repair

The engine cover doubles as air filter box and also houses the heated air intake flap.

The heated air intake is through the round hole under the box. It can be seen when the filter cover is removed from the bottom.

Thermostat

After many years of operation, the heated air intake flap usually gets stuck in hot air side fully open.This is not a big problem but there is no point in taking in warmer air than necessary.

Unfortunately it is not so easy to find replacement thermostats. The only one I could find was some Volvo replacements. Although there are 2 different Volvo models, with different prices, they seem to be exactly same thermostat. So buy the cheaper one.

Here is the link to thermostat:
https://www.skandix.de/en/spare-parts/engine/fuel-mixture-formation/injection-system/idle-control/thermostat-air-temperature/1006579/

Volvo reference numbers are 1266826 and 9142616

The Volvo thermostat itself is slightly longer than the original and also the pin is much longer. So the pin must be cut smaller. The old pin can't be used because it is not same thickness.

Had to cut about 4mm to be able to bring it to correct size. There is some slack space when the pin is completely out. One can cut it even 6mm and it should still be fine. I just cut the minimum required amount so the flap can open completely.

Thermostat Removal

The thermostat can be seen once the other cover is also removed.

Removing the thermostat is not an easy task. If you are not careful the flap will get broken! The flap may be brittle from extended periods of hot air.

It is possible to remove the thermostat without removing the flap. However the new thermostat was longer and I could not install it without removing the whole spring system. But, there are people who claim to have done it.

For removing the old thermostat, you can simply push the spring as far as it goes and let the thermostat fall out. I removed the flap first.

For removing the flap, first you will need to pry open the little cover piece in the middle. It is a ~1cm length rectangular piece. You can see it removed in the picture below.

Next step is to move the spring to side, which is shown on the picture above. After this point, the flap can be removed with a little bit wiggling and bending of the black upper part. Then the spring assembly can be removed.

Once the spring is removed, one can remove the thermostat and replace it easily. I stored the new thermostat in freezer first to be able to push the pin all the way in. It makes installation much easier.

Thermostat Installation

The installation is reverse of removal. You will need to glue/melt the rectangular plastic cap back after installation.

Testing

The mormal operation is:

      Above +23 °C, flap must close warm air connection.

      Below +10 °C, flap opens warm air connection.

The Volvo thermostat starts opening warm air at about 9C and fully opens it at about 3C. It is fully closed at about 16C. Which are pretty good values all things considered.

 

I tested mine in fridge.