Thermostats are simple heat-activated valves that let more or less cool coolant into the engine depending on temperatures. They can malfunction and are usually one of the first things to check when there are signs of overheating.
The thermostat on a Mk1 Golf is located at the lower front left of the engine, just below the alternator belts. Its just about connected to the water pump. You don't need to jack the car up to access this area the thermostat is in, but it helps.
This work should be done on flat surface to allow air bubbles to exit through the coolant reservoir at the top of the engine bay.
You'll need
Unfortunately, Mk1 Golfs usually have no petcock (tap) on the radiator to drain the coolant out, so the only way to go about this is to disconnect the lower radiator hose. I disconnect it from the engine-side first so you can at least angle it towards the catch pan. Loosen the cap on top of the coolant reservoir at the top right of the engine bay to help speed things up.
The thermostat housing appears as a short, 90 degree pipe connecting the lower radiator hose to the engine. It's usually black plastic. Remove this to reveal the thermostat. There should also be a large rubber O ring sealing the housing to the engine.
The thermostat should now easily drop out - it's just held in with friction.
Simply place the new thermostat into the block. Place the new O ring in position - its a little tricky, but be careful all sides of the O ring are in position.
Position the thermostat housing in place ensuring that the O ring is not pinched or crushed and tighten it up.
Reinstall the lower radiator hose.
Top the car up with coolant via the reservoir at the top right of the engine bay.
Start the car and check for leaks or drips.
Monitor the level of coolant in the reservoir and top up. Let the car idle and get up to temperature to ensure all air pockets are out of the system. Monitor the temperature the entire time.
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