Just read this Gaz..............
This procedure is commonly known as "burping" the coolant system. Air can get trapped in your Mk3 because the heater core is located in such a way that air can get trapped. When this occurs, you'll get a rushing or gurgling noise under the dash, possible loss of heat in the cabin, possible bouncing temperature gauge, etc. Ideally, air in the cooling system will be trapped in the radiator top, but because of how the Mk3 cooling system is designed, air can get trapped in the heater circuit or in the thermostat housing.
To remove the air, you have to get the radiator as high as possible compared to the heater core and the rest of the circuit. If you live in San Francisco, or live near the mountains (or have a convenient hill you can drive onto), just find a parking place that a) has the front of the car significantly higher than the back, b) won't freak out if (will) coolant sprays on the ground. If you don't have a natural method of elevating the front, a jack and a couple of jackstands will work too! Just put the front as high as possible. We want the air to get out of trapped areas, such as the thermostat housing and heater core, and into (and out of) the radiator.
NOTE: the coolant in your car is poisonous to kids, pets, wildlife and aquatic life, and is also attractive to such, so PLEASE make sure you flush coolant off the ground and dispose of it properly!
Next, get the bottle of coolant you'll be adding to the engine (trust me, you'll be adding coolant once the air bubbles out). Now that the car is the proper attitude, open the radiator cap (carefully if the engine is hot), and if possible, remove some of the coolant (I use a baster to remove coolant so that it doesn't all bubble out.if the coolant is clean, you can put it into the coolant bottle you've got standing by). Start your car's engine and let it idle until the engine warms up.
Once it gets to operating temperature, turn on your climate control to maximum temperature and fan. If your heater control valve is operating properly, it'll open up and allow coolant to flow through the heater core, and thus, help push the air out (remember, your radiator is now much higher than the core due to the front being so high). Let the car idle for 15-30 minutes and carefully monitor the level of coolant in the radiator. You'll see occasional bubbles (and in some cases, large burps) of air in the radiator. Keep adding coolant slowly (don't add much more than to cover the radiator core until the bubbling slows down. As you reach the 20 minute mark, you should see a steady stream of coolant without bubbles and burps. Top off the radiator at this point, fill the reservoir to the proper level, and replace the radiator cap.
_________________[You must be registered and logged in to see this image.]"Life isn't about waiting for the storm to pass...it's about learning to dance in the rain."