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As the various Haynes manuals don't mention this and it is extremely easy I thought I would post a guide as I did this at the weekend in about 20 minutes. First of all a bit of diagnosis. No hot air through the vents would indicate that the thermostat is stuck open, engine over heating would indicate the thermostat is stuck closed. In engine temperature problem I would always start by elimination the thermostat as this is usually the first to fail and definately the cheapest!

1. Open the bonnet
2. Remove the radiator cap
3. Put a bucket under the drivers side of the radiator
4. Undo the jubilee clip at the raidator end of the bottom hose, then prise the hose carefully off the radiator and point it at the bucket (this will drain the radiator and the block if your thermostat is stuck open like mine)
5. Undo the jubilee clip at the thermostat end of the bottom hose and carefully prise the hose off the thermostat housing
6. The housing is held to the front of the block by 3 bolts in a triangle shape (all 10mm socket)
7. Undo the three bolts holding the thermostat on and pull it gently towards the radiator to remove (if it was stuck closed more water will come out)
8. Reverse points 7 to 4 for installation and then re-fill the radiator from the radiator cap (about 4.5L).
9. Start the car and bring it up to temperature
10. Whilst running check the coolant level is not dropping too far, and that the thermostat opens (they are 95 degree thermostats) when the temperature guage is roughly just above the middle. (the top hose will be pressurised)
11. Switch the engine off and allow to cool, before slowly unscrewing the little screw next to the radiator cap on the top of the radiator to release any trapped air.
12. Job's a good'un :thumbsup
 

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Not forgetting to bleed the cooling system correctly afterwards, :thumbsup

Along the lines of this.

How to bleed the cooling system.
1.Jack the front of the vehicle up onto stands as high as possible,
2.Turn the ignition on to Aux 2, Engine OFF, heater on fan setting 1 and temperature fully hot,
3.Remove the rad cap/expansion cap,remove the bleed screw from the radiator and IF you have them the bleed screw(s) from the thermostat housing/top hose, return pipe or EGR thermostat.
4.Fill the radiator/expansion tank up and keep pouring till there are no more air bubbles coming out from the bleeds, using a funnel that fits tight in the expansion tank neck and filling it full of water will help push any troublesome air out, when good you can then install the bleeds and tighten down(don't snap them),ensure the rubber O-rings/seals are still on the screws/on the bleed holes.
5.Suck some water out so it's not full to the brim, start the car up and check coolant shoots out of the small hole on the inside of the expansion tank neck when the revs are held up, if it does then replace the cap and then run the car up and check for heat from the vents when the car is warm, then carry on and check if over heating still persists. You may have to bleed troublesome cars again afterwards.(but very rare).
6.If all is good then check the water level again in the morning when cool.

or

Ignition on, heater set to hot, bleed screw out.
Pull the top hose off the radiator. Pour coolant SLOWLY down the rad hose a litre at a time. When the coolant is building up in the hose, hold it above the level of the rad hose stub and keep adding it. When coolant starts to dribble from the rad, refit the hose. You may find that coolant starts to overflow from the rad cap orifice - if it does, just screw the cap back on. Once you've done that, the engine and radiator are full of coolant - the little air pockets that might be left will come through the bleed screw.

After the vehicle is bled you should check that when the revs are held up you get a flow of coolant shooting out of the hole in the inside of the expansion tank neck, this will help tell you if the water pump is circulating, then run the car up and check the lower radiator hose gets hot.
 
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