is this a petrol or diesel? Does the engine only overheat under load or all the time
as you say that the AA say there are no exhaust fumes in the coolant then it is doubtful that the head gasket has gone.Make sure there are no leaks in the system. Check the expansion tank neck and cap for any damage, make sure the seal on the cap is OK. If the cooling system cannot hold pressure then the coolant will boil at a lower temperature than it normally should. Check the thermostat this is operated by the engine management and opens wider at higher engine temperatures,this means under load the engine management opens the thermostat to allow more coolant through therefore keeping the engine at a normal temperature if the electrical part fails then the car will boil under load.
Once the coolant boils the pressure in the cooling system becomes too high and the expansion tank cap releases pressure causing the coolant to be blown out of the system.
With the thermostat out you may be able to feel the water pump impeller to make sure it hasn't come away from the pump shaft. In my experience quite a few thermostats tend to fail on this engine.
With regard to the radiator fan some cars have 2 fans some have 1 fan and a resistor to control the fan speed. If the gauge is in the red then you should hear the cooling fan quite easily as it should be going at high speed.
The lights dimming slightly can just be the electrical load changing on the alternator for example the cooling fan or air con. compressor engaging.
Let me know if you require any more assistance.
the water pump and thermostat are both located on the drivers side of the engine.
The thermostat is a little awkward to get to you need to remove the drivers headlamp to get the best access.To remove headlamp remove the front grille (1 clip near each headlamp that turns half a turn then lift up,the grille then unclips from the bumper as you pull it forward).Then pull up the two slides to release the headlamp. You can now see the thermostat just under the inlet manifold(2 hoses and the electrical connector go to it). 8mm headed bolts bolt it to the engine block,you can access it all through where the headlamp was.
The water pump is a little easier just remove the fan belt,unbolt the pulley then unbolt the water pump from the block( you may need to remove the drivers side engine mounting for better access).
As I said previously you may be able to feel the water pump impeller through the thermostat opening so saving you removing the water pump.
If you remove the water pump or thermostat you will need to replace any O rings or gaskets.
if you remove the thermostat you may be able to feel the impeller,this varies from engine to engine and I cannot remember off hand if you can on this engine but as you are removing the thermostat anyway you aren't loosing anything.
As I said previously most cooling faults on this engine are normally thermostat related.
Yes you need to access the thermostat through the headlamp aperture.
The water pump you shouldn't need the headlamp out to access.
If you are still having the same problem after the thermostat is replaced and the water pump is OK as long as there are no leaks you may well have a cylinder head or gasket fault,but as you said the AA say there are no exhaust gases in the coolant this appears unlikely.
If the cylinder head gasket has blown then you will find pressure in the cooling system very quick after start up.
Does the cooling system pressurize quickly and can you see excessive gas bubbles blowing in to the expansion tank with the engine running?
Let me know how.
a thermostat can fail open or closed, if the thermostat has failed open then yes the car would take a long time to warm up. If the thermostat fails to open or doesn't open enough then you will get an over heat condition,this engine has a 2 stage opening thermostat so it is possible for the thermostat not to open enough.
With regards XXXXX XXXXX head gasket,if the cooling system pressurizes within a minute or so from cold start when the pressure has been released previously this would point to a head gasket failure. You may also get signs of oil in the cooling system or signs of water in the oil but not always it depends where the gasket has failed (near a water jacket or oil gallery),if it has only failed between a cylinder and the water jacket then you get the pressurized cooling system but no sign of any oil.
Garages have the correct test equipment to check for exhaust gas in the cooling system so you can be certain of the result if necessary.
the ECT (engine coolant temperature)sensor is the sensor that sends the engine temperature to the engine management ECU.
If this sensor fails (open circuit or invalid reading) then as a default the cooling fan would switch on,the car would lose power and the engine management warning light would be on, but as the temperature gauge is working OK and it uses the signal from this sensor I don't think this is the fault.
As far as I'm aware the first stage of the thermostat opening is as a standard wax stat it is only at higher temperatures the second stage comes in to play.
sorry you're still having problems. Is the car still running OK? How fast is it overheating? When the thermostat was changed I assume the anti freeze strength was topped up again? Did the AA check for exhaust gases in the coolant again?
If the car is not running and blows water out when turning the engine then I would say that this is a head gasket failure.
Your local dealer should be able to confirm this straight away.
Let me know if you require any more help.
Air coming from the engine after it has been topped up would be a small amount. If it is coming out with enough force to force water out of the expansion tank then this would still point to a head gasket failure. The dealer is only trying to warn you of any potential problems that they may come across,they don't want to give you a price to find when the head is removed you then need to pay a significant amount more than quoted.As to actually proving it is the head gasket you can only get to being 99% sure this is the problem, but to confirm you will only do this by removing the head and inspecting.
The engine has a timing chain. You need tools to lock the camshafts and to locate in to the engine block to rotate the crankshaft up to to get the engine to TDC.