Hi, my VW Golf (2001) is losing coolant through the overflow on the expansion tank? is this a blown head gasket or a faulty cap? Thanks.

Wait for the cooling system to cool down. Remove the radiator cap, start engine and watch for bubbles in the tank. If you have bubbles the possibilities of a bad head gasket is suspected. To verify a suspected head gasket, leave the radiator cap on and tighten. Start the engine, using a heavy towel or rag loosen the radiator cap SLOWLY. If pressure is evident only after a minute of running.....=Blown head gasket!

get a pressure test on radiator via a radiator cap that has a pressure gauge with the engine running . The head gasket is probably blown on the engine and this can cause exhaust pressure to go into the cooling system and create this symptom you are experiencing.
you may also have more expensive problems like a warped head or cracked head . $$$$$ - remachining or welding and remachining of engine head and a blown gasket .

Have a cylinder leakage test performed to check your head gasket. It dosn't have to overheat to have a head gasket problem. A leakage test checks if looseing combustion gases to the cooling system which would presurise the system higher than normal which causes the tank to overflow.

you have a blown head gasket.. doesn't have to have coolant in the oil for a blown head gasket... if hoses are rock hard when running then you have exhaust leaking into the coolant via a blown head gasket...

If there is definately no air in the system and everything is working properly have you changed the pressure cap on the expansion bottle? Very unusual but if everything else is ok then it might be worth a shot. Caps have a pressure release mechanism biult into them.

Are you getting a lot of white smoke from the exhaust? Sounds like it might be a leaking cylinder head gasket.

replace the oil cooler first before you change the head gasket ,but normally a 10 year old vehicle often has gasket fatigue ,not made like a good old fashioned copper head gasket ,they rarely went wrong

you either have a blown head gasket or cracked head both reasons will put compression gas into coolant system since air gets hotter faster than coolant any air in system will cause very fast over heating and push coolant out. the ford 3.0 is has a record of this problem the fix is not cheap and if you do it your self or have a shop do it both heads should be done and the t stat also replacing water pump would be wise

WELL FORD HAS THE WAY THERE BUY PASS HOSE WORK YOU MAY NOT SEE IT GET HOT IN TILL ITS TO LATE I WOULD START AT THE THERMOSTAT AND SEE WHAT HAPENS IT COULD BE HEAD GASKET TO HOLD THE RAD HOSE SEE IF ONE COOLER THEN OTHER GOOD WAY TO TELL IF THERMOSAT IS OPENING DO YOU SEE AIR IN OVER FLOW WHEN RUNNING THAT MAY BE SING OF HEAD GASKET WELL HOPE THIS HELPS MAKE SHERE CHECK RAD IF HAS ANY COOL SPOTS IT COULD BE PLUGED

That's what it sounds like, a blown head gasket. Every time the piston compresses it pushes the air fuel mixture into your cooling system through the broken head gasket. Try running the engine with the radiator cap off or lose and look and smell for gas in your radiator overflow tank. Good luck and hope this helps.