How to replace thermostat

Is this a thermostat change your doing? Easiest way to find the thermostat housing is to use the radiator hoses. Look down on the radiator. Find the largest hose nearest to you on the top corner of the radiator. Follow this back towards the engine. There should should be a round 'Half ball' shaped mount with two bolts the pipe is attached to. I'm not sure for the Beetle but you might need to remove the alternator to get to it.
Bleeding the system is reasonably easy. Fill the coolant to the Max mark and run the engine for about 30 seconds. Turn the engine off and walk away for 10 minutes. This gives the coolant level a chance to settle. Check the coolant has not dropped below the 'MIN' mark. Start the engine and let it idle up to working temperature, About 10 to 15 minutes. This will get the thermostat to open and allow the radiator to refill. The top pipe to the radiator should become hot to show it's open. Turn the engine off and again walk away for 10 minutes. Coolant systems become pressurised when running so this lets it settle and cool down a bit. Recheck your coolant level and top up as necessary.

Don't drive the vehicle until you get the vehicle checked. Running a engine that is overheating can do serious damage. By serious I also mean Very expensive!
Did you replace the thermostat in the cooling system when you replaced the coolant.
Try this simple test.
**Note** You need to put your hands near the engine when it is running. BE CAREFUL you do not touch Moving or Hot parts of the engine.
With the engine Cold, open the bonnet and find the very large black hose going to the top corner of the radiator. -Carefully touch the hose. It should be cold. Start the engine and let it idle for 5 minutes.
-CAREFULLY touch the large hose. It should be starting to get warm. Let it idle for another 5 minutes.
-Again CAREFULLY touch the large hose. It should be HOT to the touch.
If the hose does not heat up the thermostat could be faulty.
-Carefully touch the hose. It should be cold. Start the engine and let it idle for 5 minutes.
-CAREFULLY touch the large hose. It should be starting to get warm. Let it idle for another 5 minutes.
-Again CAREFULLY touch the large hose. It should be HOT to the touch. If the hose does not heat up the thermostat could be faulty.

Fill the system with coolant squeezing the rad hoses to expel some of the air If u can't find a bleed plug then run engine at fast idle say around 2500 rpm turn the heaters on hot put fan on with out air on when the heaters are getting hot the water is circulating check the coolant level top up as required if heaters still hot place the coolant cap on and run until the the engine is up to temp or thermostat opened fan works as it should , turn off the engine when all the pressure has gone remove coolant cap top up and leave it off until cold most if not all the air will be gone

Possible trapped air pocket. Set to full heat position, lowest fan speed, remove rad. cap, start engine and let idle for 10 minutes, then hold gas pedal at 1500rpm for two minutes then let idle for two minutes. Shut engine off , let cool for 30 minutes. Top up coolant level and reinstall cap. Drive vehicle, should get heat now.

CODE PO128 - ECT SENSOR BELOW THERMOSTAT REGULATING TEMPERATURE CONDITIONS.POSSIBLE CAUSE LOW COOLANT LEVEL CRANK ENGINE UP LET IDLE 10 TO 15 MINUTES.CHECK BOTH RADIATOR HOSES IF BOTH ARE HOT .ECT SENSOR FAULTY.IF IT TAKE RADIATOR HOSES LONG TIME TO GET HOT.THERMOSTAT STUCK OPEN THERMOSTAT NEED TO BE REPLACED.

Are you sure that the spring side of the thermostat is toured the engine/ look for a bleeder near the top highest spot in a line or hose that may be able to release the air it may even be near the water pump side.on some water pumps for the ford's the pumps will bolt on and look almost the same but the fins on the impeller are the opposite.clock wise instead of counter clock wise or vice vars. you my wont to check with your parts supply store to see if that is possible.hope this helps give you some ideas.feel free to ask again if you would like any further assistance.thank you for asking fixya . Have a Grate Day

You say it's overheating, more info needed such as:
Is the engine temp gauge showing that it's overheating?
Is it actually 'boiling over' and losing coolant?
If it's running hot, but not losing coolant, check to see it the cooling fan is operating when the engine temp rises. You may have a bad cooling fan switch that is not signaling the fan to go on to lower the engine temp.
It could be a stuck (closed) thermostat that is preventing proper circulation of coolant after the car reaches operating temp.

The thermostat could be stuck closed.
The worst case is you over heated the engine to the point you could have internal damage to the engine . I would change the thermostat first. IF that does not work , I would take to a shop to have checked out.

When you remove your thermostat check the number stamped on the thermostat. In cold weather you want a high number and opposite cold weather. Right back with number

if its the thermostat your lucky easy fix just replace it but wait until the engine is cold the bolts are a nightmare to get to you have to take off air hose and disconnect a couple coolant hoses move them aside to get to the thermostat housing on the side of the engine tricky getting to that bolt.

if its the head gasket flush the radiator with flushing agent
first drain the coolant from bottom radiator hose replace with flushing agent run car idle about 10 minutes then
remove the thermostat replace the thermostat housing then fill the coolant back up leave room for blue devil head gasket sealant about $65.00 follow the directions to the tea run for 50 minutes if it doesn't work that baby needs to get in the shop
hope this helps don't forget to rate if I was helpful.
good luck