This is a photo guide on replacing the thermostat housing on the 2.0 FSI engine. In my case this is on a 2004 Golf MK5. The photos and descriptions aren't comprehensive but my aim here is to just give you a good idea on what's involved. Symptoms In my case the thermostat was probably stuck partly open. The coolant temperature was poorly regulated and would be very slow to reach operating temperature (centre point on gauge), if it ever did. Procedure Remove the top cosmetic engine cover. Next, we pull this cover upwards to remove, unclipping any tubes that might be secured to it. Remove this pipe. Remove this vacuum tube from nipple shown. Inspect the end of the vacuum tube for splits and cut it back or replace completely if damaged. In my case I found that a few of these vacuum tubes were split at the ends. Remove. Unclip and retract the airbox ducting from the electronic throttle body. The plastic inlet manifold assembly needs to be detached from the inlets on the cylinder head. In my case this is simple because the previous owner has replaced the factory items with Jubilee Clips. If you still have original clips fitted then you may need the appropriate hose clamp tool. It is now necessary to remove the rigid pipe that connects between the exhaust manifold area and the inlet system. (I don't know the purpose of this pipe at the moment, so I will assume that it is something to do with recirculating exhaust gas for emissions control.) There are two bolts securing the flange at each end. At the exhaust manifold end, there is a little gasket which you must be careful not to lose. The pipe is also secured to the head with a bolt. You must remove this pipe completely. Do not attempt to remove the inlet system by just forcing this pipe out of the way slightly. This is the gasket you'll want to avoid losing. Lift pipe out completely. Remove that. Unplug. Unclip. Remove the two small bore coolant hoses from the throttle body assembly. Unplug connector. Remove. By this point you should now be able to lift up and remove the entire plastic inlet system including the throttle body. To do this, there are also a couple of securing nuts for it right at the front near the radiator. I did not photograph these. The next step is to start removing bolts that secure the rigid coolant pipe going around the block. This is necessary because it needs to come forward a few inches to allow the thermostat housing to be retracted. The next nuisance is that you need to get under the front, remove the undertray, and look upwards at the metal bracket that secures the coolant pipe and various other bits to the block. You need to remove a bolt that secures the coolant pipe to the bracket. You then need to remove two bolts which secure the bracket to the block. This allows the bracket to be moved forward an inch or two to make room for thermostat housing replacement. Wish you'd not started this job yet? Now pull this sucker upwards and out. Use a spanner to rotate the alternator belt tensioner and use an Allen key to lock into position. Remove the two large bolts that secure the alternator in place. Lift alternator out. There is no need I found to disengage the wiring from the alternator because you can just rest the alternator nearby out of the way. Just ensure the battery is disconnected. Retract the rigid coolant pipe from the thermostat housing, and bring that pipe a few inches forward. Remove the two bolts securing the thermostat housing to the block. Bring the thermostat housing forward. Disengage the coolant hose entering the top of the thermostat housing. The coolant hose entering the bottom of the thermostat housing uses a push-fit connector. Remove this metal clip and then use some force to disengage the thermostat housing from the connector. Now the old thermostat housing is removed, as you can see on the lower left. At this point I'm afraid my pictures finish, but from here it is essentially the procedure in reverse to reassemble everything. Before fitting your new thermostat housing, I suggest lightly sanding the area on the block that the large O-ring seals against, just to remove powdery coolant residue and corrosion.