The brief
The Touran was running hot, the temperature climbing higher than it should, coolant disappearing, the level low, and the engine not running quite right with the odd misfire. He stopped driving it and brought it in, which is exactly right with an overheating engine. That list points at the coolant bypass pipe and connector at the cylinder head. The bypass pipe carries coolant around the head, and on these it's a plastic pipe with a moulded connector that sits under pressure and heat right up against the engine. The plastic gets brittle with age and cracks, usually at the connector or a seam, and it weeps coolant, sometimes onto the engine where it flashes off, so the level drops, the engine overheats, and the heat and the air getting into the system can upset the running. A cracked plastic pipe doesn't reseal, and the crack only spreads, so it needs replacing.
The diagnosis
A pressure test on the cooling system pinpointed it, the cylinder head coolant bypass pipe and its connector were cracked and weeping, losing pressure, which is the overheating and the coolant loss. The radiator, the water pump, the thermostat and the rest of the system held fine. That's a pipe-and-connector replacement, you don't patch brittle plastic, so the call was a complete new bypass pipe and connector with fresh seals.
The work
The cooling system was drained enough to get at it, the old cracked bypass pipe and connector removed, and a new genuine VW-spec pipe and connector fitted with fresh seals and the hose clamps renewed. The system was refilled with the correct VW coolant, bled the proper way so no air pockets were left, and pressure tested again to confirm it held with no weep. A road test confirmed the gauge sat steady through traffic and at speed, no overheating, the running smooth, and the level stayed put.
The outcome
Gauge steady, no coolant loss, the engine warming up on time and running smoothly, and the system holding pressure. The Touran went home with the overheating resolved. A cracked coolant pipe only splits worse, and the failure at the end is a sudden coolant dump and an overheat that can cost a head gasket, so changing the pipe and the connector kept it to a tidy, planned job.