If your VW Golf’s coolant temperature reaches around 90°C and the engine starts revving and eventually cuts out, it could indicate an issue within the cooling, fuel, or electronic control systems. Here are some common causes:

1. **Faulty Coolant Temperature Sensor**
– The coolant temperature sensor measures the engine’s temperature and relays this information to the engine control unit (ECU). If the sensor is faulty, it can send incorrect temperature readings, causing the ECU to adjust fuel and air mixtures incorrectly, leading to engine stalling or revving issues.
– Symptoms of a failing temperature sensor can include rough idling, unexpected revving, or cutting out when the engine reaches operating temperature.

2. **Idle Control Valve or Throttle Body Issues**
– If the idle control valve (or throttle body in some models) malfunctions, it can cause abnormal idling or revving. This part regulates the air entering the engine when the throttle is closed. Once the engine reaches operating temperature, a faulty idle control valve might cause the engine to idle higher than normal or even stall.
– Cleaning or replacing the idle control valve or throttle body may resolve this issue.

3. **Air or Vacuum Leaks**
– When the engine reaches operating temperature, air or vacuum leaks can affect the air-fuel mixture, causing high idling, stalling, or cutting out. Rubber hoses and gaskets can expand as they heat up, exacerbating any existing leaks.
– Inspect the intake manifold, vacuum hoses, and any seals around the throttle body for possible leaks.

4. **Faulty Fuel Pump or Fuel Pressure Regulator**
– If the fuel pump or fuel pressure regulator is malfunctioning, it may struggle to maintain proper fuel pressure when the engine heats up. This can lead to stalling or cutting out as the ECU attempts to compensate for improper fuel delivery.

5. **Engine Control Unit (ECU) Issues**
– In some cases, a software issue or malfunction in the ECU could cause erratic behavior, like revving or stalling, especially when combined with sensor or throttle issues. Running a diagnostic scan can help identify if the ECU is causing or contributing to the problem.

**Recommended Steps**
– **Run a diagnostic scan**: This will help identify if any error codes are associated with sensors, idle control, or fuel system issues.
– **Inspect for leaks and faulty parts**: Check hoses, sensors, and the idle control valve.
– **Seek professional help if needed**: Especially if the issue persists, a mechanic can check for deeper electronic or fuel system malfunctions.

Each of these issues can be quickly diagnosed with a scan tool, which can help pinpoint the root cause and allow for efficient repairs.