VW Tiguan Overheating: Faulty Bypass Valve Alert

Having some experience working on VWs, I faced a similar issue with my 2011 VW Golf GTI. The symptoms you describe match what I encountered, engine running hot and fault codes stored in the system. In my case, the bypass valve wasnt actually the primary issue. The root cause was a failing thermostat that wasnt detected initially. This created a chain reaction affecting the cooling system, which then caused the bypass valve to malfunction. My mechanic identified this during a diagnostic check of the entire cooling system. The workshop fixed it under warranty since the thermostat failure was traced back to a previous repair they had done on the coolant system. They replaced both the thermostat and bypass valve as a precautionary measure, ensuring the vacuum system was properly tested afterward. To better assist with your situation, could you share: Current mileage on your Tiguan, Any recent repairs or maintenance done, Specific error codes stored in the system, Whether you notice any coolant leak or loss, If the temperature gauge behavior changes during different driving conditions These details would help determine if your issue follows a similar pattern to what I experienced.

Thanks for the detailed response. My Tiguan has 136627km on the odometer, with the last service completed just 2 months ago. The engine stalling issues started appearing right after that service. Initially, I thought it was related to the vacuum system since I noticed some unusual sounds, but your mention of the thermostat makes a lot of sense. I have noticed the temperature gauge behaving erratically, especially during highway driving. I havent observed any coolant leaks, but the coolant level has dropped slightly over the past weeks. As for the error codes, I will need to get them scanned again as the previous readout was cleared during the service. Would you recommend getting both the thermostat and bypass valve checked, or should I focus on the thermostat first? I want to avoid unnecessary repairs if possible.

Based on what youre describing, I strongly recommend getting the thermostat checked first. When I had this issue with my Golf GTI, the symptoms were nearly identical, erratic temperature gauge readings and gradual coolant loss without visible leaks. The coolant level drop combined with the timing of issues after service is particularly telling. In my case, air getting into the cooling system during the service contributed to the problem, creating air pockets that affected the thermostat function. Having gone through this, heres what worked: Start with a pressure test of the cooling system, Have the thermostat operation verified while the system is under pressure, Check the air intake and vacuum system connections near the coolant passages My repair costs were around 380€ for the thermostat replacement including labor. The bypass valve was an additional 220€, but in hindsight, I could have probably waited on that since the thermostat fix resolved most issues. The engine stalling you mentioned could be related to the cooling system problems affecting the engine management. Once my thermostat was replaced, both the overheating and occasional stalling completely disappeared. If you want to monitor the situation before taking it to a shop, watch for any correlation between the engine stalling and temperature spikes. This helped diagnose my issue more accurately.

My Tiguan symptoms are very similar to what you experienced. The correlation between temperature spikes and stalling is exactly what Im seeing. I checked this morning and the coolant level has dropped another few millimeters, though still no visible leaks. I will take your advice and focus on the thermostat first. The 380€ estimate for thermostat replacement seems reasonable compared to replacing multiple parts without certainty. Your point about air pockets from the recent service really hits home, the problems did start right after that work was done. I made an appointment at a local shop for next week. They will do the pressure test and thermostat check first before suggesting any further repairs. This seems like a smarter approach than jumping straight to replacing the bypass valve. Will report back once I have the diagnostic results and know if the thermostat is indeed the culprit behind these power loss and overheating issues. Thanks for helping me avoid potentially unnecessary repairs to the exhaust system and other components I was worried about.