VW Passat Hybrid Coolant Mysteriously Disappearing

Having worked on a 2018 VW Golf GTE Hybrid with a similar issue, I can share my experience as someone with moderate car repair knowledge. The symptoms were nearly identical, check engine light and mysteriously disappearing coolant without visible leaks. After monitoring the cooling system, air pockets were discovered, caused by a faulty sensor plug connection near the water pump assembly. The repair involved: Complete system diagnostic, Cooling system pressure test, Replacement of damaged wiring and connector, Bleeding the cooling system to remove air pockets, Verifying proper engine cooling fan operation Total cost was 95€ for parts and labor. The job took about 2 hours at the workshop. Since the fix, coolant levels have remained stable and the engine overheating warnings stopped. To better assist with your situation, could you share: Current mileage, When did the coolant loss start, Any unusual engine temperature readings, Whether the cooling fan runs normally, Previous cooling system repairs These details would help determine if your issue matches what I experienced or if it might be something different.

Thanks for your input. My mileage just hit 84271km after my recent service last week. The cooling fan seems to run erratically, and I can hear an unusual gurgling sound near the radiator. I will definitely have the pressure test done to check for air pockets in the system. Did you also notice any fluctuating engine temperatures before finding the faulty sensor? The coolant level started dropping about two weeks ago, and I have to top it up every few days. The head gasket seems intact since there is no white smoke from the exhaust, but I am concerned about possible internal leaks that might not be visible from outside. What specific pressure readings should I expect during the test to confirm everything is working properly?

Thanks for the additional details. Yes, I experienced similar symptoms on my Golf GTE before discovering the issue. The gurgling sound you mention is often a telltale sign of trapped air pockets in the cooling system, which I also noticed in my case. The pressure test should maintain around 15-18 PSI (1-1.2 bar) for at least 15-20 minutes without dropping. When testing my Golf, anything below these values indicated a problem. The erratic cooling fan behavior you describe matches what I encountered before the repair, it was caused by the system getting confused by incorrect temperature readings from the faulty sensor. Before replacing the sensor and connector, my temperature gauge would occasionally spike, then return to normal. The gurgling near the radiator cap happened especially after cold starts, which was air getting trapped in the coolant reservoir. A few quick checks you can do: Inspect the coolant reservoir cap for proper sealing, Look for tiny bubbles in the coolant when engine is running, Check if the water pump pulley has any wobble, Listen for the gurgling sound particularly when the engine is warming up The good news is that since you are not seeing white exhaust smoke or getting extreme temperature readings, it likely matches the sensor issue I had rather than a serious internal problem.

Thanks for sharing those details. My symptoms seem very similar to what you experienced. I will take your advice and get a proper pressure test done, watching for those 15-18 PSI readings. The fact that your repair was relatively minor and affordable gives me hope that my coolant leak might also have a simple fix. The gurgling sound in my radiator does get worse during cold starts, just like you mentioned. Based on what you described, I will have the shop thoroughly check the water pump area and coolant sensor connections. I am relieved there is a good chance this is not a major engine overheating issue or failed head gasket, considering I am not seeing any other warning signs like white exhaust smoke. Would you recommend I continue topping up the radiator fluid until I can get it into the shop, or should I limit driving to prevent potential damage? Also wondering if there is a specific brand of coolant I should use for the top-ups in the meantime?