RS5 Overheating: Bypass Valve May Be The Culprit

Having some experience working on Audi models, I encountered a similar issue with my 2012 RS4 that showed engine overheating symptoms. The temperature gauge behavior and error codes match what you described. In my case, the root cause was actually traced back to a faulty thermostat that had been improperly installed during a previous repair. This created irregular coolant flow patterns, triggering the bypass valve warnings. Since it was a consequential error from the previous work, the workshop corrected it at no cost. Before jumping to conclusions about the bypass valve, could you share: When was your last coolant system service? Are you noticing any coolant leaks under the car? Does the overheating occur more during city driving or highway speeds? Have you checked the coolant level when the engine is cold? The exhaust system can also cause false temperature readings if certain sensors are malfunctioning. A proper diagnostic would help identify if the air intake and cooling system components are functioning correctly. Knowing these details would help narrow down whether your issue matches what I experienced or points to a different underlying problem.

Last service was 2 months ago at 193477 KM. After checking the coolant level this morning, I found it significantly low despite no visible leaks. The overheating occurs mainly during highway speeds, and there is notable power loss when the temperature rises. No puddles under the car, but I noticed a sweet smell from the engine bay. Will check the vacuum system next since several forum members mentioned this could affect the bypass valve function. Thanks for the detailed response about the thermostat, will have that checked too.

Thanks for those additional details. When I had similar symptoms on my RS4, I also noticed that sweet smell, it turned out to be coolant vaporizing through a hairline crack in the coolant reservoir. This type of coolant leak can be tricky to spot since it evaporates quickly and leaves no puddles. The power loss you're experiencing during high temperatures strongly suggests a cooling system issue rather than a bypass valve problem. On these engines, the ECU automatically reduces power output to protect the engine when it detects overheating. The symptoms you describe, low coolant level, sweet smell, and highway-speed overheating, mirror what I dealt with. A pressure test revealed tiny cracks in both the reservoir and a connecting hose that were invisible to the naked eye. The complete repair included: New coolant reservoir, Replacement of upper radiator hoses, System pressure test, Coolant flush and refill, Diagnostic scan to clear codes Total repair cost was around 650 Euro and took about 4 hours. The fuel efficiency improved noticeably after the fix, and the power loss issues completely resolved. Would strongly recommend getting a pressure test done before replacing the bypass valve. This will identify any small leaks in the system that could be causing your coolant loss.

As the original poster, I need to follow up on those findings. The sweet smell is exactly what I'm experiencing. The power loss and highway speed symptoms match perfectly with what you described. Will schedule a pressure test this week, much better to check for these leaks first before replacing expensive parts unnecessarily. The quoted repair cost of 650 Euro seems reasonable considering the complete service. The engine overheating issues got me worried about long-term damage, but it makes sense that the ECU is cutting power to protect the engine. My fuel efficiency has definitely dropped lately which fits with your explanation. Given that my car has similar mileage to your case, the coolant reservoir could be showing age-related wear. Going to document the temperature gauge readings during my next highway drive and have the air intake system inspected while they do the pressure test. Really hoping its just the reservoir and hoses rather than more serious engine cooling problems.