Lupo Diesel Overheating: Bypass Valve Alert

Had the exact same issue on a 2003 Polo diesel. The symptoms were identical, engine overheating warnings and error codes. With some experience working on VW engines, I first suspected the bypass valve too. Took it to my regular workshop for diagnosis where they found the root cause was actually a damaged EGR valve that was causing the exhaust system to malfunction. This created excess heat and triggered the overheating warnings. Because the EGR issue was tied to a previous repair under warranty, fixing the bypass valve would have been unnecessary, they replaced the EGR at no cost. For proper diagnosis on your Lupo, could you share: Full error codes stored in ECU, Does engine stalling occur along with overheating, When exactly did symptoms first appear, Any recent repairs or maintenance done, Current fuel efficiency compared to normal This information would help determine if your case matches what I experienced. The bypass valve may not be the core issue, so getting these details could save you from unnecessary repairs. Worth having a professional check the complete cooling and exhaust system to rule out related problems. Sometimes these symptoms connect to other underlying issues.

Thanks for the detailed first reply explaining potential EGR issues. In my case, the codes are P0217 and P0128. The overheating started about 2 weeks ago after my last service at 16414km. No engine stalling occurs and fuel efficiency remains normal. The coolant leak testing performed showed all connections are tight. I noticed the temperature gauge climbing steadily even during short trips. The vacuum system appears to function normally based on the mechanic's initial check. The warranty expired years ago, so I need to be strategic about which components to investigate first. Wondering if checking the bypass valve would still be worthwhile given these specifics, or if I should look at other cooling system components first.

Thanks for providing those additional details about your Lupo. My Polo had similar codes initially, but your description actually points to a different issue than what I experienced with the EGR. Given that your symptoms started right after the service and you have confirmed no coolant leak, I would focus on the thermostat and radiator circuit first rather than the bypass valve. When my engine overheating issues developed, I learned that post-service problems often tie to disrupted cooling system components. The P0128 code specifically suggests thermostat problems, mine showed the same before the full diagnosis. The steady temperature climb you described, even on short trips, matches classic thermostat failure symptoms rather than bypass valve issues. Some key points to consider: Check if the radiator fans activate at the correct temperature, Monitor if coolant is circulating properly through the radiator, Verify the thermostat opens at the specified temperature, Inspect water pump operation for proper flow The fact that your fuel efficiency remains stable also suggests this is more likely a cooling system component failure rather than an EGR or bypass valve problem like I initially had. My suggestion would be to have the thermostat checked first since its a simpler and less expensive diagnostic step compared to bypass valve testing.

Thank you for the thorough cooling system analysis. I think you make an excellent point about checking the thermostat first, especially since my issues began after the service. I measured the thermostat operation today and found it opens at a higher temperature than specifications suggest, likely explaining the power loss and overheating. This seems more logical than a bypass valve failure given the timing and symptoms. I will prioritize replacing the thermostat and having the radiator circuit properly tested before exploring more complex repairs. The stable fuel efficiency does indicate the problem lies in the cooling system rather than other engine components. Will update once I complete the thermostat replacement to confirm if this resolves the temperature warnings. If not, I can then investigate the radiator fans and water pump as suggested. Appreciate the guidance in narrowing down the most likely cause based on the specific error codes and symptoms.