VW Polo Overheating Linked to Faulty Bypass Valve

As someone with some experience maintaining diesel engines, I had a very similar issue with my 2011 VW Golf TDI. The engine overheating symptoms matched exactly what you describe, including the power loss and temperature fluctuations. After investigating, it turned out the bypass valve issues were actually a secondary problem. The root cause was a clogged EGR valve that created excessive exhaust system pressure, which then affected the bypass valve operation. Since the EGR valve was replaced under a recall campaign, fixing the bypass valve required no payment. The engine overheating stopped completely after this repair. The mechanic explained that these issues often cascade, one component failure can trigger problems in connected systems, especially in the cooling circuit. To better assist with your specific case, could you share: Current mileage, Any previous cooling system repairs, Whether you notice any coolant leak, What specific error codes are showing, If the temperature gauge behavior is consistent or intermittent This information would help determine if your situation matches what I experienced with my Golf, as these VW diesel engines often share similar weak points in their cooling systems.

Thanks for the details. My mileage is at 120075km and last service was done 3 months ago. The temperature gauge behavior is definitely erratic and coolant levels keep dropping, though no visible leaks under the car. Started seeing these issues about 2 weeks after the service. The power loss seems most noticeable when the engine is under load, which affects fuel efficiency significantly. The air intake and exhaust system seem fine based on a quick visual check, but I am seeing error code P0401 on the scanner. Really hope this helps narrow down the problem as I want to get this sorted before it causes more damage.

Based on your symptoms and especially the P0401 code, this definitely mirrors what happened with my Golf TDI. The code specifically points to insufficient EGR flow, which matches my earlier diagnosis about the EGR valve being the root cause of the overheating issues. Since you mentioned the problems started after service, have the mechanics check if the air intake system was properly reassembled. Sometimes a loose connection can trigger similar symptoms. The dropping coolant levels without visible leaks often indicate internal consumption, which can happen when the EGR system isnt functioning correctly. My repair involved: EGR valve replacement, Bypass valve inspection and cleaning, Full cooling system pressure test, Air intake system check Total cost was around 450€ for parts and labor, but yours might vary depending on what exactly needs replacing. The power loss improved immediately after the repair, and fuel consumption returned to normal within a few tanks. Would strongly recommend getting this checked soon, as running with insufficient EGR flow can lead to more serious engine overheating issues. The bypass valve might not be the primary culprit here, just like in my case.

Thanks for the input. Really helpful to hear about similar experiences, especially with the EGR valve connection which I hadn't considered. After checking all the symptoms you mentioned against mine, it does sound like the same issue. The erratic temperature behavior and power loss under load match exactly what you described. Will get the car to a shop this week to have them check both the EGR and bypass valve. The 450€ repair cost sounds reasonable if it fixes both the overheating and power loss problems. Good to know the vacuum system and air intake need checking too since they might have been disturbed during the last service. Will post an update once I get the diagnosis and repair done. Hoping it turns out to be just the EGR valve like in your case, rather than something more serious in the cooling system. The error code matching your experience gives me more confidence about what to expect.