VW Polo Diesel: Mixed Temp Signals Point to Bad Thermostat

Had the exact same cooling system issues with my 2004 VW Golf TDI. The engine overheating warnings were unusual, and cold starts would take unusually long to reach operating temperature. The thermostat housing was indeed the culprit. The thermostat was stuck partially open, causing inconsistent coolant flow. This explained both the slow warm-up and periodic overheating. The coolant leak around the housing gasket was another telltale sign that became apparent during inspection. The repair involved replacing the entire thermostat housing assembly since these parts tend to become brittle over time. While not a critical emergency, it needed attention within a few weeks to prevent potential engine damage. The job also included a coolant system flush to remove any debris that could have accumulated. The cooling fan operation should also be checked when diagnosing temperature regulation issues. In my case, the fan was working properly, but its a common related failure point with these symptoms. Would strongly recommend getting it checked soon. These temperature fluctuations can lead to increased fuel consumption and potential engine wear if left unaddressed.

Thanks for sharing your experience with the Golf TDI. The symptoms you described match mine exactly, my Polo also struggles with temperature control between cold starts and overheating warnings. I got an appointment for next week to check the thermostat and cooling system. The cooling fan seems to work fine, but I will have them inspect it too just to be safe. Quick question, do you remember roughly how much you paid for the thermostat housing replacement and system flush? Also, has everything been running smoothly since the repair?

Yes, I got the thermostat housing and cooling system fixed about 6 months ago. The repair cost came to 350€ total, which covered replacing the defective map thermostat and a complete system flush. The temperature control has been perfect since then, no more engine overheating warnings or slow warm-ups. The mechanics found the thermostat cooling circuit motor was defective, which explained all the issues. Once replaced, the coolant flow regulation returned to normal. The repair was worth every cent since proper engine temperature management prevents long-term damage. The fuel consumption also improved slightly after fixing the temperature sensor system. Most VW models from that era tend to develop similar thermostat issues around the 250000km mark, so the timing fits with your Polos mileage. Getting it fixed now will likely save you from more expensive repairs down the road. Just make sure they do a thorough coolant leak check while theyre at it. These temperature regulation problems can sometimes hide small leaks that should be addressed at the same time.

I took my Polo to the mechanic yesterday and got everything sorted. The thermostat housing was indeed faulty, just as suspected. The total repair came to 380€, which included replacing the thermostat, a new temperature sensor, and a complete coolant system flush. The mechanic showed me the old thermostat, it was stuck partially open, explaining both the slow warm-up and overheating issues. They also found a small coolant leak at the housing gasket that needed fixing. The car runs perfectly now. The temperature control is steady, warm-up time is normal, and no more warning lights. The engine even feels smoother overall. Definitely worth getting it fixed before it could cause any serious damage. Thanks again for the advice. Without it, I might have waited longer to get it checked, potentially risking more expensive repairs.