VW Sharan Diesel: Erratic Temperature Mystery Solved

I dealt with nearly identical symptoms on my 2008 VW Touran 2.0 TDI last summer. Having some experience with cooling system issues, the symptoms you describe immediately sound familiar. After monitoring the erratic temperature behavior for a few weeks, I finally took it to my regular workshop. The diagnostic showed a defective map thermostat, which was causing inconsistent coolant flow regulation. This explained both the overheating and the slow warm-up issues you mentioned. The repair involved replacing the thermostat unit and a complete cooling system flush. Total cost was 350€ including parts and labor. Since the fix, the temperature has remained perfectly stable. Before jumping to conclusions though, could you share: Current mileage? Any warning lights on the dash? Have you noticed any coolant leaks? When was the last cooling system maintenance done? This information would help confirm if your symptoms match what I experienced. While the map thermostat is often the culprit with these symptoms, its worth checking the water pump and cooling fan operation as you mentioned. A proper diagnosis is essential since multiple cooling system components can cause similar symptoms.

Last service was done 3 months ago where I checked transmission fluid and brake pads. Current mileage is 62523km. No warning lights appear on the dash. The temperature sensor readings fluctuate though, making it hard to tell if the coolant system is functioning properly. I have been monitoring closely and noticed small puddles under the thermostat housing area, suggesting a possible coolant leak. I will check the cooling fan operation as suggested and get the system pressure tested before proceeding with any repairs. Thanks for sharing your repair experience and cost estimate, helps set expectations if I need the thermostat replaced.

Thanks for the additional details. Those coolant puddles near the thermostat housing area are definitely concerning and match what I saw on my Touran before the repair. The fluctuating temperature sensor readings combined with the visible leaks strongly suggest a failing thermostat assembly. My engine overheating issues started similarly with small leaks. The coolant loss gradually got worse until the temperature variations became more extreme. A pressure test confirmed the thermostat housing was cracked, allowing coolant to seep out. For reference, after fixing the thermostat and sealing the housing properly: Engine now warms up consistently in about 5-7 minutes, Temperature stays stable even in traffic, No more coolant loss, Temperature sensor shows steady readings Given your relatively low mileage, its worth having the entire cooling system inspected while they replace the thermostat. In my case, the cooling fan and water pump were still in good condition, but its best to verify all components while the system is open. Keep monitoring those coolant levels closely until you can get it repaired. Low coolant from the leak could lead to more serious engine overheating issues.

Based on your detailed feedback, I think youve nailed the diagnosis. The coolant leak near the thermostat housing matches exactly what you experienced. My temperature control issues and the small puddles are getting worse, just as you described. I scheduled a full cooling system inspection for next week. Will have them pressure test everything and replace the thermostat assembly. The 350€ estimate helps with budgeting. Good to know the water pump wasnt involved in your case, though I understand why they should check it while doing the work. Really appreciate your time explaining how the repair resolved similar symptoms on your vehicle. Will make sure they do a complete system flush as you suggested. Having the engine warm up properly and maintain steady temperature readings will be a relief after dealing with these erratic overheating issues. Thanks for helping me avoid potentially more expensive engine damage by addressing this promptly. Will update the forum once the repairs are complete.