The cooling system in your Volkswagen consists of a number of interconnected parts that direct coolant flow through the engine. If any of these parts fail to perform their given function, your engine temperatures will climb to unsafe levels, which will then show on the gauge on your dash. Blockages can also cause the temperature reading on the gauge to spike. Allowing your engine to overheat for any length of time could damage the block and head in addition to the gaskets between the metal mating surfaces. As a result, if you notice your car starting to overheat, it is wise to immediately turn off the engine until you can track down the problem. You can inspect and replace the three following components to restore the functionality of your cooling system.
When you first start your engine, the coolant does not fully cycle through all of the cooling system components. Instead, the thermostat blocks flow until your engine reaches an appropriate operating temperature. At that point, the valve opens, allowing coolant to flow into the radiator for immediate cooling. If the valve on the thermostat fails to open, the coolant stuck in the head and block will continue to rise in temperature.
You can pull your thermostat out of the housing, which is often attached to the bottom hose of the radiator, and perform a quick test. To make sure the thermostat opens and closes properly, you simply boil it in plain water for 15 seconds and watch for movement of the valve. If the valve doesn't open, replace the thermostat with a new one.
Most Volkswagen vehicles feature a contained cooling system that keeps air from entering the lines and passageways. You may notice that your overflow tank attaches directly to the radiator cap. Therefore, you can only fill up the radiator by pouring a 50/50 mix of coolant and water directly into the overflow tank. If the two-piece overflow tank separates enough to allow air into the system, your car will overheat.
The overlapping sides of the tank will rarely separate enough to allow fluid out, just air, so this problem is difficult to spot with a visual inspection. You can test the system by releasing air accumulation using the coolant bleeder valve on the head. If you notice steam rushing out of the head, you should plan on replacing the overflow tank immediately.
Oil Cooler Lines
Your Volkswagen utilizes an oil cooler to drop high lubricant temperatures before damage can occur. The oil cooler processes both coolant and oil to perform this important function. If the oil cooler lines grow brittle from constant heat transfer, a leak could develop. Leaking coolant from these lines is quite difficult to spot at first, as it seems to just briefly drip when the car reaches operating temperature. Any damage to the oil cooler lines allows air to enter the system and leads to high fluid temperatures.
To accurately spot leaks, and pinpoint their exact origins, you must warm up your engine in the driveway until it reaches at least halfway on the gauge. At that point, turn off the vehicle and slide a piece of cardboard under the engine bay. Come back out in 10 minutes to see if you can spot any fluid drips on the surface of the cardboard. You can look above the leak point to find the parts or lines allowing fluid to escape.
Sourcing The Problem
If you cannot find or resolve the cause of the overheating, bring your car down to a local Volkswagen service center for help. Technicians skilled in diagnosing and repairing these unique vehicles know just where to look for advanced problems, such as faulty internal parts or passage obstructions. Volkswagen technicians also have special tools on hand for the required testing and part replacement procedures. Once your tech identifies and fixes the cause of your overheating issue, you will notice your temperature gauge rarely reaches halfway, even while sitting in traffic.