The cooling system needs to be flushed every now and then to prevent corrosion in the engine and radiator. While ethylene glycol doesn't break down, the additives used to keep water from attacking and corroding metal will wear down. The organic acid additive coolant used by VW should be good for ~5 years/150,000 miles.
In my case, I flushed the system to switch from G12 to Prestone Extended Life - more info here.
Part Information & Sources:
Volkswagen G12 coolant - ~$20/gallon
Prestone Extended Life coolant - ~$8/gallon
Distilled water (preferably reverse-osmosis/deionized water) - 3 gallons
Tools Needed:
- 19mm socket
- Torque wrench
Draining coolant:
- Start by propping up the car on ramps or jack stands.
- Get under the car and remove the noise insulation panel (belly pan).
- Remove the driver side bumper panel to get access to the radiator drain (it just pops out). If there is a flexible piece of plastic in the way, move it aside to access the drain valve. (illustration)
- Remove the coolant tank cap.
- Attach a hose to the radiator drain leading to a collection container. Open the drain valve, allow to drain, and close the valve. (illustration)
- Once the radiator is drained, it's time to drain the engine block. Look under the car and find the water pump drain plug - it's a large 19mm bolt. Open up the bolt, and have a container ready to catch the draining coolant. Clean off the bolt, reinstall and tighten to 22 ft-lbs/30 N-m. (illustration)
Flushing the system:
- The cooling system needs to be bled while refilling to get air out of the coolant passages. Look for two rubber hoses covered by a black plastic sheath to the right of the battery. Pull down the sheath (it's rather annoying to work with) and the hoses should be exposed. Unclamp the left hose with the white mark surrounding a hole in the hose. Pull the hose off the connector until the hole is exposed. (illustration)
- Unscrew the coolant tank and set it as upright as possible - this sets the coolant level above the hose bleeding level. (illustration)
- Set the interior temperature control to maximum heat - this allows coolant to flow through the heater core.
- Fill the system with water until the bleed hole begins to seep.
- Push the bleed hose back onto the connector, close the coolant tank cap, and start the engine.
- Let the engine run for a few minutes until the lower coolant hose becomes hot (until the thermostat opens), and then let it run for a few more minutes. You can speed this up by running the engine at 1800-2200 rpm.
- Shut down the engine. You'll need to wait a while (up to an hour or two) for the engine to cool down before you can drain and refill the system. The issue here is that draining the coolant soon after shutting down the engine leaves the engine hot. If the system is refilled with fresh cold coolant, there is a large temperature differential and the block may warp or crack. It's unlikely, but possible. Waiting a few hours for the engine to cool down is one way to get around this. The other is to simply heat up the replacement coolant before adding it to the system - I'm impatient, so this is what I usually do.
- This cycle can be repeated as much as you'd like - if the old coolant was clean and free of contamination, I just flush with water once. If I'm working on an old car that has scale and gunk in the cooling system, I flush until the water leaving the system is as pure as when it went in.
Refilling the system:
- Drain the system as before.
- Mix up a batch of coolant - use a 1:1 mix ratio of water and coolant. Note that you may need to have a bit more coolant in the mix to compensate for the water that remains in the engine block (even after opening up the radiator and water pump drains).
- Pull out the bleeder hose and refill the system until coolant seeps out.
- Reseat the bleeder hose, clamp it, and replace the plastic sheath.
- Recap the coolant tank and reinstall it.
- Run the engine until the lower coolant hose is hot again. The coolant level can drop a bit as air is bled out of the system. Add coolant until it's at the high mark on the tank if needed.
Done! Check for leaks, then go for a drive...