2003 Volkswagen Passat Coolant Runs Out Too Quickly

If the coolant is running out, where is it running from? If the leak is too small to tell, you can add a small bottle of dark purple dye to it, then search a little while later with a black light. The dye will show up as a bright yellow stain that you can follow back to the source.

With no visible leak, the most likely suspect is a leaking cylinder head gasket. There's three tests that can verify that. The first one is that dye I mentioned previously. If the head gasket is leaking, you'll find traces of the dye inside the tail pipe.

The second test involves drawing air from the radiator, while the engine is running, through a glass cylinder with two chambers partially-filled with a special dark blue liquid. If combustion gases are present, the liquid will turn bright yellow. Usually you have to find a mechanic with that tool, but if you have an auto parts store that rents or borrows tools, they will likely have it, but they typically will make you buy a bottle of the fluid. That's because antifreeze will contaminate it. That, and freezing will render it inoperative. To insure against that, they make you buy your own bottle of fluid.

The third test is the cylinder leakage test. That involves forcing compressed air into the cylinder through the spark plug hole, with a test gauge, then looking for signs of air escaping from four places. If the head gasket is leaking, you'll see bubbles in the radiator.