* Corrosion: Windshield washer fluid is primarily water with added detergents and, importantly, often methanol or isopropyl alcohol. These alcohols are not compatible with the materials in your engine's cooling system (hoses, seals, water pump, etc.). They can break down these materials, leading to leaks and potentially catastrophic engine damage. The detergents aren't designed for the high temperatures and pressures inside the cooling system either.
* Reduced Coolant Effectiveness: The washer fluid will dilute the coolant, reducing its ability to transfer heat and maintain the correct operating temperature of the engine. This can lead to overheating, potentially causing serious engine damage, including warping the cylinder head or cracking the engine block.
* Pump Damage: The washer fluid's different viscosity and properties can damage the water pump.
* Seals and Gaskets: The chemicals in washer fluid can attack seals and gaskets, leading to leaks.
In short, don't do it. If you've accidentally done so, flush your cooling system immediately with clean water, then fill it with the correct coolant mixture for your vehicle. This might require professional assistance, depending on how much washer fluid got in and how long it was there.
