When you're filling your car up with some coolant, it's vital that you pay close attention to the coolant level. In short, you have to be wary to not under or over-fill the coolant. In the case of the latter, too much coolant is actually a very bad thing. For starters, too much coolant may substantially increase the pressure within the cooling system. Thus, potentially causing hoses, seals, and seams to rupture. Otherwise, it might even cause damage to your car's electrics. Most cars have an overflow valve or hose which excess coolant will be released from. At the very least, you'll see the overfilled coolant start to leak out underneath your car. Yet, some cars have these valves and hoses placed close to your car's wiring harness, so leaks there might damage the wires.