will running tap water in coolant system in 2004 pontiac grand prix cause it to overheat?

I have run plain water in the summer without a problem, but it is not the best ideal. Make sure you get some anti-freeze in there by fall. Do not use Dexcool!!!!!

Plain tap water is fine but it needs to be mixed with coolant as has already been said. This is important year round. If you're going to switch from the factory Dexcool your entire system needs to be properly flushed first to remove all of it as it is not compatible with other types of non- dexcool coolant. GM still uses it in their cars.

Tap water is actually better at transferring heat than 50/50 antifreeze, but it will boil at a lower temperature than the mix, and that can cause problems. So as a temporary emergency fix, tap water is okay. But do not leave it in the engine. The minerals in tap water will corrode internal engine parts and can quickly clog up small water passages with deposits. When I say 'quickly' I mean within a few weeks. Some people use distilled water to avoid this, but the anti-corrosion additives in anti-freeze are a vital part of the cooling system and shouldn't be neglected.