As an absolute last resort, you could � but it�s strongly advised that you don�t. The tap water in most parts of the UK contains a lot of chalk � and this is likely to clog up your coolant system and need lengthy repairs or a complete overhaul. Your best bet is to get into the habit of having spare coolant, engine oil, and screen wash in your car boot at all times. But running an engine without any coolant at all is likely to cause real trouble. So if you�re caught short and have to use water, opt for distilled water rather than tap water. Be sure to keep an eye on your engine temperature and drive slower to stop your engine from overheating.