I just had coolant replaced with the required coolant for a Jaguar XJ8 2 days ago. Today my coolant warning says "coolant low" My mechanic said to put a cup of water in there. Is that going to dilute and change the coolant that is already in there. Is that a wise thing to do?

 

I don't think it'll damage the engine if that is what you are worried about. You can buy the pre-mix radiator fluid too or keep an additional quart of Jag fluid on stock. Use distilled water if going just the water route.

Does your mechanic have any leftover fluid?

 

You should be fine. The coolant is diluted with water anyway. There is a level bar inside the resevoir tank you can check to see how low it is.