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I have a 2004 Nissan Xterra 3.3 liter V6. Tonight, it overheated while idling. I found out the coolant reservoir was empty. I filled it with what I had handy which was just cold water and this cooled the engine down substantially.

After driving it in 35-degree weather, the temperature gauge went all the way down, the coolant remained fairly decent (for a 20-mile trip I lost maybe an inch in the reservoir).

Is this the water pump or what is causing the issue for the safety of driving this vehicle?

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First advise: never never add cold or even ambient temp coolant/water to an overheating engine; there is a very high risk of cracking the engine's head or block. Always turn it off, and let it cool itself down to say 60C minimum.

You need then to find where the coolant is leaking: could be external or internal. External leaks are easy to spot: you will see a puddle of coolant under the engine, maybe it is not dripping right down because the coolant may travel over parts, plastic covers, etc but at least you can see puddles of coolant somewhere. Look also for rusty trails, if there is a leak, it may be rusting stuff internally and/or externally. Look in the hoses, squeeze them while the engine is idling and moderately warm, look around the water pump, specially the shaft where the pulley is, also more complex to inspect, look around the heater hoses and radiator.

In fact, sounds silly but also look inside the car under the front floor carpets. Any sign of rust and humidity could point to a leak in the heating system. Pain in the neck to spot.

As for internal leaks, meaning the head gasket is gone, you may see transparent/white fumes in the exhaust, or a kind of mist there. But more common is: look the oil cap in the engine, coolant leak will blend with oil and make "mayonnaise", a kind of identical glop you can see inside the oil cap. Oil stick may not get this, but you will see oil like fogged.

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You need to find where your coolant is going. Find that leak. Overheating while idling could have been caused simply by your low coolant level.

Keep an eye on the temperature gauge, if it doesn't overheat again, you only have a minor issue. Water pumps do leak when they get worn out, but there's other things that could be leaking.

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You may have fixed your problem already, but this is important to know. You shouldn't fill up your coolant with straight (tap) water if it's not an emergency situation. It doens't contain the components that prevent the internals of your engine to rust and other beneficial components, it's calcified, and contains other minerals you don't need in your engine. Lastly, it dilutes the coolant, increasing the freezing point. Flush your engine with proper coolant when you get the opportunity.

As others already mentioned, keep an eye out for leakages, but there's other possible reasons for you to lose coolant. Remember, the cap on your coolant system is like a pressure cooker pan. It keeps the system pressurized to a certain threshold, at which it starts to allow coolant to escape as steam. If the spring or seal is broken, coolant will evaporate it the air when it gets hot. That will start happening already at room temperature, but will get exponentially worse as temperatures rise.

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