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I have a 2007 Mazda6 with 3.0L V6. I had coolant changed while in for routine maintenance two months ago. Since then I have noticed the radiator fans running under conditions that they never did before. So I checked the coolant reservoir and found it effectively empty (i.e., below top of low hose):

Virtually empty coolant reservoir

I added distilled water to the reservoir to bring it to the F line. After a few days I checked and it was back to virtually empty. Today I filled it with distilled water again, immediately drove just 2 miles, popped the hood and yet again the coolant was back at the level shown in the picture above!

I haven't noticed any coolant leaks where the car is routinely parked.

The ECU has not indicated overheating even on long drives at high loads in extremely hot summer weather.

Do I have a problem if the coolant is persisting below the L line in the reservoir? If so, what is recommended to diagnose what's going on?

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    Use a 50/50 pre-mixed when filling up the tank. You are effectively diluting your coolant. Continue on a daily basis to fill up the coolant reservoir. If after a few days it is still doing the same thing, you might have an issue. If it settles out, you should be golden. It could be the system never properly bled, so you are just in the process of getting it back to completely full. I don't like to jump to conclusions, so take care of what you know, then go from there.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented Sep 16, 2018 at 17:29
  • yeah, use mixed coolant, not pure water. If you smell maple syrup while driving you have a coolant leak into the cabin.
    – Jasen
    Commented Sep 16, 2018 at 20:08

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Assuming that there was enough coolant in both the engine block and the radiator to start with (on average cars can store near 5L in each), if you're losing that much coolant at that rate, you are either:

  • leaking it. Inspect the engine drain plug, the radiator drain plug, any bleeder values, the condition of the radiator and the condition of the hoses. If you have covers on the bottom of your car, the leak may not be obvious as the fluid may be pooling on the cover.

or

  • losing it into your oil. Check your oil level, is it increasing? Is the oil watery?
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    I was also told that if coolant makes it into the oil it will turn it a milky color.
    – feetwet
    Commented Sep 23, 2018 at 17:29
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I had a leak at the water pump; it only leaked at highway speed so there was never any leak evidence or drips showing when I stopped and looked under the hood. Something to check.

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  • How did you detect that leak?
    – feetwet
    Commented Sep 17, 2018 at 16:04
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When a water pump starts failing, the first sign is it'll weep fluid out near end of the pump. The seals/bearings are going bad... initially it isn't bad enough to notice while sitting, but while driving there is enough pressure to leak fluid. In fact, many pumps have a "weep hole" in the housing out near the output shaft. Eventually (if not already) you'll notice a little play in the water pump output shaft/pulley.

So, I think your pump is just due for replacement.

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