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I drive a 2007 Subaru Outback. Recently, I've noticed that when I use the air conditioning, my coolant level drops ever so slightly. The rate of loss seems to be related to the amount of time the A/C is on.

I'd like to fix this myself before summer hits, so my question is, which part of the the AC system could be causing the coolant loss?

  • Considering they are two completely separate systems, I'm not seeing how one can affect the other directly (or indirectly for that matter). I'd suggest it is coincidence, but couldn't tell you why. – Pᴀᴜʟsᴛᴇʀ2 Jun 3 '18 at 23:12
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    Day is hotter, load on engine is more as ambient T is higher and A/C is running... – Solar Mike Jun 4 '18 at 5:08
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Here is a possible way the two cooling systems are coupled:

  • With the AC on the vehicle has to reject additional heat to the atmosphere (sourced from the cabin).

  • This heat is rejected through the condenser, which is usually right next to the radiator.

  • The end result is that the coolant in the radiator is hotter than before, resulting in higher cooling system pressures. At a high enough pressure this can send some of the coolant to overflow, resulting in loss of coolant.


This behavior could be due to a number of factors, such as a radiator cap that isn't holding its rated pressure, or a clogged radiator but it could also occur just due to extreme outside heat.

I find this occurs on some of my vehicles on extremely hot days (45+ °C); I just need to top it up afterwards and make sure the level doesn't go too low.

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    Radiator cap/header tank cap is the first and cheapest way to go. OP, try changing that cap first (or both if you have both a radiator cap and an header cap). Also, condensers and radiators eventually get dirty and lose efficiency, try cleaning them. – Al_ Jun 4 '18 at 12:26
  • Great answer, thank you. I will ask a follow on question about the other effects of higher than usual cooling system pressures. – WeakMech Jun 4 '18 at 20:46
  • @Al_ Do you mean exterior clean of the radiators or flush them with some kind of chemical? – WeakMech Jun 4 '18 at 20:46
  • Many cooling systems have a second radiator fan that turns at higher temperatures or when the a/c is on. Verify that both fans run when the engine temp is high or when the a/c is on. – mikes Jun 4 '18 at 21:46
  • @OP: exterior cleaning. You won't have to do any interior one if, for the radiator, you only used the recommended coolant mixture and, for the condenser, the compressor didn't seize or you lost a great amount of refrigerant oil. Moreover, if the compressor seized, you would have to change the entire condenser since you can't flush dirt the new parallel flow type (only leftover oil, through closed loop flushing with liquid refrigerant). – Al_ Jun 5 '18 at 7:45
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If you have automatic temperature control, the heater is also used to control the vent temperature. Try turning the heat alone on and see if it does the same. If yes, the heater core (or its piping) is probably leaking and has a solenoid valve that closes, slowing the leak when the heater is not in use.

  • Thanks for the info. I use the heater all the time (much more than the AC) and I've never noticed the coolant level drop as a result. – WeakMech Jun 4 '18 at 10:02

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