I have issue with engine coolant rapid level drop in my Toyota Avensis 1.8 vvt-i petrol engine. I did some inspection and noticed these things:

  1. When engine coolant temperature is still low, some bubbles rising from bottom of coolant expansion tank.
  2. When thermostat opens and fan starts, about 1/4 of volume of coolant disappears from the tank.
  3. When thermostat shuts off, coolant level stays the same. So after ~4 cycles of thermostat opening, all coolant in tank disappears.
  4. All hoses seems to be filled and not flat
  5. No leakages visible from any of components - hoses, water pump, thermostat, radiator, radiator cap valve, coolant expansion tank.
  6. Engine never overheats, temperature is within limits.

I am a bit confused, because what I believed is that first the level of coolant in expansion tank should rise when thermostat opens and pressure in radiator increases, hence opening radiator cap relieve valve and pushing coolant back into expansion tank. When thermostat opens (I assume it opens when fan starts), after ~5-10 seconds coolant level starts dropping in expansion tank, which means that pressure in radiator/cooling system is below the pressure in coolant tank (below atmospheric). That normally should happen when pressure in radiator drops due to liquid being cooled, but in normal circumstances coolant should be pushed into expansion tank first due to high pressure through radiator valve and only then same amount of coolant should be sucked back. My only guess is that somehow when thermostat opens, cooling circle always has lower pressure than atmospheric, hence sucking coolant from tank. Is it possible that there is just not enough coolant in the system or maybe there is some air there? I already filled about 4-5 liters of coolant and it seems it's never enough. I can't find any leaks and I would doubt it has something to do with engine gasket as coolant level drop won't be so rapid and there is not residue in oil system and no white fume from exhaust. Any thoughts on this?

  • Welcome to Motor Vehicle Maintenance & Repair! Does the coolant reappear when you shut the engine off and allow it to cool? When was the last time the coolant was changed? Commented Jul 7, 2023 at 19:07
  • @Pᴀᴜʟsᴛᴇʀ2, it never reappears. I think it's been quite some time since last coolant change. At least couple of years. Do you offer to drain and refill system completely to check if this this behavior continues after coolant change?
    – Andyally
    Commented Jul 7, 2023 at 19:14
  • So this is completely new behavior? Commented Jul 7, 2023 at 19:25
  • @Pᴀᴜʟsᴛᴇʀ2, can't tell exactly, but I used to check coolant level more often and year ago it was all normal, always between high and low. Then I stopped checking it so often as engine temperature was fine. Recently fan started working more often than previously but I though it has something to do with higher temperatures outside and mostly driving in city with some traffic jams. Also I seemed to me that car does not heat up as fast as it used to during winter period (when temperatures outside ~ 14F) which is not common for petrol. This is why I guess there is not enough coolant in system.
    – Andyally
    Commented Jul 7, 2023 at 19:44
  • Also, is it possible that there are some leak in radiator that can't be noticed due to coolant evaporating? But I doubt this theory as there should be some fumes visible when loosing 1/4 of coolant tank capacity in 30 seconds.
    – Andyally
    Commented Jul 7, 2023 at 19:46

1 Answer 1


Sounds like it's time to have a coolant pressure tester hooked up to this. It uses dye in the coolant and pressurizes the system. Any leaks can be found using a blacklight which illuminates the dye. Special attention is used when checking the heater core for leakage. They are harder to discover. Many people will use pipe cleaners and stick them up the AC condensate drain and check the pipe cleaner for dye. If you can't find a tester for rent somewhere or it's not fiscal to purchase one, a shop can do it. If there no external leaks, then a compression test is done to check for leaking head gasket.

  • A leaking head gasket leads to milky looking oil, does it not? One can simply check the dipstick for that.
    – FreeMan
    Commented Dec 5, 2023 at 19:11
  • @FreeMan coolant does not always get into the oil when a gasket fails.
    – Jupiter
    Commented Dec 5, 2023 at 20:00

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