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1998 Subaru Forrester, 4 cyl. I don't remember the exact specs of the engine.

It's using what seems to be a lot of coolant -- probably 2 qts in a 30 day time span.

There's no leak (I've left cardboard under it overnight to confirm).

When I leave the car running with the radiator cap off, I can see the coolant moving in the radiator, so it doesn't seem like a water pump issue.

We've replaced a lot of the basics in the cooling system recently -- thermostat, some hoses, radiator cap -- in an effort to diagnose the overheating.

It seems like a head gasket leak is the most likely culprit. However, 2 things seem odd to me about this:

  1. The oil seems normal. There is no evidence of coolant in the oil (it isn't milky looking), and there is no evidence of oil in the coolant.
  2. The reservoir never drains down. I add coolant to the radiator directly, and the reservoir is always full.

The car doesn't smoke, or smell bad when it runs. It does run rough, and is probably skipping.

The fact that the reservoir never seems to empty (at all), makes me think that I might have a vacuum issue somewhere. But I could be completely crazy, and grasping for something that is not a head gasket. :-)

  • Welcome to Motor Vehicle Maintenance & Repair! IIRC, the Subarus are known for head gasket problems. You can leak water directly into the cylinder without it going into the oil. As far as the reservoir, this would mean to me your radiator cap may have an issue (isn't working as a two-way-valve). – Pᴀᴜʟsᴛᴇʀ2 Oct 7 '18 at 13:28
  • Hi Kirk - having had many Subarus, I can support Paulster's comment. Have a look at my post on a different but related Forester issue to see why it can be tricky to check coolant. I had an airlock until I properly opened both caps and shook the car. mechanics.stackexchange.com/q/27991/37 – Rory Alsop Oct 8 '18 at 7:33
  • Wait a second. 2 radiator caps?!?! This car doesn't have a turbo, I don't think. I've had 0 luck finding a Haynes manual for the thing. Does this second cap look like a normal radiator cap? I've burped the colling system multiple times, but every time it has felt like an airlock somewhere, but none of the normal stuff I have done has alleviated the issue. – Kirk Oct 17 '18 at 15:21

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