I have a 1996 Subaru SVX with a leaking heater core and I wanted to know or confirm two things.

First of all, I started the car and allowed it to idle up to warm and fluid levels were normal. I came back 3 hours later and found the overflow tank had lost a fair amount fluid but the radiator was full. There was no coolant below the overflow tank and none had leaked from the cap. My question is, while the engine was cooling down, would the leak from the heater core cause the coolant from the overflow tank to displace the leaking coolant? I also found a fair amount of air bubbles in the radiator (after noticing the drop in levels in the overflow tank) and have just burped the system.

I originally thought the car had a bad headgasket because my radiator cap was leaking after replacement but after burping the system and adjusting the fluid levels (overflow was overfilled and radiator was filled all the way to the top), it had stopped leaking from the radiator cap too. After using a combustion leak tester on the radiator, the fluid stayed blue (no exhaust gasses leaking into cooling system). My second question is, if there is air in the radiator (from leaking heater core), would that air build up enough pressure as the engine warms up to break through the radiator cap and let coolant leak out from there?

I am in the process of removing my dash and fixing it up. Additionally, there is no white smoke coming from the exhaust, and while I was burping the system there were no additional bubbles coming from coolant in the radiator filler neck while I was revving it. The oil is fine and levels have not risen as well. Coolant is green and does not have a rainbow colored film above it.

Any help or information is appreciated and thanks in advance!

  • Welcome to Motor Vehicle Maintenance & Repair! On a side note ... I took an EG33 out of an SVX and put it in my son's 96 Impreza which he'd blown up the EJ22 ... it was a chore, but made for quite a car :o) Mar 28, 2021 at 0:23
  • Thank you! I really hope my motor doesn't have a broken headgasket.
    – Jawad Haq
    Mar 28, 2021 at 1:12

1 Answer 1


To answer the 2nd question: No. There has to be pressure in the system for the coolant to press past the radiator cap. Think of this as a controlled leak. If there is a leak in the heater core, this would be considered an uncontrolled leak. The uncontrolled leak of the heater core would take precedence and allow fluid to flow. Why would it do that? Because there's nothing restricting it. The radiator cap won't allow fluid to flow out unless there is more than (IIRC) 16psi of pressure in the system. If there's a leak in the heater core, the pressure is going out there. Air can also be introduced into the system through there as well (which should answer the first).

  • I should have worded the second question better. I meant would the pressure built up from the engine warming up cause the air in the system to break through the radiator cap? I have edited the main post already.
    – Jawad Haq
    Mar 28, 2021 at 0:28
  • @JawadHaq - No. If there is a leak in the heater core, there won't be enough pressure in the system to push past the cap. Some pressure will build up, but it escape from the busted core before it would push past the cap. Mar 28, 2021 at 0:35
  • @Paulster2 I'm sorry but I'm still confused about the following: why was the radiator cap leaking before I had burped the system and the overflow tank was overfilled? The only thing that is coming to my mind is that the overflow tank being overfilled led to the excess pressure (from the air in the radiator neck area) being releived through the cap since there was too much weight in the overflow. The main reason I doubt my thoughts is because I am unsure if the overflow has enough to break through the 13 psi cap for the SVX.
    – Jawad Haq
    Mar 28, 2021 at 1:10
  • The overflow has nothing to do with the internal pressure of the radiator and system. It could not lead to pressurizing the system. If the system has too much pressure, some coolant will flow out into the overflow. If the cap was leaking, then there could have been a lot of fluid go out from the radiator. Mar 28, 2021 at 1:14
  • That makes a bit more sense. One more thing, any idea why the overflow tank continues to reduce in fluid Everytime I shut down the engine and let it cool down? Am I correct to say that it is related to the heater core leak?
    – Jawad Haq
    Mar 28, 2021 at 1:46

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