Considerations related to H6 head gaskets and cooling system

Posts specific to the 3.0 litre NA H6 engine

Re: Considerations related to H6 head gaskets and cooling sy

Postby Yowie » Fri Aug 06, 2021 8:59 am

I suspect "all Subarus".

When I re-routed my throttle body coolant path through a temp sensor assembly I hardly lost any coolant - indicating that the throttle body coolant hoses are very high up in the system.
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Re: Considerations related to H6 head gaskets and cooling sy

Postby JezzaH6 » Fri Aug 06, 2021 7:38 pm

bigBADbenny wrote:That’s a great tip, where’d you find that?

Does it apply to H6 only or all Libs/Subaru?


I suspect any Subaru with a coolant hose going to the throttle body :dntknw: - although the turbo 4's with the throttle body underneath the intake runners the effect is going to be a lot less noticeable. For those it'd just be a matter of finding whatever coolant connection is the highest point and removing it!

I didn't see it posted anywhere if that's what you're asking, but I highly doubt I am the first one to think of it either. Just noticed one time while filling the coolant that the hose going to the upper throttle body is the highest one in the system, and therefore that is where all the air that normally gets trapped would be. Looking at the coolant routing too removing that hose from its upper barb offers an easy way for air to get out of the heater core.

Since I started doing that I have had zero issues getting the system bled first time :-D

Yowie wrote:I suspect "all Subarus".

When I re-routed my throttle body coolant path through a temp sensor assembly I hardly lost any coolant - indicating that the throttle body coolant hoses are very high up in the system.


Interesting that you decided to put a temp sensor there- what made you choose that location?
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Re: Considerations related to H6 head gaskets and cooling sy

Postby bigBADbenny » Fri Aug 06, 2021 11:48 pm

Love it.

Add the swirl pot to the throttle body loop then :)

Presumably a flushing T on the heater return would be a convenient solution to bleed the heater coolant loop & flush the core.
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Re: Considerations related to H6 head gaskets and cooling sy

Postby Yowie » Sat Aug 07, 2021 7:35 am

Interesting that you decided to put a temp sensor there- what made you choose that location?


I had wanted to do the throttle body coolant bypass thing anyway (not needed - Queensland weather).

The other alternative location for a coolant temp sensor seemed to be cutting a radiator hose and adding that large bore adapter in this style https://www.carmodsaustralia.com.au/coo ... apter.html, disadvantages including:

- messy with a lot of coolant lost;
- obvious 90s ricer mod when the bonnet was up;
- probably less accurate reading of the then-current engine coolant temperature (especially between cold-start and steady-state warm) if the sensor is out on the wrong side of the thermostat.


Conversely, with my selected approach:

- very little coolant lost (as it turned out);
- hidden from view on top of the gearbox bell housing;
- subject to about 2 degrees difference between ECU coolant temp reading and my gauge thermistor reading - this location shows a temp rise in lockstep with the ECU reading from cold start.


Once all the little fittings etc. are taken into account, my version is not cheaper than the large bore adapter approach.

Pictures for context:

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Re: Considerations related to H6 head gaskets and cooling sy

Postby JezzaH6 » Sat Aug 07, 2021 8:50 pm

fair enough that makes sense!

What is the benefit of the throttle body coolant bypass? I have seen intake temps above 60c on track- I would assume a bypass has the possibility of reducing that? Any downsides?
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Re: Considerations related to H6 head gaskets and cooling sy

Postby Yowie » Sat Aug 07, 2021 9:12 pm

My impression of the matter is that in very cold places you can get the throttle body freezing - possibly not helped by the vacuum being generated behind a mostly-closed throttle. Pumping hot coolant through the throttle body would prevent freezing.

In such a cold place, removing the coolant loop would have a down-side. In Queensland not so much.

The nominal benefit of taking hot coolant away from the throttle body would be slightly cooler intake air temps, but that would probably be an immeasurably small difference on account of the small interior surface area and short dwell time of air in that space.

I didn't bother until I needed the coolant path to plug a temp sensor into.
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Re: Considerations related to H6 head gaskets and cooling sy

Postby bigBADbenny » Sun Aug 08, 2021 1:50 pm

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Re: Considerations related to H6 head gaskets and cooling sy

Postby JezzaH6 » Sun Aug 08, 2021 8:54 pm

Yowie wrote:My impression of the matter is that in very cold places you can get the throttle body freezing - possibly not helped by the vacuum being generated behind a mostly-closed throttle. Pumping hot coolant through the throttle body would prevent freezing.

In such a cold place, removing the coolant loop would have a down-side. In Queensland not so much.

The nominal benefit of taking hot coolant away from the throttle body would be slightly cooler intake air temps, but that would probably be an immeasurably small difference on account of the small interior surface area and short dwell time of air in that space.

I didn't bother until I needed the coolant path to plug a temp sensor into.


Makes sense thanks for the explanation! I am curious to try and see what a difference (if any) it makes on track, but I agree looking at the design its going to make a tiny, if negligible, difference.

bigBADbenny wrote:Various bleeders:
https://xwebforums.com/forum/index.php? ... eas.21993/

Can even spend $100+ on fancy brass bleeders :shock:
http://montehospital.com/shop/index.php ... cts_id=499
Ouch!


They look nice though!
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Re: Considerations related to H6 head gaskets and cooling sy

Postby bigBADbenny » Mon Aug 09, 2021 5:08 pm

How about interchilling your throttlebody by running a loop by your ac evaporator? ;)

You’d have to include a temp control so as not to freeze it. :P
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