Considerations related to H6 head gaskets and cooling system

Posts specific to the 3.0 litre NA H6 engine

Considerations related to H6 head gaskets and cooling system

Postby bigBADbenny » Sun Aug 30, 2020 6:38 pm

Based on research, here's what I've learned about H6...

The first thing I'd do would be to install an engine watchdog device, so you know as soon as the coolant system is over temp.
Or get the BtSsm app or even Torque app, and set an alarm for same.

H6 are notorious for developing air pockets in the heater core, or from an improper bleed after maintenance, and the result is usually a blown head gasket, usually around the $3000 mark to fix.

Other preventative measures include changing the coolant on time, replacing the stock radiator every 100-150k km, dont mix coolant colours, changing out any bulging when hot hoses, back-flushing the heater core both ways (the in and out sides) and bleeding the cooling system maybe once or twice a year: cant hurt.

If your heater starts playing up, not heating, intermittent heating etc, waterfall sound behind dash (not to be confused with weird sound inside dash: hvac blend door actuator fault), take action immediately.

Other than that, plus timing chain guides, avcs and alvs issues, they're lovely cars, I cant wait to own one :good:

Screen Shot 2020-08-30 at 6.33.21 PM.jpg


Source: https://www.alldrivesubaroo.com.au/serv ... t-repairs/ See H6 section.
Last edited by bigBADbenny on Tue Sep 15, 2020 11:57 am, edited 1 time in total.
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Re: Considerations related to H6 head gaskets

Postby bigBADbenny » Tue Sep 15, 2020 11:56 am

Here’s my correct coolant system bleed faq:

An incomplete coolant bleed will cause issues.

Luckily for me I made a note on the correct procedure.

Bleed coolant

Check that the cooling system has been bled properly:

Raise front of car on slope or ramps.

Jam a funnel into the turbo coolant tank, (or radiator for n/a cars) support it with a wire hook hanging from bonnet, fill up funnel with coolant.

I use 2 SCA funnels, the inexpensive two piece type, one unit complete, and the flexible section of the second jammed into the first for extra length/volume, but one will do the job.

For n/a cars, it’s worth buying a proper jam funnel, as the core is closer to the top of the radiator.
I used tape and an inch of old garden hose to modify an sca funnel to jam in an n/a radiator.

Idle car with heater on full blast to help purge the heater core.

Raising the front of the car on stands, ramps or a hill or kerb will help, as will making the radiator, or turbo coolant tank, and funnel the highest point in the cooling system.

Get up to temperature and massage top hose, then rev engine.

Repeat till no bubbles in funnel.

Remove funnel, replace cap and wash off excess coolant.

Take your time and be patient, use a welding glove , rag or oven mitt to massage the upper hose as the coolant comes up to temperature.

Once you no longer see bubbles in the funnel whilst massaging the hose, you should be good to go.

Wash all excess and dried coolant from the engine bay, this will make spotting slow leaks from eg a cracked or separated radiator easier to spot.

Otherwise a pressure bleeder is a good tool to have for the job.


If the cooling system is completely bled and you still have bubbling noises in the dash: check, refurb or replace the hvac blend door actuators.
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Re: Considerations related to H6 head gaskets and cooling sy

Postby nvmylh » Tue Sep 15, 2020 3:22 pm

Would it be possible to modify the H6 heater system to run in parallel instead of series with the engine cooling system?
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Re: Considerations related to H6 head gaskets and cooling sy

Postby bigBADbenny » Tue Sep 15, 2020 4:34 pm

I did think of that, and looping the hoses is a temporary fix for a blocked heater core, eg prior to flushing or replacement...

Its possible you’d never get heat in the core with a dual bypass, but maybe a pressure relief valve might work in the loop?
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Re: Considerations related to H6 head gaskets and cooling sy

Postby nvmylh » Fri Sep 18, 2020 10:22 am

I just had another look at the technicians reference info for the H6 cooling. The heat core does not actually flow in SERIES to the engine.

Coolant flows through the block, then heads and exits towards the radiator. The heater core and throttle body feeds are taken off the head coolant exit pipes and return directly to the pump, not through the radiator.

Its hard to see how a blocked heater core would actually cause any damage to the engine. Maybe the reduced flow could slow down the whole sytem causing the pressed metal impeller to not work as efficiently? I wonder if the blockages cause issues as the heater return is plumbed straight into the pump and not through the radiator.

I also wonder if its possible to reverse the flow through the heater core now and then to help remove any build-up of crap? But then where would this crap go??

EDIT: very interesting info on the thermostate operation and heater core! If this info is true then its all starting to make sense - Blocked heater core flow prevents the thermostate from opening, engine overheats.
https://legacygt.com/forums/showthread. ... 20477.html
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Re: Considerations related to H6 head gaskets and cooling sy

Postby bigBADbenny » Fri Sep 25, 2020 8:43 pm

Cheers for the link. I’ll take a peek.
On another note, and something I want to discuss with actual cooling system experts, is the formation of steam pockets In the block or cooling system.

Someone on Subaru Club of Melbourne FB group said that they’d been chasing the source of spiking coolant temps for months, eventually the source was identified as “micro cracks” in the stock radiator.

My local mechanic insists that air cannot enter the cooling system as its under pressure when its hot, pressure raises the boiling point etc.
But when the hoses and or radiator are worn and starting to fail... ?

There’s a radiator specialist nearby in Richmond, I’ll have to go pick their brains :P
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Re: Considerations related to H6 head gaskets and cooling sy

Postby bigBADbenny » Mon Sep 28, 2020 5:41 pm

So the “solution” in the link is to tap the coolant crossover pipe and run it to a swirl pot, or I guess, a remote bleed valve.
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Re: Considerations related to H6 head gaskets and cooling sy

Postby montyLalor » Mon Nov 09, 2020 10:57 pm

What's everyone's thoughts on something like this?

Image

By cutting and shutting this into the heater lines, the heater can still function, but coolant is flow-path promoted to return to the engine...

Been thinking about this, too :lol:

It's a bunch of standard ND15 316 stainless steel butt weld fittings TIG welded together. I'm about to detail its drawing and send it off for a quote.

Its weight should be about 400 grams.
Stuck behind those that maneuver with the eloquence of a fiery wall of disintegrating fuselage...
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Re: Considerations related to H6 head gaskets and cooling sy

Postby montyLalor » Tue Nov 10, 2020 7:26 am

Actually, which is the in / out for the heater hoses? i.e. is the top one the inlet or the outlet for the heater core?
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Re: Considerations related to H6 head gaskets and cooling sy

Postby bigBADbenny » Tue Nov 10, 2020 7:32 am

Coolant flow path is in the FSM.

You could add a solenoid fluid valve that could be triggered via a coolant temperature activated controller.

Or use a ball valve on the bypass circuit to get the proportioning between bypass and heater core circuit correct.

Or potentially use an adjustable inline pressure relief valve on the bypass circuit.
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Re: Considerations related to H6 head gaskets and cooling sy

Postby montyLalor » Tue Nov 10, 2020 10:23 am

To my mind, with that contraption I presented, the primary coolant flow running back to the engine should create enough low pressure to 'drag' the heater core outlet flow into returning to the engine as well. Subsequently, the pressure differential created on the heater core outlet side should make a sufficient portion of the primary flow 'pull' into the heater core inlet at the peak of the 'U' shaped return. I'm hoping this will be self-governing somewhat, satisfying the flow needs of the engine and heater core, but also critically, avoiding turbulence that can promote air pocketing.

Fingers crossed. It's looking like it'll need to be thoroughly bench-tested :roll: Big clear tubs full of still water and pressure-fed food die with video capture... I can hear my Missus already, "Another project! :angry2: "

I will have a chat with an engineer mate of mine to find holes in the design. At the moment, I've tried to minimise manufacturing costs by utilising off-the-shelf bits as much as I can...
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Re: Considerations related to H6 head gaskets and cooling sy

Postby bigBADbenny » Tue Nov 10, 2020 12:25 pm

Bernoulli’s effect at work?

GDT cylinder 4 cooling mod comes to mind for H4 T.

A lot of testing went into the correct sizing of the bypass orifice to get the desired effect, but its still a simple tee, iirc.
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Re: Considerations related to H6 head gaskets and cooling sy

Postby nvmylh » Wed Nov 11, 2020 11:41 am

My initial interest in this was because I bought a spec B, h6 auto with 183xxxkm and noticed the temp gauge going way up on long hills. Engine oil looked perfect, no real sign of a head gasket but I did notice a gurgling sound in the dash so was worried about a clogged heater core, air lock and/or head gasket. Plus I was loosing some coolant out the overflow bottle.

About 2 weeks ago I ordered a pair of 5/8 x 1/2inch brass tees (https://www.ebay.com.au/itm/333433867586 ), drained the coolant, checked the flow through the heater core with the garden hose (definitely no blockages there!), installed the tees and 1/2" bypass hose before the heater core, drilled a 3mm hole at the top of the thermostat beside the jiggle pin thing and everything has been sweet since.

To bleed the coolant I:
- Jacked up the front as high as possible.
- Took the top hose off the throttle body (this looks like the highest point in the system) and waited until coolant came out of that (while filling the radiator) before reconnecting.
- Then did the usual jam a funnel in the radiator, run until it gets hot, squeezing top and bottom hoses etc. which took about 20-30min i think.

Since then I haven't had any issues, the temp gauge does move very slightly but no where near like it did before and so far I have not lost any coolant from the overflow bottle. Also the heater works fine.

Does anyone else's temp gauge (EZ30's) move at all when driving?

Also note: I think the car was serviced just before I bought it, the coolant looked VERY clean and the thermstat looked brand new. Fair chance the mechanic didnt bleed it properly in the first place.
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Re: Considerations related to H6 head gaskets and cooling sy

Postby bigBADbenny » Thu Nov 12, 2020 12:04 pm

Excellent stuff, another H6 saved! :P

Did you find the test mode connectors to see if both fans work?

I need to add in-tune mods for fan activation and there may be a hack to get both fans working all the time or: research.
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Re: Considerations related to H6 head gaskets and cooling sy

Postby nvmylh » Thu Nov 12, 2020 1:31 pm

Didn't need to, I've seen them both running before.
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