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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