Coolant Bleed, Cooling System Bleeding

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Coolant Bleed, Cooling System Bleeding

Postby bigBADbenny » Sun Oct 11, 2020 7:01 am

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 brand 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, or more likely, to let you know if warm coolant is circulating through 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.
Revving the engine is optional, some say it just cavitates air and coolant.

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 you’re changing the coolant, eg every 2 years, it’s a good opportunity to use a garden hose to flush the heater core via the inlet and the outlet, eg flush and back flush.
This is extremely important for subarus without a heater tap eg H6 and some older models.

For H4’s this is a good opportunity to install the GDT cylinder 4 cooling mod, details available online.

The stock composite radiators are known to seperate with age, generally around 150k km is a good time to replace them. If there’s no sign of dried coolant spray adjacent and you’re having persistent issues, we’ve seen “micro cracks” allow air to enter the cooling system.

If you’re replacing the stock radiator with an aftermarket item, Koyorad and Golpher are trusted brands.

If you still get rising temperature under load, eg long hills, lookup how to find your test mode connectors under the dash, this mode will cycle the radiator fans so you can quickly verify if both are plugged in and the ecu is controlling them.

If you still have issues after a complete bleed, do a TK test kit for revealing compression gasses in the coolant.


If the cooling system is completely bled and you still have bubbling noises in the dash: check, test, refurb or replace the hvac blend door actuators, aka “weird sound inside dash”.
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