Mitsubishi PHEV fix cabin heating or 4 way valve, rinse and clean coolant system

DIY and tips on how to bleed your car's cooling system, fix cabin heating, 4-way valve thermostat and rinse cooling system completely without causing bleeding or air pocket issues.
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Please read steps on this description too:

Mitsubishi Outlander PHEV seems to have manufacturing issues related to aluminum engine casting and having residual metal flakes interacting badly with the cooling system components and the coolant itself. Problems may happen with just a few year old cars already. Symptoms being lukewarm cabin heating or no heat at all.

Mainly targeted for PHEV's with electric heater, esp. MY2016 which I got here, but others may find this useful as well.

Steps for complete system cleanup and coolant change:
1. Drain engine & heating system's coolant
2. (Optional: Rinse system)
3. Fill system up (with 7% citric acid concentrated water)
4. Drive for 15 minutes
5. Drain again (see step #1)
6. Rinse thoroughly with tap water (remove citric acid and gunk)
7. (Optional: Fill up with tap water + drive + drain)
8. Fill system up (with approved coolant, see step #3)
DONE

Video also has tips on how to check coolant filter, clean up 4-way valve's thermostat and a quick demonstration on how dirty your system may be even if your filter and coolant looks quite clear.

NOTE: When your ICE (combustion engine) is hot, and you set maximum cabin temperature, your cabin fans should produce so hot air that you cannot keep your hands in front of the grill for more than 5 seconds. If this is the case, then your system is performing like it should.