Eric has a video on that procedure, it’s a ‘Leakdown test’ in the States. If it’s your head gasket, sometimes you can also see the coolant extra oily when this is the case, as well as your engine oil might appear as a creamy sludge when you open the oil fill cap.
I had a similar problem, engine overheating and heat cabin wouldn’t go above 90°-95°F (35°C). Car would drive great as long as we didn’t turn the heater on, then turn the heater on and within minutes the engine was overheating. I flushed the heater core ’till the water coming out was clear, topped off the coolant and cabin heat would improve to maybe 105°F (40.6°C), but after some time driving the engine would overheat again and vent heat would drop to the 80’s (27°C). Since I’d gotten some improvement flushing the core I kept repeating the flushing and kept bleeding the system of air to no avail.
I’d bleed and bleed and bubbles still came out. I did the leakdown test and a cooling system pressure test and every other test and found nothing wrong and of course, the problem still wouldn’t go away. I’d even tried bleeding the air in the cooling system using a spill-free funnel while parking the car on a hill to help the engine burp the remaining air bubbles to no avail. Finally I flushed the core again, only this time, instead of pouring water ’till it came out clear on the other end, I did the following:
Note: Before flushing the heater core, you must first turn your ignition to the on position, then the A/C/Heater temperature setting to the hottest setting, then turn the key back to off.
1. I connected clear vinyl hoses on both the inlet and outlet heater hoses (so I could see the water coming out), the inlet hose I pointed into a catch-basin on the floor and clamped it secure to the basin to avoid spilling the flush.
2. To the outlet hose I attached a funnel, and poured about 8-12oz of distilled water (I used the outlet hose for back-flushing)
3. After pouring the water, I removed the funnel and pressed a high-pressure air gun (connected to my air compressor of course) to the hose and squeezed the trigger in spurts first and then fully opened. Make sure the other hose is well secured to the catch basin or the hose will thrash around like a stabbed basilisk (Harry Potter knows this). The basin should also be covered with a towel, not sealed, and somewhat secured as the water coming out is at high pressure and it will knock the basin around (took me 20 minutes to clean all the spilled coolant water around my garage) . I noticed that the water coming out was clear but also copper/rusty sand and crud was coming out in handfuls as well.
4. I repeated steps 2 and 3 until I got no more crud/sand, this took almost 10 gallons’ worth (no kidding on this one).
5. Re-attached the hoses back to the proper connections on the car and voila! high heat from the vents, around 168°F (76°C) and no more overheating.
What I presume is that the sand/crud was creating air pockets in the heater core. So the moral is, just cuz your flush-water is coming out clear after a flush, don’t assume you have a clean system. Also, in my case, gravity alone didn’t flush out the crud, I had to use highly compressed air to push it thru.
Hope this helps.
Leakdown test video (scroll to bottom of page):
https://www.ericthecarguy.com/faq/solving-automotive-performance-issues/leak-down-testing
Eric has mentioned and recommended the spill-free funnel (highly agree): http://a.co/84zKe2C
Air gun I used: http://a.co/gkuBTOq
Similar catch basin to the one I used: http://a.co/a1pZIUR