Update on the problem: while the car was running looked inside the engine bay to see if I can spot anything out of the ordinary. Decided to remove a connector going to a small black box on the intake hose right after the airfilter. The idle immediately became normal (around 700 rpm) and no more heavy black exhaust. So I decided to buy a new airfilter and installed it. With the connector installed I started the car and idle was fine/normal. No more stall, no more rough idle even on cold weather. This went on for at least 5 days. Now it is having rough idle again and stalling. I'm thinking of cleaning the sensor that the connector goes to, which I am assuming is the MAF sensor. I will update again when I have done this. Anybody who can give me advise on this problem is more than welcome to do so.