Engine rough idle and low rpm

[77i9]

I have a 98 Cedric w/ 136000 km. Lately the car has been acting up during start up. It would idle at around 200-300 rpms and causes the car to shake. Also the exhaust comes out excessive and blacker than usual. At times it would even die out, especially when I am coasting to make a turn and I let go of the gas. It would start up fine though and if I give it enough gas it would not die. Acceleration is very sluggish when the idle is real low. Then without doing anything to the car it would just go back to normal operation. I mean the rpm is around 700, the exhaust is normal, there's no shaking and acceleration is normal. I have taken it to the dealer but all they told me is the the O2 sensor should be replaced soon. I had another mechanic take a quick look at the car and he told me that it could be the idle air control valve. I have searched thru the forum here and found nothing specific for my car. Can anybody give me guidance on how I should go about troubleshooting this? Any help will be greatly appreciated.

[77i9]

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.

[77i9]

More update: after getting tired of the sh**ty running condition of my car (the constant stalling, the excessive black exhaust, and the poor gas mileage) I have decided to take a look at what I believe was the MAF sensor. First I loosen the clamp that holds part of the air filter housing to the air intake hose. Then I unclip the top cover of the filter housing. Then I took the assembly inside, especially since it was raining when I decided to do this. The actual sensor is located after a metal mesh towards the intake. It has this metal thread-like filament that looks very flimsy. All I did was use canned air to blow any dirt or grime of it. Installed the assembly back and fired up the car. The idle was normal and no more excessive black exhaust. It has been running good for 4 days now. Keeping my fingers crossed that I've finally fixed the problem.