black smoke will be emitted from exhaust

adriana22 · Post by **adriana22** » 14 Apr 2005, 08:33

When I start my 850/1996/155k a puff of black smoke will be emitted from exhaust. I know the Black smoke is an indicator of very rich mixture. After the car starts it still do black smoke but less. Mileage is very low 16-18 /gallon. Car runs great with the exception of this.
I have no CEL light, NO CODES, no vacuum leaks,air metter is clean, all maintenance completed including new battery, plugs, , MAF and fuel filter. I have cleaned the TB and IAC as well. I have a new front oxygen sensor installed also.The rear o2 is old but does not affect the mixture ?!
What else it can be?

VolvoFreak · Post by **VolvoFreak** » 14 Apr 2005, 09:31

How's the oil level? Do you need to add any?

Jot · Post by **Jot** » 14 Apr 2005, 13:40

The rear O2 sensor reads the exhaust that has passed through the cat. If OK then no code. I have seen on some site that it is reccommended a change of sensors at 100,000 miles. The front sensor reads the exhaust and tells the computer to adjust the fuel in conjuction with the MAS reading. With the mileage you report and no codes my best guess it is the MAS. The O2 sensors are partially doing there job thus no codes

White850Turbo · Post by **White850Turbo** » 14 Apr 2005, 18:09

I've heard of instances where the coolant temperature sensor can send a bad signal resulting in a very rich A/F mix, causing poor gas mileage and black smoke.