I just noticed recently a substantial increase in the amount of black dust or soot on the inside of the exhaust tip. I am used to having a slight dusting, but now it gets completely black. I was wondering what could cause this condition and should I be concerned. Thanks!

 

Black soot in the exhaust is a sign of an over-rich mixture. Does the car start and run well? Idle well? Have you noticed a rotten-egg odor from the exhaust? Is the fuel mileage good?

 

The car starts fine, it does idle high upon startup at 1200rpm but lowers to 900 or when put into drive. Then once it warms up idles at around 600-700. No rotten eggs smell but it does smell like unburned gasoline. The fuel mileage has been somewhat lower but the other car has decreased as well and I dismissed as something being added to the fuel.

 

Is the engine is coming up to normal operating temperature? I had a van a few years ago that had the thermostat stick open, causing the computer to keep it running rich.

If yours seems to be warming up properly, check the air filter, it may be possible for a really dirty one to cause it to run rich.

 

The "smell like unburned gasoline" is another indication of an over-rich mixture. This may be caused by
- sticking fuel injectors
- faulty fuel pressure regulator
- obstructed fuel return line

Please attend to this promptly. Raw gas in the exhaust system will cause the catalytic converter to run abnormally hot, shortening its life. In the extreme case the converter can catch fire.