Dear Tom and Ray:
When I was a kid doing my own tune-ups, I understood that the color of the deposits in the very end of the tailpipe was an indicator of how well-tuned the car was. Tan-colored deposits were good; black and sooty deposits were bad. My new car doesn't get the 24 mpg highway that it should, and the tailpipe has a black sooty deposit. I told the dealer the black soot means that the mixture is too rich, and that causes the poor mileage. He disagrees. Am I right, or is the dealer?
TOM: Well, in the old days, a black, sooty tailpipe WAS an indication that your mixture was too rich (too much gasoline, not enough air and, therefore, lots of excess carbon).
RAY: But nowadays, with computer-controlled engine-management systems, your "check engine" light would come on long before your tailpipe turned black.
TOM: Modern cars all have at least two oxygen sensors that check the mixture by measuring how much oxygen is in the exhaust. And they check the mixture continually -- every moment the engine is running.
RAY: So whatever caused the inside of your exhaust pipe to turn black, unless your check engine light is on, is unlikely to be a fuel-air mixture that's too rich.
TOM: And besides, what are you doing looking up the inside of your tailpipe, Jerry? What if your neighbors catch you doing that?
RAY: If you're still not convinced, Jerry (and I can tell you're not), the easiest way to answer your question definitively is to get an emissions test. An emissions test will tell you immediately whether the mixture is off.
TOM: I'm guessing it's fine, and that there's some other explanation for your mileage (like cold weather, a stuck thermostat, underinflated tires, a heavy driver's foot, a nonaerodynamic roof rack or an overly optimistic EPA highway number), but an emissions test will tell all.
RAY: Ask the dealer if he'd be willing to test the emissions for you, and then you guys can continue the mileage discussion from there. Good luck, Jerry.