Hi,
Just want to find out what causes the black soot on my tail pipe and smell gas fumes during cold start ups. I have a feeling my mixture is rich? My gas comsumption is 17mpg city and 24mpg hwy. I already did a tune up- change plugs, wires, rotors, flame trap, new air filter, service TB, usual change oil....add some injector cleaners....i don't get any codes for the O2 sensors yet. Lately i noticed that there were times the tachometer reading would go up and down and engine would die and i would start it up and rpm goes back to normal??
I really want to see my gas mpg improve, what with the ridiculous cost of gas.
Help....Thanks in advance for your inputs...

94 850 wagon n/a - 197K

I think unless the car is running poorly or there's a diagnostic code there is probably nothing wrong. My 95-850 with 130K gets only a little better mileage than you do. Are you an agressive or easy driver, how much load do you have in the car, and what kind of terrain do you drive on?

I have a Ford Taurus whose size and weight are very similar to the Volvo and has been carefully maintained with no indications or history of fuel or other problems. It runs smoothly, passes emission tests and gives off no exhaust smell except at cold start. At 58K miles, it gets 14 mpg city and 23 highway.

I have the black soot too and was concerned until I looked at the tail pipes of cars where I park at work (Volvos and many others) - every single one has a tail pipe which is black and sooty inside.

When you start the car (cold engine) the fuel mixture is highly enriched. The normal control of the mixture via feedback from the oxygen sensors doesn't go into effect until the car has been running for a minute or so. Until that happens I think the fuel smell is normal - it has been present at startup on various cars I've owned.

If the engine coolant temp sensor is significantly out of spec the car will not run right once it is warm and you will probably see the check engine light.

Regarding the air pump/air injection I am unfamiliar as I do not have this on my car.

that's a relief. thanks

Replace the ECT, it's reasonably cheap and easy to do. Also check that the cold start air pump is working properly. I have just pinched the air line going from the air box to the pump to feel for pulsing (air is being pumped to the exhaust).

Klaus
--
98 V70Rawd(108Kmi), 95 854T(88K mi), 75 164E(173K mi)

Are your O2 sensors the origionals? They are suspect in a rich condition although they usually fail gradually and don't set a code right away. Other possibilities high on the list are vacuum leaks and a shot ECT sensor that makes the engine computer "think" it's stone cold all the time.

jB

thank you

When you see black smoke on acceleration you probably have a bad fuel pressure regulator. Good luck.