Mitsubishi Repair: black powder coming out of exhaust pipe, air fuel ratio, secondary computer

Question
My son just bought a 1997 Eclipse 2.0 GST spyder, manual tranmission. When he is idling in the driveway and revs up the engine black powder and what appears to be water shoots out of the tail pipe. This car had a new head installed recently before he bought the car. The previous owner had NOS hooked up to this car, but it was removed before my son got the car. There is also a fairly strong exhaust odor coming from the car ( gas/exhaust odor). What might the problem be. Thanks  Troy

Answer
Troy,
It sounds like the engine is running very rich (i.e. too much or unburned fuel getting through the system).  Common causes of running rich are malfunctioning O2 sensors, worn catalytic converter, and a clogged air filter.  Running rich can also wear out your spark plugs prematurely as well, which enhances the problem.

If the previous owner was running NOS, that would indicate that they also had to force a lot more fuel through the system so the NOS system would not cause deteonation, which could destroy the engine.  The previous owner may have also installed secondary computer to control the air/fuel ratio to assist in getting more power out of the engine and help prevent detenation.

But stock parts can suffer from these upgrades like the exhaust system, emission system, and parts of the fuel and ignition systems.  You may want to get some more information from the previous owner (if you can) to find out any and all upgrades they might have done.  But definitely have your O2 sensors, catalytic converter, and spark plugs inspected to see if those are operating properly.
Good luck!