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1994 Ford Ranger failed smog check because running too rich. How do i fix this?

My Ford Ranger failed the smog inspection as a gross polluter. Smog technician said the gas is raw and it is running too rich. Can you tell me why it failed? It does not miss at all and it runs really smooth.
Answer:

A smog check repair station will first have to find out why your Ford Ranger is running rich, and more then likey producing high CO. The diagnosis for this fault should begin with inspection of your Ranger's fuel injection "feedback" system. This includes ensuring the oxygen sensors are working properly, your Ford's emissions computer is receiving the signals from the oxygen sensor, the computer is computing the data properly and sending the correct signals to the fuel injectors to either increase or decrease fuel delivery to the combustion chambers. The feed back test will utilize a 5-gas analyzer (usually a smog machine) and propane. The smog technician should introduce propane to your Ford Ranger's intake system and ensure the oxygen sensors are reading this introduction as an increase in CO, and ordering the computer to decrease fuel delivery at the instant propane is added to the system. This test should be done after the engine has sufficiently warmed up and while it is running at idle. If no change in fuel delivery is seen we know we have a feedback problem. Then the diagnose should turn to which part of the feedback system is defective. Using a voltmeter the technician should observe the voltage output of the oxygen sensor as propane is added. Voltage should increase. If there is no increase we have a defective O2 sensor. The sensor should be replaced and the test started over. If voltage increase is present at the sensor output the technician needs to ensure voltage increase is also present at the computer input terminal. If voltage is present at the terminal as well next the technician must ensure the computer output terminal to the fuel injectors are lowering "injector pulse rates". At the end of the feedback test it will be know whether the rich fuel fault is electronic or mechanical, and the technician can further diagnose the fault. posted by SmogTips Support

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