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1991 Chevrolet Suburban Failed Smog Test Because Running Rich

I have a 1991 Chevrolet Surburban that won't pass the smog test because the smog technician says it is running rich at high idle. We have tried all this below:

New air filter
New spark plugs and spark plug wires
Oil change
Complete tune up including ignition timing correction
New sensors
New muffler and pipe from catalytic converter back (CAT checked out ok)

Please tell me you know what it could be. I've gont to the smog station 10 times, and 10 times they tell me to fix something else. The bottom line is it runs rich at idle.

Answer:

Our first and most important recommendation is to pay for and obtain an accurate smog check failure diagnosis; which should pin point your Chevy Suburban's rich mixture problem. Following a smog station's "suggestion" on what to repair could end up costing you hundreds in unnecessary repairs. The repairs you've conducted so far should help lower emissions however if a rich fuel condition exists due to an ECU error, meaning a fault which is commanding the fuel injection system to delivery more fuel than necessary, only an accurate diagnosis of all your vehicle's emission sensors will suffice; specifically the oxygen sensors. We recommend after a basic visual look over and verifying mechanical functionality (ie. spark plugs, air filter, fuel pressure, cylinder compression) a smog station conducts a "fuel feedback test". This test includes ensuring the oxygen sensors are working properly, your vehicle'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. Along with checking the oxygen sensors, the MAP (manifold absolute pressure) sensor, TPS (throttle position sensor) sensor, and ECT (engine coolant temperature sensor) sensor will be inspected as well. More on high CO below... What causes high CO? posted by SmogTips Support

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