How to Fix GM NOx Failure

GM MPFI 3200, 3800, ... engines failing NOx caused by lean condition. General Motors Multiport Fuel Injection engines may fail emissions standard testing due to excess Oxides of Nitrogen levels in the exhaust gas. Here are some steps to determine and correct the cause of high NOx emissions.

Steps

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The richer the air to fuel ratio (less than 14.7/1), the cooler the combustion will be, the leaner the air to fuel ratio (more oxygen), the hotter the combustion is and more NOx is produced. These particular engines either equipped or not with an EGR system may produce excessive NOx due to a lean condition.
This device is located underneath the car in the exhaust system, between the exhaust manifold and the muffler.
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Make sure the EGR (Exhaust Gas Recirculation/Re-combustion) system is functioning properly, and particularly, that the EGR ports are clear. To check if the EGR ports are clear, start the engine and let it idle; using hot-gloves or a rag, reach underneath the EGR valve and push the diaphragm actuator rod upward. If the engine stalls or runs very rough, then the EGR ports are clear. If no (or very little change) occurs in engine RPM, then the EGR ports are plugged and must be cleared first (See repair manuals or Google it). Retest as mentioned.
Air volume entering from the air duct through the air filter is measured by the MAF (Mass Air Flow) sensor, usually located just before the throttle body and held down by three small screws.
Remove the three screws and gently remove the sensor, careful not to damage the small resistor and wire at the opposite end of the sensor.
Using carburetor cleaner or brake cleaner, spray the resistor and wire thoroughly, you can use a toothbrush but be careful not to break the tiny wire.
Have the vehicle's emission gas retested to make sure it is in compliance with state guidelines.

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My 2000 Ford Expedition passed the first time on emission test for the NOx GPM and failed on the HC GPM and CO GPM. I took it back today and it passed on everything except the NOx GPM; what can I do?
ASEmastertech
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NOx is usually high due to a problem with the EGR valve, its related components that operate it, or clogged pipes between the EGR valve and the exhaust pipe, or, less commonly, between the EGR valve and intake manifold.
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Note the purpose of the MAF sensor. The MAF sensor measures the amount (hence, mass) of air, flowing through the air intake ductwork to the throttle body. The small resistor and wire detect the volume of air, sending this information to the CPU, which adjusts the fuel to air ratio. If it gathers dust and fuzz after many miles of driving it will not be able to sense the proper amount of air flow. It tells the computer a less amount of air is passing through to the intake than what really is; The computer reacts by injecting less fuel to the amount of actual air, causing a lean condition.
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Warnings

Carburetor cleaner and brake cleaners in aerosol cans are highly flammable, do not use near open flames or other ignition sources.
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This procedure involves hot areas of the engine, so be careful not to burn your self.
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