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Failed Smog Test

1991 Ford Mustang 5.0L Failed For High NOx

I have a 1990 Ford Mustang and I can't seem to get it to pass the smog check. I've tried changing just about everything... including the O2 sensors, catalytic converter, new radiator, EGR position sensor (the EGR valve works). I still can't seem to pin point the problem. If you could help out that would be great. Thanks.


It sounds like your Ford Mustang might be running lean. Have you checked for any vacuum leaks? Make extra sure all vacuum lines are connected and that none are broken or brittle. Just as a vacuum leak (after the throttle plate) will lean the fuel mixture, low fuel pressure (dirty fuel filter/defective fuel pump) will do the same. So check fuel pressure as well.

If the oxygen sensor wiring is intact, and since you mention the oxygen sensors are new, we'll assume the O2 sensors are sending the correct voltage to the PCM, and that the PCM is trying to control air/fuel ratio... so we'll move on to the most important sensor in fue control.

Next, have your checked your Ford's MAF (Mass Air Flow Sensor)? If you're certain the EGR system is working right, and you've already changed the catalytic converter, we'd recommend focusing on fuel control, specifically the MAF.

Make sure the Mass Air Flow sensor has 12v and good ground, and that it is producing the correct voltage output.

At idle and with your Ford Mustang's engine at normal operating temperature, the MAF sensor should produce a signal voltage of .09 to 1.6 volts. The MAF sensor's entire operating range should be between .0 to 5 volts. After conducting some tests you may notice the MAF is producing below normal voltage, and this will inicate the MAF is defective.

The MAF should be tested while the engine is at idle and 2500rpm, and without removing the MAF harness. Below we indicate which wires should be tested using a voltmeter. Your Ford Mustang's MAF sensor should be imprinted with the following letters; indicating the wires function.

Wire A: 12 Volts.

Wire B: Ground.

Wire C: Ground from PCM.

Wire D: MAF Signal.

Wire E: Air Temp. sensor circuit (N/A on 4-wire MAF sensor).

Wire F: Air Temp. sensor circuit (N/A on 4-wire MAF sensor).

Check and cross check for voltage and ground, both between the battery terminals, and the PCM. If all checks out, test the signal wire. While the engine is idling check for voltage from the MAF signal wire and a good known ground. Ensure as engine speed increase, so does voltage; and the increase (or drop) is smooth and steady.

posted by SmogTips Support 07-01-2015 04:02 PM

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