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Failed Smog Test

1987 Ford Mustang 5.0 HC Mildly Elevated With Very Low CO and NO?

My 1987 Ford Mustang failed the smog check. It has 122k on motor. Pretty much stock. OEM Ford Catalytic Converters. Prior to smog test I replaced the following smog parts: vacuum lines, EGR, spark plugs, cap, rotor, wires, distributor, ECT sensor, MAP sensor, TPS sensor, Idle air ctrl valve, fixed exhaust leak. Tuned engine: running strong and no more running rich of misfires.

2 years ago: CAT-back exhaust Walker Dynomax system and mufflers.

Test history: 1st test: prior Ran: 3 gallons 91 fuel with 12 oz isopropyl alcohol. Barely failed HC both 15 and 25 mph. (180 deg thermo).

2nd test: put old EGR, 195 thermostat, 8 gallons E85 with 1 bottle CRC Guarantee to Pass. Seaformed engine. HC fail (all numbers mildly elevated, but CO and NO still within compliance).

What to do next? Catalytic converters? Anything before then?


While running 91 octane, not to mention the with isopropyl, you're running your Mustang too lean, more than likely causing a slight misfire. The CO numbers appear good because they are low, but that's not necessarily a good thing. We have to recommend you run Ford required fuel octane (which is more than likely 87) and also not use any additives which could lean the fuel mixture.

Should your Ford Mustang still fail the smog test with 87 octane we'd recommend conducting a fuel feedback test; basically ensuring the oxygen sensors are functioning properly and commanding the ECU to increase fuel delivery.

The final emissions component to suspect (if needed replace) should be the Catalytic Converter. If the CAT is defective it will most definitely cause a smog check failure by not reducing HC enough to to pass.

posted by SmogTips Support 08-02-2014 10:30 AM

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