95 mustang emissions issues

Cannon77550

Active Member
Jan 7, 2018
42
33
28
Galveston, TX
Hey guys... got a question for you guys and looking for advice. Have a 95 gt with 5.0 and i am having problems passing emissions. They are doing a dyno emmisions test by putting it on a dyno and probing the exhuast fumes at 15 and 25mph. The first time i failed hydrocarbons at both speeds an failed nox at 25mph. I replaced my coil since the connections were a little corroded and that got me past the nox but i was still failing for hydrocarbons. I had read that #1 cause for high hydrocarbons was timing so i changed the timing a little bit and the hasnt helped with emissions but she does run better now. I checked my codes since i noticed my CE light cycling on and off on my way back from the emissions test. I got a code for 02 sensor on right side. Now that would make sense but the exhaust is a true dual and they were probing the left side. Oh and im barely failing but less than 100 points. Any help or suggestions would be helpful. Thanks guys
 
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A bad 02 sensor can cause the computer to think you are running lean, and add fuel which will make your car run rich. It doesn't matter which tailpipe they were probing, if the computer richens your fuel mixture due to a bad sensor your whole engine will be rich. Since you ran your codes and it shows the 02 sensor is bad replace it and you will probably fix your issue.
 
Umm, no. There are left & right fuel trim adjustments managed by the computer corresponding to each of the 2 O2 sensors. In the case of true duals, they're only testing half of your engine.

Lean or Rich conditions are your most likely cause. If you're getting one O2 sensor failure code, change them both (they probably need it). You can have a lazy O2 sensor that isn't technically broken enough to throw a code but is causing mixture problems.

You should set timing to the factory value - making sure your spark plugs and wires (and cap & rotor) are in good shape. If you have an EGR delete you'll want to retard a bit from factory for NOx. That may slightly increase hydrocarbons.

Second most likely is a failed / gutted / missing catalytic converter. Might be time for a new one(s).

This all assumes a stock setup (injectors, engine parts, tune). If you're non-stock with all kinds of add-ons, then your tune may be off and needs to be fixed.