Barely Passing Smog

Barely Passing Smog
Hello Madera Ranchos. I received my first question for my new Ranchos Independent Automotive Questions and Answers Column. 93636 resident Larry contacted me over concern his vehicle barely passed its previous smog inspection. You’ve come to the right place, Larry. Emissions are something I understand and can help.

Different Gasses
The tail pipe emission gases monitored during a smog inspection are: Hydro Carbons (HC), Carbon Monoxide (CO) and Nitrous of Oxides (NOx). These three emissions are usually generated from differing causes, have different solutions and they don’t always play well together. When you lower CO for example, expect the NOx to rise. Correcting one emission gas is not extremely hard, it’s achieving the balanced condition where all three emission gases are lowered together (stoichiometric) and the engine runs well. That’s difficult.

HC is normally a symptom of incomplete combustion, CO an “overfueling” condition and NOx an “underfueling” condition.

Larry’s concern is elevated CO emissions (overfueling). Since 70,000 miles has elapsed since Larry’s last tune up, he decided to try that prior to the next smog inspection. Here is my conversation and answer to Larry:

“Is the engine running poorly?”
His general answer was “no.”
“A tune up is probably a good idea but do not expect it to solve an overfueling condition. The better approach is to address the fuel injection system which is where the cause for excessive CO is usually found. Is the air filter dirty and restricting available oxygen from entering the engine? Is a fuel injector internally leaking? Is the air temperature sensor out of calibration and effecting the air density calculation? Is the oxygen sensor lazy?”

Larry, these suggestions and more should be considered but the oxygen sensor is number one on my list to test. Here’s why. The oxygen sensor measures the oxygen content in the exhaust and generates a voltage based on that measurement. That information is feedback to the PCM (powertrain control module) and the PCM uses it to adjust the air to fuel ratio. If the oxygen content in the exhaust is very high (lean), the O 2 sensor will generate a low voltage (near 0 volts). If the oxygen content is very low (rich), the O 2 sensor will generate a high voltage (near 1 volt). Just like any generator, O 2 sensors wear out. When they wear out they struggle to generate the higher voltages. The low voltage signals from a worn out O 2 sensor will be misinterpreted by the PCM as high oxygen content and will command the fuel injectors to stay open longer and thereby cause an overfueling condition.

A secondary ignition tune up does not even address CO, an overfueling condition. A secondary ignition tune up is a routine maintenance issue usually performed at about 100,000 miles. It should not be postponed until there is a problem. If you wait too long and the vehicle develops a misfire, kiss your catalytic converters GOODBYE! It may be too late. $$$$$

Two Converters? I asked Larry if he had ever replaced his catalytic converter. He answered, “Yes, twice.” TWICE!?! Catalytic converters theoretically last forever. Catalytic converters should not get old and die. A defective converter is a symptom, not a cause. If you don’t correct the cause prior to replacing the catalytic converter, the converter is doomed. NEVER, NEVER, EVER replace your catalytic converter without verifying the engine is in “Fuel Control.” Who is in charge of verifying “Fuel Control”? The Oxygen sensor. He is king of “Fuel Control.”

Sometimes a technician will blame the catalytic converter for the emissions failure and recommend replacement. I’m sure the converter is bad but if the condition that caused the converter to fail is not identified and repaired prior to its replacement, your new catalytic converter will soon go bad also. Hopefully you will pass the emission test before it fails and the process begins all over again. Hope this information helps 93636.

Again, guessing is the most expensive way I know to repair the modern vehicle. Accurate testing is much more efficient and cost effective. I am here to answer your questions, give you a second opinion and give you an explanation you can understand. If you have a question or comment for me, please contact me at complete_car_care@hotmail.com or send them to the Ranchos Independent. Thanks again and God Bless.

Warren Parr
California Smog License #EO139887
California Star Station #117312