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Failed Smog Test

Chevy S-10 produced high CO during CA smog inpection

I own a 1991 Chevy S-10, 4.3 engine 4x4 5 speed manual transmission.

My S-10 passed the smog check at a California Smog Test Only Station in all areas except for the 15 mph CO%, the readings should have been; MAX 0.51 and GP 2.03. My Chevy S-10 produced 2.76 percent.

Where should I start looking? Thank you.


Your Chevy S-10's high CO indicates a rich fuel mixture. You'll need to begin by looking at things which control fuel delivery to the combustion chambers. The problem can be electrical, mechanical or both. You want to ensure the oxygen sensors are working properly, and the ECU is computing the data properly and sending the correct signals to the fuel injectors to either increase or decrease fuel delivery to the combustion chambers. Additional sensors which also determine air/fuel ratio) are the Engine Coolant Temperature (ECT) sensor and the Mass Air Flow (MAF) sensor. The ECM calculates how much gas to send the combustion chambers based on the ECT signal. Insuring the ECT is sending the correct voltage to the ECM is important for correct air/fuel ratio (optimum is 14:7). Your Chevy's engine requires a richer fuel mixture when cold. The MAF sensor measures the amount of air entering the intake manifold. The MAF sensor must send accurate information to the ECM as well, in order for the ECM to calculate exactly how much fuel to add to the air entering the combustion chambers. At the end of the feedback test it will be know whether your Chevy S-10's high CO fault is electronic or mechanical. A few mechanical problems your Chevy might be experiencing and which should be inspected are all points of "fuel entry"... such as and most predominantly the Fuel Injectors. It is not uncommon to a stuck "open" or "sticking" fuel injector to cause high CO. One other mechanical fuel related concern is that of a malfunctioning "fuel pressure valve". This valve during normal operation will return all unused fuel to the fuel tank.

posted by SmogTips Support

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