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1994 GMC Sonoma 4.3 litre v6 vortec with a throttle body.
I took it in to get tested, and it failed emissions. So, I took it to have the catalytic converter changed, spark plugs, spark plug wires, oil filter, oil, and put a new exhaust pipe and tip on it to make it look nice and easier to shove that wire thing in it to smog it. I went back to test it again, and it read 10 times worse. literally. I can't remember what exactly he said, but before it was around 2.8% which was over emissions standards, then it went to 12.3% I'm confused, he said to change my gas cap, and my oxygen sensor and that may help, what do you think?
Answer:
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The smog technician was probably referring to your Sonoma's CO emissions. A smog check repair station will first have to find out if your GMC is in proper fuel control. This includes ensuring the oxygen sensors are working properly, the computer system is receiving the signals from the oxygen sensors, the computer 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.
Your GMC Sonoma has additional sensors which should be inspected for proper operation (and which also play a significant role in determining air/fuel ratio) as well, such as the Engine Coolant Temperature (ECT) sensor and the Mass Air Flow (MAF) sensor.
When your GMC is cold the ECM is programmed to deliver a richer fuel mixture (more fuel). As the engine warms up, the ECM will lean out the mixture. Insuring the ECT is sending the correct voltage to the ECM is important for correct air/fuel ratio (optimum is 14:7).
The MAF sensor measures the amount of air entering the intake manifold. This 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 vehicle is in proper fuel control, and a smog technician can further diagnose the fault.
posted by SmogTips Support
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