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Failed Smog Test

1991 Chevy Silverado Failed Smog Check Due to High NOx

My ASM emission test results - numbers are as follows: No (PPM) MEAS 1722 at 15 MPH and a score of 873 at 25 MPH. My question is what is my next step after two failed attempts at trying to get this vehicle to pass. After my first failed attempt, I did take it to an automotive repair shop and they changed a couple of air filters and made some other adjustments to it. I had also added the STP fuel system cleaner and a Cumout - All in One fuel system cleaner (before the 2nd attempted smog check) in an effort to get this vehicle to pass the 2nd time. What do you suggest that I do next?


Have you check the operation of the EGR valve? It's the main component which controls NOx emissions. Here's a quick way to test the EGR valve.

Testing the EGR Valve: This test only applies to vacuum controlled EGR valves only. A defective EGR valve will not open or close when vacuum is applied. We'll need to find out of the EGR valve on your Chevy Silverado is functioning and whether or not its passages are plugged with carbon deposits.

The way to test this out is to start your Silverado and let it run at idle. Disconnect the EGR valve's vacuum line and apply direct vacuum using a hand vacuum pump. When you apply vacuum, the EGR valve should open. When the valve opens; if the passages are clean and unobstructed you should notice a large RPM decrease. If the passages are plugged up, you will not see any RPM difference. If there is a RPM decrease, then the passages are free, and the next step is to inspect the vacuum signal which is used to open the EGR valve. It may be possible your Chevy is not opening the EGR valve by applying vacuum at the right time, or at all. If there is no RPM decrease, the passages may be plugged up. You will need to remove the EGR valve and clean all passages within the valve and within your engine's intake and exhaust passages, eliminating any obstruction. Please note, if the hand vacuum pump does not open the valve, it is also possible that the exhaust back pressure is not great enough (since the vehicle is stationary). You may want to either partially plug the exhaust pipe during the EGR test or use an object to depress the diaphragm inside the EGR valve, physically rather than using vacuum.

Air/Fuel Ratio: If the EGR valve is operating properly, and you've ensured vacuum exists at the EGR valve vacuum supply line, we'd recommend having a smog check repair station ensure your Chevy Silverado is in proper fuel control. Running lean will also cause high NOx. You'll want to make sure the engine's air to fuel ratio is proper (14.7:1, respectively), as well as rule out any overheating problems.

Basically, NOx is created when internal combustion temps rise above 2500F. The two main things that keep temps low... EGR valve which introduces an inert gas to the fuel mixture, and air/fuel mixture control (we need to keep air/fuel mixture just rich enough not to increase NOx but still allow the catalytic converter to do it's job and remove HC and CO). Controlling the air/fuel mixture is the job of the engine control computer and the various sensors which report engine data.

posted by SmogTips Support 09-27-2017 03:27 PM

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