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Discussion Starter #1
I have a 1992 Chevy Silverado fullsize pickup with a 350. Two years ago it passed the emissions test with readings of 1.04 GPM Hydrocarbons, 13.9 GPM Carbon Monoxide, and 1.85GPM Oxides of Nitrogen. I haven't made any changes to the engine since then other than preventative maintenance. Last week, I took it in for it's emissions test, and it failed. The readings were 1.42GPM Hydrocarbons (passed), 13.65GPM Carbon Monoxide (passed), and 6.60GPM Oxides of Nitrogen (failed - must be under 4.000GPM). I discovered that the vacuum tube going to the EGR valve was broken, so I replaced it, and took the test again today. It failed again, but the failure was due to hydrocarbons this time. The readings from today were 7.44GPM Hydrocarbons (failed - must be under 2.000GPM), 37.42GPM Carbon Monoxide (passed, but much higher than last week), and 1.02GPM Oxides of Nitrogen (passed – best reading yet). I didn’t make any other changes other than replacing that tube. Why did replacing the vaccum tube on the EGR valve increase the emissions readings for hydrocarbons, and Carbon Monoxide. What can I do to get all of the levels to be acceptable. This test is required in Maryland.

Thanks,
Paul V.
 

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Over two years a lot of things can happen. You might try running something like Seafoam through the engine to clean away any deposits etc. Before getting it rechecked, change the oil and filter, make sure the fuel is fresh and make sure the truck is up to operating temp. Also, check your electrics, cap, rotor, ohm the plug wires and check the plugs. I have also heard that adding 104+ will help.

Good luck
 

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That would be carb'ed with OBD I, right? Check Engine/ Service Engine light come on? Have it checked for stored diagnostic codes. Might be time for a fresh cat. converter.
 

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put a new o2 sensor in it and it will be better and help drop the hc and co a bit.

when nox goes down co goes up. and vice versa.

also are the doing a dyno test or a idle test?
 

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Just went to have my car inspected this year and no sniffers, they just plugged into my computer...I had a ford explorer that failed similar to yours on the sniffer and i had the cat and o2 sensors changed out and passed....
 

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Discussion Starter #6
The truck has TBI with OBD I. Maryland checks the computer on all OBDII cars, but does a treadmill test on everything older than that. The thing is, the truck passed the test for hydrocarbons and carbon monoxide the first time, but failed oxides of nitrogen. I discovered that the vacuum line going to the EGR was broken, so I replaced it. During the second test, the oxides of nitrogen were very low, but the truck failed on hydrocarbons and had much higher carbon monoxide readings then before the EGR valve was connected properly. Hooking up the EGR valve caused problems elsewhere. I'm trying to figure out where the new problems are and how to fix them.
 
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