Question about 1990 Chevrolet C1500

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1990 CHEVY C1500, 350 V-8 HAS HIGH HC & CO AT IDLE. HIGH SPEED EMISSIONS ARE OK. CHECK ENGINE LIGHT IS NOT ON. WHAT DO I HAVE TO DO TO FIX IT?

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  • james_panace Jul 22, 2010

    THANKS FOR THE SOLUTION, I WILL IMPLEMENT IT ASAP,
    JIM

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Hello
High CO and HC at idle means in most cases that the catalytic converter is worn out, also very contaminated engine oil can cause this as well as factor's like the engine running cold due to a faulty thermostat, so replace the thermostat, change the oil if needed and replace the cat converter and it will most likely pass, my experience? 35 years as a Class A smog inspection and repair technician at a GM dealer.

Posted on Jul 21, 2010

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First, thank you for asking me for my input on this. It just so happens, that I used to own a Ford ranger that had the same engine as yours. It was a 1990, but there are few changes to these model years.

Anyway, there is only one reason that your HC levels would be high. Your engine has WAY too much unburned fuel in the exhaust. The same reason applies to high CO levels. Too much fuel and/or not enough air.

Below is a link that you may find very helpful. It explains exactly what we are talking about here and also lists some of the possible causes.

EMISSIONS ANALYSIS

Incidently, when I had the same emissions failure on my truck, replacing the air filter and replacing a failed MAP sensor fixed it. - I did NOT have to replace the catalytic converter. If the numbers are as high as you listed above, you will only destroy a new converter anyway because there is obviously not enough oxygen in your exhaust system for the catalyst to function properly. Those numbers are nearly DOUBLE the allowable maximum. If the engine is running correctly and the catalyst is good, your numbers will be WAY BELOW those maximum numbers.

Please understand that I am NOT telling you to replace your MAP sensor without proper diagnosis. There are many possible causes here. Please click the link above and read the article. It will help you to understand what is going on here.

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Change spark plugs,wires,cap and rotor if they are over 24,000 miles
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Make the smog check appointment for the EARLY MORNING. Colder air will help the fuel burn better.

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Clean or change the air filter and pcv valve.

Clean Mass Air Flow sensor with CRC Mass Air flow cleaner or electronic part cleaner.

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Do a quick wipe down on the wires and cap to take the moisture off.

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Check spark plug wire contacts for corrosion.

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Fill the tank to around 3/4.......just in case your fuel pump is weak.

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Ran the truck for 20+ minutes on highway before the test.

========

Do not carry excessive cargo on the truck bed.


These are the little things I would do to get the HC and CO down.


Please post more information on the comment window.

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