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Failed Smog Test

1982 Chevy Caprice High HC and CO Flipping at Idle

Year: 1982

Make: GM

Model: Caprice

Engine: 4.4 v8 (267) Can

Carburetor: Rochester 2bbl DualJet E2MC

Trying to figure out why my HC and CO flips fail on idle and high idle 


Idle - Fail

HC ppm: 685 (220 passing) Fail

CO%: 0.18 (1.20 is passing) PASS

CO2%: 12.2

O2%: 3.3

High idle - Fail

HC ppm: 65 (220 passing) PASS

CO%: 1.38 (1.20 is passing) FAIL

CO2%: 13.5

O2%: 0.0

1. CAT was installed about 3 year ago.

2. New MAP sensor about 2 years ago.

3. New O2 sensor installed about 2 year ago

4. New ECM & PROM installed about 2 years ago

5. New EGR valve installed about 2 years ago.

6. Several vacuum leaks fixed.

7. New air cleaner

8. New PCV value installed

8. Distributor, rotor, Plugs, Wires installed less than 3 weeks ago. (one wire was found burned through from touching the manifold).

9. check engine light is not on.


Perform a cylinder compression and cylinder vacuum test on your Chevy Caprice. Low compression can be due bad valves, piston rings, and possible weak valve springs. A vacuum test will verify a weak valve problem. Look for stuttering needle on your vacuum gauge. If your see the needle bouncing around, you more than likely have a valve(s) which is not seating properly/tightly enough. Fix that problem and you'll have decreased HC. The second main fault to look for regarding high HC is going to be ignition and ignition timing related. Ensure timing is set properly, and that electronic timing advance is functioning. HC above 500 ppm indicates a guaranteed misfire, either timing is off or there isn't enough compression to ignite the air/fuel mixture.

So you've checked the issues mentioned above... move on to the carburetor. If you plan on doing this work yourself you'll be needing a tool to measure CO output from the tailpipe, while you make adjustments to the carb. At the smog station they use the smog machine set to "5 gas analyzer". This repair can not be done without a tool indicating how your adjustments are effecting emissions. Let me explain the smog test results... (both idle and 2500rpm are based on the same principle). 1.38 Carbon Monoxide (CO) is an indication the carburetor is dumping just a bit more fuel into the engine than needed (or if not the carb, some how the combustion chambers are getting more fuel than needed... maybe from a defective EVAP system; saturated charcoal canister, defective purge solenoid.

posted by SmogTips Support 10-22-2015 08:04 AM

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