1991 Chevy Camaro wont pass smog

  • V8
  • 2WD
  • 187,000 MILES
I replace my engine with a new 305 2 years ago and passed with flying colors a year ago smog cam up and I failed on everything but gas cap check I fixed the timing and it is close now on most but I cant seem to get the numbers done I have been working on cars for 13 years and im baffled I replaced the cat two years ago egr and all sensors my injectors might have a drip but im not sure smog pump is original along with charcoal filter and so on with 187000 on them for all I know they have never been replace but I dont know if they have anything to do with why im failling I do DRIVE the car and im pretty ruff on her so im wondering if something could be clogged like the cat even though I replaced it 2 years ago or tubes, I also smell strong gas smell when I shut it off, and gets worse when I open the hood I replace plugs wires cap rotor set timing changed oil filters and ran car for 30 min before I took it besides any ideas on parts to replace are there any other tricks to passing like retarding the timing ect ect ect I need help the no registration tickets are stacking up thanks
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have the same problem?
Tuesday, September 16th, 2008 AT 2:47 PM

1 Reply


When a vehicle fails an emissions test, the motorist usually receives a printout that show the test results of the vehicles emissions as well as the applicable cut points. From this, you can determine if too much HC and/or CO caused the vehicle to fail.

Hydrocarbon failures mean unburned gasoline is passing through the engine and entering the exhaust. The three most common causes include ignition misfire, lean misfire and low compression (typically a burned exhaust valve). Ignition misfire can be caused by worn or fouled spark plugs, bad plug wires or a weak coil. Lean misfire results where there is too much air and not enough fuel, so check for vacuum leaks, dirty injectors or a fuel delivery problem. In addition to these, hydrocarbon failures can also be caused by oil burning due to worn valve guides, valve guide seals and/or rings.

Carbon monoxide failures indicate an overly rich fuel mixture. On older carbureted engines without electronic feedback controls, look for things like a stuck choke, misadjusted or fuel saturated float or a rich idle mixture adjustment. On newer vehicles with electronic carburetors or fuel injection, the system may not be going into closed loop because of a bad coolant or oxygen sensor.

If both HC and CO are high, the vehicle may have a bad catalytic converter or an air pump problem.

NOX failures are usually EGR-related, since the EGR system is primarily responsible for reducing oxides of nitrogen. But NOX emissions can also be caused by a bad three-way converter or a computer control system that remains in open loop.


There are four things you should always check on every vehicle that has a computerized engine control system:

1. Scan for fault codes. Any codes that are found need to be dealt with before anything else.

2. Make sure the system is going into closed loop. No change in loop status often indicates a coolant sensor problem.

3. Confirm that the system is alternating the fuel mixture between rich and lean. This is absolutely essential for the converter to function efficiently. You can do this by observing the O2 sensors output with a scan tool, or directly with a digital storage oscilloscope or voltmeter. If everything is okay, the sensor should be producing an oscillating voltage that flip-flops from near minimum (0.1 to 0.2v) to near maximum (0.8 to 0.9v). O2 sensors in feedback carburetor applications have the slowest flip-flop rate (about once per second at 2500 rpm), those in throttle body injection systems are somewhat faster (2 to 3 times per second at 2500 rpm), while multiport injected applications are the fastest (5 to 7 times per second at 2500 rpm).

4. Confirm that the system responds normally to changes in the air/fuel mixture. To test the system response, pull off a vacuum hose to create an air leak (not too large or the engine will die). You should see an immediate voltage drop in the O2 sensor output, and a corresponding increase in injector dwell or mixture control dwell from the computer. Making the fuel mixture artificially rich by injecting some propane into the intake manifold should cause the O2 sensor output to rise and the computer to lean out the fuel mixture.
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Tuesday, September 16th, 2008 AT 2:56 PM

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