Failed Emissions, High HC

Did you change the oil? First thing I would do with an engine with that history is decarbonize the engine (get a can of Combustion Chamber Cleaner or BG 44K and follow the instructions) then change the oil and take it for a good drive with hard accell followed by closed throttle decell to clean out the cyls (carbon) - The old "Polish tune-up". It also helps get the cat back to operational.

I have a theory, and I want to see what you think.

The O2 sensors should be sensing the high HC output and leaning out the Air/Fuel ratio, correct? That is the whole purpose of the O2 sensors, right?

If the EEC is not able to lean out the mixture sufficiently, then the O2 sensors should be setting a "Too Rich" code, right? There is no "Too Rich" code, which seems to point to an O2 sensor reading wrong, and falsely believing that the mix is in compliance.

Maybe the O2 sensors aren't that sensitive, but I'd think they have to be. Otherwise they'd never be able to keep the emissions numbers in compliance.

What do you thing guys? I'd have thought that if the problem was anything else, there would be a "too rich" code from one of the O2 sensors.

CJB

Except O2 sensors only measure O2 content - not HC, and high HC does NOT necessarily mean running rich. Quite the opposite sometimes. Excessively lean makes for poor combustion, which makes high HCs (and not high CO). Too rich makes high CO, and SOMETIMES high HC

They are VERY sensitive - to Oxygen. Absolutely NO sensitivity to Hydrocarbons.

You may have a cracked PCV hose making the engine run lean, causing a lean misfire on at least one cyl - pumping raw HCs into the exhaust along with unused O2 - which makes the engine think it is ean, so it richens up the rest of the engine trying to compensate. End up with half the engine, or more, running rich to get the O2 concentration to where the engine thinks it should be, while several cyls could still be lean, pumping out unburned HCs from the lean misfire. Would show up worst at idle but not go away on the 25mph test.

High HC along with high CO failures are common, and strongly suggest a bad O2 sensor/s. Other things are possible, but the O2 sensor/s should be checked first.

Bad O2 sensors erroneously tell the computer it's engine is running too lean. The computer in turn (dumb and dutifully) compensates by commanding rich mixtures... driving the HC & CO high. Mileage figures suffer as well.

Also,a failed coolant temperature sensor can tell the computer it's engine is running cold. The computer will run the engine rich... as in mimicking the choke function of carburetors. A bad thermostat (or one of a too cold value improperly installed) will have much the same effect.

Your cylinder #8 issue is probably not O2 sensor related... hopefully just a bad plug wire or plug...

Good Luck!

Erik

Thats right, (working) O2 sensors only report exhaust gas O2 levels to the onboard computer. As far as exhaust gases go, the onboard computer doesn't understand, or care about anything other than O2 content.

It's stationary smog test gas analyzers that read HC, CO and other gases.

A failed O2 sensor stops producing a signal, which the onboard computer interprets as the engine running too lean. It in turn compensates by commanding rich mixtures in response.

Rich fuel air mixtures produce high HC, CO and reduced fuel mileage.

Here's something to read...

Erik

Just a thought

I believe the catalytic converter combined with the smog pump and air diverter valve are the main components in holding down your HC, CO numbers.

For what its worth

Yes, but the HC & CO will still go high if the computer decides to run the engine rich due to erroneous inputs from a bad O2 sensor.

Erik