Quote:
Originally Posted by slartibartfast
High HC indicates a very rich mixture, which means either too much fuel for the available air or too little air for the fuel supplied. I have no idea what you were thinking when you wrote that. High O2 is the opposite condition of what would cause unburned HCs, i.e. lean condition and would appear in your NOx section.
Technically, It's "NOx" and it means "oxides of nitrogen", of which there are several species that result from combustion. NO, NO2, NO3, N2O2, N2O3, etc.
Not so. Highest NOx production happens at about 16:1. I offer "The Internal Combustion Engine in Theory and Practice, Vol #2" by Charles Taylor as my supporting material. He has a really nice graphic that shows the interrelation between fuel mixture and NOx production.
Otherwise, you wrote a comprehensive post. Why not spend a few minutes editing it for spelling, punctuation and capitalization for a really spiffy post.
[edited 30-Sep-06. Finally got my copy of Taylor's book back and checked the NOx graphic I mentioned.]
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High HC's with high CO means rich mix.
High HC's with low CO means lean mix.
Two very different problems with the same high HC result.
There isn't one right answer.
Vacuum leaks cause extra O2 and a high O2 readings, which cause lean conditions and high HC's. The high HC's are unburned fuel and the unburned fuel is caused by either a lean missfire/incomplete burn or a rich condition.
CO tells you what your mix is, and HC's tells you how complete the burn is. A rich condition will also leave unburned fuel but will increase CO readings. My smog liscence has been expired for a few years but I was liscensed for 10 years and been repairing cars for 21 years. The only reason I let it expire is because I only wrench part time now, so it wasn't worth it to me to continue with re-training and re-testing.
The most common problems causing high NOx(oxides of nitrogen) are,
1. heavy carbon deposits on pistons, raising compression ratio/combustion temp.
2. Weak CAT
3. Inoperative EGR system. The sole purpose is to lower combustion temp. but the EGR is funtion checked on a smog check so that should be obvious if that is the problem.
Slightly elevated NOx can simply be a bad/weak CAT with no underlying problems and almost all engines even when running properly produce some NOx.
Carbon deposits may raise the level just enough to uncover a weak CAT because a good CAT will clean up alot of NOx. I've seen many cars fail for high NOx that passed after a Sea Foam treatment. If you O2 sensor is reading properly, that means the mix is correct. If cleaning the carbon doesn't change the NOx you need a CAT. This is all assuming your EGR system funtions properly, but like I said this is function checked on a smog test in California.
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