"If it were good I'd expect only for hydrocarbons. NoX are a function of overheating/carbon buildup/wrong timing and CO poor ignition."
This is such an incomplete statement that it is inaccurate.
Actually, I'm kind of disappointed with the collection of disinformation on this thread.
Let's start by saying that a healthy well tuned clean engine will usually have less trouble passing a smog test. Frequent or at least adeqluate oil change intervals help with this, so so does other maintenance such as cleaning the combustion chambers, replacing worn spark plugs and plug wires, cleaning injectors, a clean unrestricted air filter, and so on.
Lets go through the gasses read on a smog test.
HC is unburned fuel. High HC the universal measure of ineffeciency. It can be high from the fuel mixture being way too lean ans well as from being way too rich. It can happen if there is an ignition misfire. It can happen from compression loss problems such as a burned valve or badly worn rings. In extreme cases it can be from fuel contaminated engine oil. It can happen from overly advanced ignition timing.
CO is a measure of incomplete combustion. On a healthy engine, it will rise as the mixture becomes richer.
CO2 is the universal measure of effeciency. The more effecient the engine is running, the higher the CO2 reading. A healthy cat will raise the number even more. Perfect fuel mixture is near where the CO2 reading peaks. The only reason for reading CO2 on a smog test is to prove that the probe is plugged into a tailpipe. If it doesn't reach a certain threshold, the test will fail. A low reading can be from a poor running engine as well as from excessive exhaust leaks.
O2 is high on a lean engine and becomes lower on a rich engine.
NOX is generated when the combustion temps get too high. The engine must be under load (dyno) for any significant NOX reading. Basically, the normally inert nitrogen of the atmosphere gets scorched as it goes through the combustion chamber if the temperature is too high. There are three basic ways to cut NOX: 1) retard ignition timing, 2) exhaust gas recirculation (exhaust that has already been through the combustion chamber is nearly inert and becomes inert filler), and 3) a reducing catalyst.
HC and CO can be reduced with an oxidizing catalyst. On todays cars, most cats are "three way" cats and help to reduce NOX, HC, and CO.
Your best chance of passing a smog test is to 1)have a healthy engine 2) be up to date on appropriate maintenance, 3) have not tampered with any of the smog devices, 4) have the engine AND THE CATS well warmed up when attempting the test.
On the PCV valve. As long as it is clean and has not fallen apart it will funciton properly. I prefer not to change the thing just because of high milage because the odds are that you will be given the wrong one and may cause more harm than good. If you use good oil and change the oil at proper intervals, the valve may never go bad, but it doesn't hurt to clean it once in a while as well.
Anyway, my $.02 on several items raised on this thread.
This is such an incomplete statement that it is inaccurate.
Actually, I'm kind of disappointed with the collection of disinformation on this thread.
Let's start by saying that a healthy well tuned clean engine will usually have less trouble passing a smog test. Frequent or at least adeqluate oil change intervals help with this, so so does other maintenance such as cleaning the combustion chambers, replacing worn spark plugs and plug wires, cleaning injectors, a clean unrestricted air filter, and so on.
Lets go through the gasses read on a smog test.
HC is unburned fuel. High HC the universal measure of ineffeciency. It can be high from the fuel mixture being way too lean ans well as from being way too rich. It can happen if there is an ignition misfire. It can happen from compression loss problems such as a burned valve or badly worn rings. In extreme cases it can be from fuel contaminated engine oil. It can happen from overly advanced ignition timing.
CO is a measure of incomplete combustion. On a healthy engine, it will rise as the mixture becomes richer.
CO2 is the universal measure of effeciency. The more effecient the engine is running, the higher the CO2 reading. A healthy cat will raise the number even more. Perfect fuel mixture is near where the CO2 reading peaks. The only reason for reading CO2 on a smog test is to prove that the probe is plugged into a tailpipe. If it doesn't reach a certain threshold, the test will fail. A low reading can be from a poor running engine as well as from excessive exhaust leaks.
O2 is high on a lean engine and becomes lower on a rich engine.
NOX is generated when the combustion temps get too high. The engine must be under load (dyno) for any significant NOX reading. Basically, the normally inert nitrogen of the atmosphere gets scorched as it goes through the combustion chamber if the temperature is too high. There are three basic ways to cut NOX: 1) retard ignition timing, 2) exhaust gas recirculation (exhaust that has already been through the combustion chamber is nearly inert and becomes inert filler), and 3) a reducing catalyst.
HC and CO can be reduced with an oxidizing catalyst. On todays cars, most cats are "three way" cats and help to reduce NOX, HC, and CO.
Your best chance of passing a smog test is to 1)have a healthy engine 2) be up to date on appropriate maintenance, 3) have not tampered with any of the smog devices, 4) have the engine AND THE CATS well warmed up when attempting the test.
On the PCV valve. As long as it is clean and has not fallen apart it will funciton properly. I prefer not to change the thing just because of high milage because the odds are that you will be given the wrong one and may cause more harm than good. If you use good oil and change the oil at proper intervals, the valve may never go bad, but it doesn't hurt to clean it once in a while as well.
Anyway, my $.02 on several items raised on this thread.
