NOx can be formed by many component failures and system malfunctions - anything that allows the combustion chamber temperature to exceed 2300 degrees Fahrenheit. For example, if the cooling system is utilizing pure water, it will absorb combustion heat and obtain its boiling point rapidly, leaving an air pocket around the combustion chamber. The air pocket will become a hot spot allowing for the combustion chamber temperature to rise. Equally bad is 100% antifreeze, which forms a blanket around the combustion chamber, keeping heat in and allowing the combustion chamber temperature to rise. Rust surrounding the cooling jacket surface will create the same blanket around the combustion chamber. Poor flow through the radiator, as a result of a blockage or poor circulation, or a partially closed thermostat or limited flow from the water pump, will prevent the high temperatures from escaping through the cooling system. This temperature increase will cause preignition, allowing the oxygen to prematurely oxidize the hydrogen, reducing the available air and fuel during spark ignition. As a result, oxygen will be limited to complete the oxidation of the hydrogen and formation of carbon and oxygen into carbon dioxide will be limited. In this case the gas meter will read high HC, low CO, low 02, low C02, and high NOx.
As the piston rises during the compression stroke, the opposite force of compression in the upward direction and the downward force created as a result of preignition will cause the piston to rock, slapping the piston skirt against the cylinder wall, causing a knock. As a result of an engine knock, we should see the scanner knock reading indicate yes, with a command to retard ignition timing. The secondary ignition scope should have a higher than normal firing line. With excessive preignition, the early flame front comes in contact with the spark ignition flame front. This will result in high HC, low CO, high 02, low C02, and high NOx. The horsepower of the engine will suffer as well.
A lean condition as a result of a false signal from the oxygen sensor, an out-of-calibration MAP sensor, plugged injector, low fuel pressure, low fuel pump volume, or a vacuum leak will cause NOx as well. Under these conditions the oxygen will oxidize the hydrogen and the carbon, creating extreme combustion chamber temperature. This acts much like a cutting torch used to oxidize the carbon atom of the metal. As the temperature climbs the nitrogen atoms will separate and form with the oxygen atoms to create NOx. As the oxygen is used to form NOx, the oxygen will be used up, causing a lot of hydrocarbons to be unused. The gas analyzer will read high HC, low C0, high 02, low C02, and high NOx. The secondary ignition scope should have a higher than normal firing line, and longer than normal spark duration. The scanner would read zero to low oxygen sensor voltage and a lean condition. As a result of running lean, the additional HC from a richer mixture is not there to absorb the heat. Therefore, the CO molecule needed in the catalytic converter to cause the catalyst to reduce minor levels of NOx to nitrogen and carbon dioxide will not be created. In no way will a catalytic converter reduce excessively high NOx levels.
If the EGR system were not functioning properly, as a result of a plugged or partially plugged passage, inoperative EGR valve, inoperative vacuum control system, or an electronic malfunction, the controlled exhaust flow would not occur. This would allow the combustion temperature to rise. The gas analyzer would see high HC, low CO, low O2, low CO2, and high NOx as result of a faulty EGR.
Carbon build-up on top of the piston or on the cylinder walls may also cause preignition. This would give similar results to the cooling system issue. The carbon build-up could be as a result of running extremely rich for any length of time. Therefore, when repairing high CO emissions failures, always assume that carbon has formed. This type of carbon can even be caused by oil consumption, which will also cool the combustion chamber temperature. Carbon on the throat of the valve will absorb fuel, causing a lean condition and giving a similar result as to the previous lean condition. This type of diagnosis may require a borescope to visually inspect pistons, cylinder bores or valves. To repair this, a chemical top end cleaner may help. If the vehicle is not running rich, the gas analyzer will read high HC, low CO, high O2, low CO2, and high NOx. The reason for the high O2 is that the carbon will assimilate a lean ignition misfire. The scanner would read low oxygen sensor voltage. The secondary ignition scope should have a higher than normal firing line, and a possible appearance of second firing line in the spark line.
A worn or slipping timing belt can certainly increase internal temperatures. If the timing belt or timing chain has excessive slack, the cam timing will be retarded. The camshaft will be behind the crankshaft resulting in the camshaft lobes not opening the valves in the proper relationship to the piston. The intake valves during the intake stroke will open late, causing the air to continue entering the cylinder later than required. As a result, the compression pressure will increase at the top of the compression stroke and temperature will reach its maximum later on into the stroke causing a longer oxidation period which results in extreme combustion temperature. As a result of late intake valve opening, the vacuum will be low, causing the MAP sensor to see a load adding more fuel, causing high CO. The gas analyzer will read high HC, high CO, low O2, low CO2, and high NOx.
If the base timing is advanced too far, the spark plug will ignite the air fuel mixture early, causing the combustion temperature to rise as the compression stroke continues, causing NOx to form. With a very early flame front in the compression stroke, as a result of advanced ignition timing, the temperature created as a result of early oxidation will cause the temperature created during compression to rise extensively.
The exhaust valve seating surface is not just to seal the cylinder airtight, but to provide a means of removing heat from the valve and disbursing it to the cooling system. An exhaust valve that is improperly seated will not transfer heat. As a result, the valve and its seat will rise in temperature, causing preignition. This will result in high HC, low CO, high O2, low CO2, and high NOx.
So as you can see, excessive NOx is caused by high internal temperatures. Unfortunately, there are any number of factors that can increase combustion chamber temperatures past the magic 2300 degrees. Fortunately, with your trusty five gas analyzer and a little reasoning ability, you're in great shape to isolate the problem, replace the faulty components or make adjustments as needed, and send the motorist on their way with a better performing and lower polluting vehicle.
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