Smog Failure Possible Causes

What causes high HC (hydrocarbon)? High HC is caused by incomplete combustion; in other words, the air and fuel that went in to the combustion chamber was not completely burned, now the unburned fuel has ended up in the tailpipe. Problems that can cause incomplete combustion include:

· An ignition misfire. If a spark plug does not spark, the air/fuel mixture in the combustion chamber does not burn and then goes out the tailpipe.

· Incorrect air fuel mixture. If there is not enough fuel in the combustion chamber to ignite, the fuel that is there goes out the tailpipe.

· Low compression. If the compression is too low the air/fuel mixture will not ignite and the unburned fuel will go out the tailpipe.

· A bad catalytic converter. The converter lives in the exhaust pipe and burns unburned gas as it passes through.

What causes high CO (carbon monoxide)? High CO is caused by a rich air/fuel mixture. The air fuel ratio should be 14.7 parts air to to 1 part fuel. Problems that can cause a rich mixture include:

· Problems with the feedback system (the system that regulates the fuel being added to the engine based on the amount of oxygen in the exhaust).

· Problems with a computer sensor such as the coolant temperature sensor, air flow meter, MAP sensor, etc..

· Problems with the fuel pressure. Fuel pressure that is too high can cause a rich mixture.

What causes high CO2 (carbon dioxide)? High CO2 is the product of a good running engine. The better your engine runs, the more CO2 it produces. The only way to reduce CO2 emissions is to buy a fuel efficient car. The bigger your gas guzzler, the more CO2 it makes. This is what's so sad about the recent SUV trend. In addition to depleting the worlds oil reserves, and putting our country in a position where "we will go to war to protect our oil supply", the trend of building increasingly inefficient behemoths is making the planet a less hospitable place for our species (and others) by contributing to global warming.

What causes high O2 (oxygen)? High O2 can be caused by a lean air/fuel mixture, but most of the time, noticeably high O2 is caused by either a leak in the exhaust pipe allowing air to enter, or air injection pumping air into the exhaust (this is done on some cars to aid in the burning of HC). High O2 is not a problem unless the smog machine decides the exhaust sample is too diluted to use; in which case, the car may fail the smog test for dilution.

What causes high NOx (nitric oxide)? High NOx is caused by high combustion temperatures and pressures. Problems that can cause high NOx include:

· An inoperative EGR system. The EGR system was designed specifically to reduce NOx; that's it's only function. The EGR, which stands for Exhaust Gas Recirculation, allows exhaust gas to enter the combustion chamber through the intake. The exhaust gas has a cooling effect on the combustion chamber.

· Over advanced ignition timing. The more advanced the ignition timing, the higher the combustion chamber temperatures.

· Overheating. If the engine temperature is too high the NOx emissions will go up.

· Lean air/fuel mixture. If the air/fuel mixture is too lean the combustion temperature will go up.

· Compression over specification. Normally when we think of engines aging we think of the compression going down. However, there are a couple of ways the compression can increase as the engine is used. One is carbon buildup on the tops of the pistons. When carbon forms on the tops of the pistons, the combustion chamber area decreases and the compression ratio increases. The carbon can be manually scraped off the tops of the piston to correct this problem, but that would require disassembling the engine.

· Mysterious reasons. Sometimes NOx is too high and there's no obvious cause: the EGR works, the timing and advance work normally, the engine temperature is within range, the air fuel mixture is good, and the compression is within spec.. When this happens you can try a little tweaking. If there are two thermostats listed for the car, use the cooler one. If there are two spark plugs listed for the car, use the cooler one. Retard the timing 2 degrees (the maximum allowed). Clear out the EGR passage even if the passage seems fine. Cross your fingers.