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" Your car is running beautifully, and all seems right with the world. You have no trepidation whatsoever when the time comes to take it in for your state or local emissions test. After an interminable wait in line at the inspection station, a smiling, snappily dressed functionary shoves a probe up your tailpipe and asks you to gun it. Moments later, the inspector smiles wearily as he hands you an envelope--not a new inspection sticker for your license plate, but an envelope that says &amp;quot;FAIL&amp;quot; in bright red letters. You have only a month to do your homework and pass inspection. Heck, getting into college wasn&amp;#039;t this hard. Wasn&amp;#039;t your car designed to meet these emissions standards? Haven&amp;#039;t you done your best to maintain it with regular oil changes and tuneups? What&amp;#039;s gone wrong? And, most important, how can you make it pass? Take a minute to read the rejection slip carefully. Typically, it will give you the test results for your car along with the acceptable limits. Note which of the &amp;quot;terrible trio&amp;quot;--hydrocarbons (HC), carbon monoxide (CO) or oxides of nitrogen (NOx)--is out of bounds.  Raw Gas Hydrocarbon emissions are simply unburned fuel being pumped raw into the exhaust system. Misfiring is the most likely culprit, and that can come from an ignition problem, or an internal engine failure that reduces compression. Another possibility is a mixture that&amp;#039;s too lean to catch fire dependably because of either a fuel system malfunction or a vacuum leak. If a high HC number isn&amp;#039;t from a miss, it&amp;#039;s probably due to an overly rich mix. So, worn or fouled spark plugs are the first things to suspect. If you can feel a dead miss at idle, use insulated pliers to pull plug wires one at a time to see if one in particular does not have any effect on smoothness (don&amp;#039;t do this on &amp;#039;96 and newer cars with OBD II, as it will set a misfire code and turn on the Malfunction Indicator Lamp on the dash). Remove and inspect that spark plug. If it&amp;#039;s okay, use an ohmmeter to find out if the plug wire is open--you should meter 5000 to 20,000 ohms, then examine the distributor cap for heavy corrosion in the sockets, cracks, etc. With the plug out, do a compression test--anything much less than 100 psi should be suspect (a valve may be burned). Also, look over the intake manifold to see if there&amp;#039;s a vacuum tap in the runner leading to the problem cylinder. A leak here can lean out the mix to the point of misfire at idle. There are a couple of other things that can supply enough extra gasoline to put hydrocarbons above the limit. One is the evaporative-emissions control system that traps gasoline vapors that would otherwise escape into the atmosphere. It then meters the vapors into the intake during off-idle modes to be burned. If it flows at idle, the mix will be too rich. Another potential culprit is engine oil that&amp;#039;s diluted with gasoline. Fuel vapors are drawn through the Positive Crankcase Ventilation system and into the intake manifold. Smelling the dipstick may tip you off to this, and an oil change will correct the problem--temporarily.  Older cars might benefit from tweaking the idle mixture screw no more than a half turn in either direction to get the idle mixture setting correct.  Many cars no longer have adjustable ignition timing, but you should check it with a timing light nonetheless. Killer Compound Carbon monoxide is produced by incomplete combustion--the air/fuel charge is ignited, but isn&amp;#039;t burning properly or entirely. In most cases, this is due to fuel-system troubles that make the engine run rich, but intake air restrictions could affect it too. It&amp;#039;s important to note that some combustion is required for CO to be created. So, you won&amp;#039;t see a high CO reading if there&amp;#039;s no spark. With a carburetor, richness might be due to an idle mixture screw that&amp;#039;s open too much, a heavy float (the foam type may absorb gasoline, and the brass pontoon type can develop holes and fill up with fuel), a choke that stays on after the engine has warmed up, or a power valve that sticks in the open position. With fuel injection, perhaps the fuel pressure in the rail is too high due to a faulty fuel-pressure regulator, or there may be a cracked vacuum hose to the regulator. Of course, just about any car built within the last two decades will have an electronic engine management system that controls the air/fuel mixture (among other things), so consider the sensors that feed information to the computer. This, however, graduates into professional-level troubleshooting. On a precomputer carbureted car, one way to get under the HC and CO limits is to retard the ignition timing. This will slow the idle and provide a longer, hotter burn. Then, bring the idle speed back up to specs with the throttle stop screw or idle stop solenoid, which will admit more air. Performance will be off, but you&amp;#039;ll probably pass the test.  A cracked vacuum line can cause the charcoal canister to malfunction, increasing emissions.  A misfiring cylinder may have bad rings or valves, which can be spotted with a compression test. Hard Nox NOx is a tough one. It forms when nitrogen (78 percent of the air we breathe) combines with oxygen (21 percent of the atmosphere), which can happen only at very high combustion temperatures. The trouble is, many of the things that were initially done to cut HC and CO involved dramatically increasing the heat inside a car&amp;#039;s cylinders. That heat created plenty of NOx. Fortunately, it was found that only the peak temps (over 2500\u00B0F) produced this smog-promoting gas, not the long, hot burn that oxidized the other two pollutants. Introducing Exhaust Gas Recirculation (EGR) knocked the top off the temperature curve, thus putting NOx within bounds. One of the main reasons electronic engine management systems were developed in the late 1970s was to allow a 3-way catalyst, which adds rhodium to the 2-way cat&amp;#039;s platinum and palladium to further diminish NOx. The first things to suspect are an inoperative EGR valve (perhaps the vacuum line is cracked, or the internal diaphragm has ruptured) and carbon-clogged EGR passages. Try applying vacuum to the valve at idle. If the engine starts to run roughly or stalls, the valve is okay and the passages are at least partially open. Remove the valve and dig out all the deposits you can. There&amp;#039;s a good chance that the catalytic converter is no longer capable of reducing NOx, but determining that requires high-tech diagnosis best left to a professional. There&amp;#039;s another distinct possibility that you can deal with, however: heavy carbon deposits in the combustion chamber and on the backs of the intake valves. These can raise the compression ratio enough to cause the hot flash that forms NOx, and will also interfere with the carefully engineered swirl pattern of the incoming mixture that makes it burn evenly. Numerous treatments have been devised to avoid the huge job of removing the head and manually cleaning the combustion chambers, piston tops and valves, but the only one suitable for the do-it-yourselfer is chemical treatment. The most basic approach is to pour a suitable additive into the gas tank. A more thorough job can be achieved by adding detergent- or solvent-based cleaners directly into the intake stream. Too much heat is another thing to think about. Is the engine running hot? Is the intake air overheated due to a jammed Early Fuel Evaporation valve, or is a Thermostatic Air Cleaner door in the snorkel stuck closed? Ignition timing that&amp;#039;s too far advanced can result in detonation that causes NOx to rise. A switch to premium fuel may reduce it. A final note: Moisture-laden air can lower NOx output by as much as 30 percent, so you might consider visiting the emissions station on a rainy day. "^^<http://www.w3.org/2001/XMLSchema#string>
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