Too many people don't understand emissions systems, or why they exist, and waste time & effort removing or disabling them. Early systems were inefficient, cumbersome, unreliable, and produced adverse effects on engine performance, causing many people to think that emissions systems were inherently bad. They aren't. Modern systems are transparent to the operator, and do a fair job of reducing smog & improving air quality, considering the number of vehicles that exist, and the number of miles they're driven. Some even HELP engine performance.

Here's a list of Automotive Terms & Abbreviations.

WHAT ARE VEHICLE EMISSIONS?
Anything that escapes from a vehicle is an emission, including sound. Mufflers reduce sound emissions; open headers, dump valves, & glasspacks don't, and most jurisdictions have laws against excessive noise, so sound emissions ARE regulated. But the term "emissions" in this context more often refers to chemical emissions generated by the fluids used in vehicles, & the operation of internal combustion engines. Fluid leaks are the worst type of emission since they're so concentrated & toxic, but oddly, they're the least-regulated in the US. Even a small coolant leak can kill several animals within hours, but I've never heard of a ticket being issued or an inspector failing a vehicle for fluid leaks. The only harmless fluid leak is A/C evaporator condensate, since it's just cool, dirty water. Windshield washer fluid often contains poisonous alcohols (as surfactants & antifreeze), so it would be an unhealthy leak. But the emissions that get the most attention are fluid vapors & exhaust gases. The EPA has mandated an 8-year/80Kmi compulsory emissions warranty on all vehicles sold in the US since 1995.

EVAPORATIVE EMISSIONS
Recently, these systems have become combined, but originally, there were 2 distinct evaporative systems: fuel tank vapor, and PCV. The PCV system was developed to stop the early practise of simply venting crankcase vapors (and often fluids) out of a valve cover, where they were allowed to drip to the ground, or blow away. Fuel tank vapor has always been a concern since it represents a loss of fuel, which has become increasingly expensive. It's also a fire hazard, and extremely noxious & toxic.

PCV
No matter how new or well-made an engine is, the piston rings (or seals in a Wankel rotary) can't capture 100% of the combustion gases. There will always be some blowby, resulting in contamination of the crankcase oil. These contaminants most often include water (the ideal result of combustion, which remains a vapor at normal engine temperature), fuel (fuel molecules are smaller than oil molecules, so they pass by the rings more easily), soot (which turns the oil black), and various acidic gases. To reduce the accumulation of these contaminants (which rapidly affects the oil's viscosity & effectiveness), the crankcase must be positively ventilated. This means forcing a draft of air through the crankcase to carry these vapors out. But rather than venting them under the hood, the vapors are contained within the PCV system and routed into the intake system to be burned in the engine.

Since the system is powered by negative pressure (engine vacuum), I'm going to describe it in reverse. The PCV system ends with a tube carrying the vapor-laden (& often oil-laden) airstream into the intake manifold to be burned. This tube comes from the PCV valve, which regulates the quantity of air "leaking" into the intake, and also contains a one-way valve to prevent backfires in the intake from burning into or overpressuring the crankcase. (The valve or the tube may include another port where the fuel tank vapor system is combined.) The PCV valve is installed either in an oil separator chamber outside the crankcase, or in a valve cover which often contains an oil separator, or sometimes midway in the tube to make access/replacement easier. The valve must be replaced regularly because its mechanism is lightweight (generally gravity-operated), and is easily fouled by normal engine operation. The oil separator is necessary to prevent crankcase oil from entering the intake system, fouling sensors, coating the valve stems (which accelerates wear on the valve guides), fouling the spark plugs, or increasing HC emissions. Because most oil separators are not designed to be easily serviced (and rarely if ever appear on any maintenance list), their benefit is typically lost on high-mileage vehicles, and the inside of the intake manifold suffers. The airflow thru the separator comes from the crankcase, where undesirable vapors have boiled out of the oil. On engines with 2 banks of cylinders (V or flat), the airflow is generally into one valve cover, down thru the oil drainback journals in that head, into the crankcase in the block, up the other drainbacks, & into the 2nd valve cover. On inline engines, the flow is most commonly in one end of the valve cover & out the other, but some have the oil separator on the side of the block near the pan so flow is down from the valve cover to the crankcase. The airstream enters the valve cover either thru a dedicated nipple on the cover, or thru a vented oil filler cap. In either case, the airstream originates with a fresh-air "breather" filter, usually inside the engine air cleaner housing, but sometimes simply mounted directly on a valve cover.

Several failures are common in the PCV system; the most-often noticed is oil contamination in the intake &/or the air filter housing. Oil in the intake generally indicates that the oil separator has become restricted, which might be caused by gelling of the oil from moisture buildup due to insufficient PCV flow because the valve hasn't been changed on-schedule. But infrequent oil changes or overheating, or any combination of these conditions can contribute to oil in the intake. Oil in the air filter housing is almost exclusively caused by reverse-flow in the fresh-air tube, which is often the result of worn/stuck rings, hardened exhaust valve stem seals, or a ruptured head gasket. But it may also result from low-quality oil, incorrect viscosity oil, or excessive oil. An often-overlooked failure in the PCV system is cracking of the hoses, resulting in vacuum leaks & contamination of the engine oil. All vulcanized rubber (tires, hoses, bushings, etc.) ages & deteriorates, so it must be replaced as needed. A symptom that shocks many people is the presence of light-colored foamy oil residue inside the filler cap, or in the valve covers. And while it's possible that this effect can be produced by severe engine damage (like coolant in the crankcase), it's much more likely that it's caused simply by the vehicle being used only for short trips, during which time the engine never fully heats up to boil the water out of the oil. The moisture naturally condenses in the coolest parts of the crankcase, which is the thin upper sheet metal valve covers & filler neck. It may also be noted in the top of the dipstick.

Fuel Tank Vapor
Gasoline is extremely volatile in almost all environments, and even diesel is aromatic. Since these vapors can be flammable or noxious, they must be contained & routed to the engine to be burned. But they are produced even when the vehicle is unused for long periods, so a simple tube from the fuel tank to the engine would still allow them to vent out the air filter. Also, during hot weather or violent maneuvers, the quantity of vapor generated can exceed the engine's capacity at low RPM, so the vapors must be stored & their flow regulated.

The system begins in the fuel tank where one or more valves are used to vent vapor pressure, but also to exclude liquid from the vapor system due to overfilling, slosh, or rollover. There may also be a pressure sensor to monitor the system's operation & effectiveness, and/or a vent valve (CANV solenoid, or built into the cap) to allow fresh air into the fuel tank or vapor system. As vapor exits the tank, it flows thru a tube to a canister containing carbon (activated charcoal), which absorbs the fuel vapor, but allows air to pass. Depending on the size of the fuel tank, there may be several canisters, or a larger canister. Older canisters are vented, but they're known to collect water, so most modern canisters are sealed. Another tube leads from the canister toward the engine's intake, but it may contain a regulator valve (CANP solenoid, or VMV). The vapor system may also combine with the PCV system at this point.

Being virtually a zero-maintenance system, most faults are simple valve failures, hose leaks, or mechanical damage (collision, road debris, etc.).

Faults in the evaporative systems are usually detected by the use of a special machine which pumps a non-toxic non-flammable high-visibility smoke into the vapor lines to make leaks evident. But a common source of evaporative codes on '97-04 vehicles is the operator not securing the fuel filler cap. Earlier vehicles didn't detect this, and later vehicles are designed to exclude this from turning on the CEL.

EXHAUST EMISSIONS
The obvious source of a vehicle's chemical emissions is the engine, where fuel is first atomized (to speed vaporization) & mixed with air (which is ~75% Nitrogen & only ~15% Oxygen) and then ignited in some way to produce pressure which acts on the pistons to turn the crankshaft & propel the vehicle's mass. The chemical process of combustion is often oversimplified to CxHxOx + O2 = CO2 + H2O (the stoichiometric ideal), but there's actually MUCH more going on inside an engine. Not all the fuel is vaporized; the fuel molecules aren't always perfectly paired with Oxygen molecules; other chemicals participate or interfere; the heat released by the reaction can trigger OTHER reactions; tolerance & wear on the mechanical components doesn't always result in ideal combustion; the environment (weather) can affect the process; and the electronic controls (including the ignition system) may not operate exactly as intended.

HC
Unburned fuel is probably the single biggest concern in vehicle emissions, not only because it's the most detrimental to the environment, but also because it's a waste of money. As engine management technology has progressed, a continually-increasing proportion of fuel is burned within the combustion chambers where it produces useable energy. Possibly the single biggest step in this direction is EFI, which results in MUCH more precise control of fuel flow, MUCH better atomization, and consequentially higher engine efficiency & reliability. Electronic engine management has also contributed significantly by instantly adjusting fuel delivery to the engine's exact state, and to the operator's needs. But overfuelling still occurs frequently (for several reasons), resulting in unacceptable HC emissions. The earliest attempt to reduce these emmissions was the addition of a device to "re-burn" the exhaust & consume this fuel (a "thermactor"). Engineers found that pure Platinum metal facilitated the reaction between fuel molecules & oxygen in the hot exhaust stream, without consuming the Platinum (meaning that it "catalyzes" the reaction). So powdered Platinum was mixed with ceramic clay & formed into honecomb-shaped tube extrusions to be incorporated in the exhaust system. Given its high surface area, the vast majority of the unburned fuel could be catalyzed before being emitted, but the Lead that was being added to gasoline as an anti-knock agent coated the Platinum, requiring UNleaded fuel to be produced. (The anti-knock agents in unleaded fuel are cheaper than Lead, but oil companies recognized the opportunity to gouge consumers & priced the new fuel accordingly.) But the high cost of Platinum & the expenses associated with developing the technology caused early designers to undersize catalytic converters, resulting in exhaust restrictions that noticeably reduced engine performance. Their initial solution was to add air to the exhaust (secondary air) using a belt-driven pump so that the fuel would burn more easily. But again; those early systems were too complicated (vacuum controls) & poorly designed for the typical mechanic to understand, so they were often neglected, modified, or sabotaged causing most people to think secondary air was counterproductive or unnecessary. Over time, and with the development of EFI, the cost of producing catalytic converters has come down, and the quality of their construction has gone up, making them very reliable & effective. So effective, in fact, that most now don't require the addition of downstream air. They have also been improved with additional catalyst chemicals that reduce CO & NOx emissions (3-way cats). Currently, the single biggest threat to a catalytic convertor/thermactor is probably mechanical damage. Collisions, road debris, improper service technique, & fording can shatter the delicate ceramic structure, causing exhaust restriction, noise, & increased emissions. But another significant threat is severe overfuelling (either because of fuel delivery or misfiring) which can overheat the ceramic substrate to the point that it powders & erodes. Modern engine management systems include dedicated downstream Oxygen sensors to monitor the catalysts' performance, but this performance generally has no impact on engine performance (exhaust restriction being the main exception).

The 2ndry air system is known to fail in a wide variety of ways. The check valves that prevent hot exhaust from entering the rubber hoses age, rust, leak, & crack open melting the plastic TAB & TAD valves, creating exhaust leaks that can damage other components, raising exhaust oxygen levels (setting lean codes or rich adaptive limit codes), and making rattling noises. The hard steel tubing between the exhaust & the check valve can rust or crack (especially the infamous "crossover tube" on the backs of V8 heads). The vacuum controls leak (including the "coffee can" reservoir on the R wheelwell), get misrouted during other repairs, or the diaphragms rupture. The electronics that control the vacuum controls can fail electrically or mechanically, or the wires can be damaged. But all of these failures are either A) relatively cheap & easy to repair, or B) cheap & easy to prevent with normal inspection & maintenance.

NOx/EGR
To get the maximum power & efficiency from an engine, most designers set the fuel/air mixture slightly lean, and advance the ignition timing. But these adjustments also result in very high combustion temperatures, which allow the formation of oxides of Nitrogen (air's 2 main components). These compounds dissolve into rain to form acid which affects agriculture, lakes, & even stone buildings and paint. Another way to increase the engine's power is to reduce its moving mass by using Aluminum & its alloys for the pistons & connecting rods. But the high temperature can even oxidize the Aluminum & burn through the pistons, causing catastrophic engine failure. (Aluminum heads don't suffer as badly since they're water-cooled.) So to permit this increased performance, AND to reduce emissions, engineers found that introducing a metered quantity of inert exhaust gas back into the intake would significantly reduce the combustion temperature, WITHOUT a corresponding reduction in power or efficiency. As with other emissions systems, early implementations of EGR had problems that lead to a common misconception about its practicality. The engineers designing the alloy pistons weren't necessarily using the same design parameters as those developing the EGR systems, so both were overly conservative, and performance suffered. But modern engine management systems are more synchronized, and EGR is actually beneficial when properly maintained. Modern catalytic converters are also designed to reduce NOx emissions. Engines designed without EGR are either running rich (to keep the combustion temps down), or are using exceptionally-precise operating parameters to minimize NOx formation.

Failures in the EGR system commonly result from the same type of vacuum leaks & wiring damage that can affect the 2ndry air controls, but excessive soot in the exhaust can block the EGR journals in the intake, resulting in insufficient EGR flow. Also, the EGR valve's pintle can crack, allowing exhaust to pass even when the valve is commanded closed. There is a common misconception about water contamination inside the PFE/DPFE, but that water is safe & insignificant; the actual cause of that problem was a design flaw in the sensor itself, which has been corrected. On some older engines, the EGR's external tube is known to crack or rust allowing an exhaust leak, but modern tubes are stainless & much more reliable.

VECI
Take a minute to open your hood & REALLY look at the stickers & labels. There should be one either on the hood, the air filter cover, or the core support that gives the tune-up procedure, spark plug type & gap, codes for the emissions systems & components, and the vacuum map. If it's aging & fading, take a GOOD photo of it to keep in your records, or find the exact same one onlnine so you'll always have it. It can take a little while to figure out what the map's colors & abbreviations mean, but the map is laid out BASICALLY the way you see the engine when you open the hood. Ford always uses the same color for a given function on every vehicle of every year model. For instance, a green vacuum line is ALWAYS for the EGR; red is always manifold vacuum; black is always vacuum supplied thru a check valve &/or reservoir. So even if you don't copy the original vacuum system precisely, connecting all the lines of the same color will PROBABLY work.

SO WHAT DOES ALL THAT MEAN?
If you look back at each system, you'll notice that each is virtually independent of each other, which means that a fault in one doesn't mean that ALL the emissions systems have failed. And you should also note how simple each one is, when you understand what each component does, and how it's designed to work. So even if you're not required by LOCAL laws to maintain your vehicle's emissions systems (FEDERAL laws always require it, and don't think your local laws will never change), there's no reason to start ripping & shredding what you don't comprehend. You're more likely to HURT the engine & make it perform WORSE than to solve any symptom you've noticed. Fix the problem - don't create new ones. A few minutes & dollars spent repairing an emissions system will pay itself back RAPIDLY in engine reliability, and in the maintainability of a stock system. If you've ripped out a bunch of random vacuum lines & weird parts, you'll probably find it VERY difficult to get helpful suggestions over the internet since NO ONE (including YOU) will know what you've screwed up.

Here's a list of Automotive Terms & Abbreviations.