Emissions Testing and Exhaust: Everything you need to know

28 Oct Emissions Testing and Exhaust: Everything you need to know

Emissions Testing and Exhaust: How Do These Systems Work?

When it comes to Emissions Testing, the goals of the emissions and exhaust systems are really one and the same. Their goal is to reduce the amount of noise and harmful pollutants coming from your vehicle. After combustion occurs in the cylinders, the resulting gas is expelled into the exhaust system, where it encounters a series of chemical reactions, which helps to turn the poisonous gas into more inert material. The various parts of the exhaust system then baffle this gas and, while some of this gas is recycled into the system, most is expelled out the tailpipe in a (typically) pleasing note.

The main components of the exhaust system are the exhaust manifold, the catalytic converter, the oxygen sensor, the resonator and muffler, and the remaining components that comprise the emissions system are the PCV valve, the EGR valve, the evaporative controls and the air pump. In the exhaust system, the exhaust gases flow through the exhaust manifold, encounter the catalytic converter, and exit through the resonator and finally the muffler.

The Exhaust Manifold

The exhaust manifold, to break it down to its simplest explanation, is a series of tubes (not unlike the internet) that collect the gases from the engine and deliver it to the exhaust pipe. Sometimes, you’ll hear exhaust manifolds referred to as “headers,” which is essentially just a synonym. Exhaust manifolds are typically made from cast iron, stainless steel or ceramic. While cast iron is strong and somewhat inexpensive, they’re prone to rusting and can become brittle over time. Stainless steel is smoother, providing for better exhaust flow, but can be a bit on the heavy side. Ceramic, unlike steel, is lightweight, but can sometimes crack under exposure to extreme temperatures.

The goal of aftermarket headers is to decrease the resistance to exhaust flow (a.k.a. backpressure), and to increase the volumetric efficiency of an engine, resulting in a gain in power. An increase in backpressure (due to small exhaust piping, large mufflers and restrictive catalytic converters) forces the engine to work harder to expel the exhaust gas, reducing power. Headers are the first important step in reducing backpressure.

The following picture is a set of custom FMU headers for a Mercedes C63 AMG:

The Catalytic Converter

Some of the gases in a car’s exhaust can be dangerous, such as carbon monoxide and various volatile organic compounds. The job of the catalytic converter (known for brevity’s sake as a “cat”) is to take those harmful components of the exhaust and convert them into less dangerous compounds using chemical reactions.

Inside the cat, there are two different types of catalysts at work, a reduction and an oxidation catalyst. Both catalysts consist of a ceramic structure coated with a metal catalyst. These metal catalysts are occasionally made of precious metals, including platinum, rhodium, palladium and/or gold. Some of these materials can be expensive, which is why replacing a cat can be egregiously expensive, depending on the vehicle.

The reduction catalyst’s goal is to reduce nitrogen oxide-based emissions; it uses platinum and rhodium to convert the NO or NO2 molecules into pure oxygen and nitrogen components. The oxidation catalyst reduces the unburned hydrocarbons (present in gasoline) by oxidizing them over a platinum and palladium catalyst.

Since it only works at a fairly high temperature, it tends to do very little with cold exhaust, typical when starting a car in chillier temperatures. If you notice a more pungent exhaust during this time, it’s totally normal.

The following is a picture of a catalytic converter on a BMW 135i:

The Oxygen Sensor

All new cars (and most that have been produced within the last 30 years) have an oxygen sensor, which feeds data to the engine’s computer. Its goal is to help the engine run as efficiently as possible and to produce as few emissions as possible.

When a gasoline engine burns gasoline and oxygen, the amount of oxygen relative to the amount of gasoline is referred to as the air/fuel ratio (AFR). There is a “perfect” ratio for this mixture, which means that it’s a mixture that works best with the engine and emissions systems. If there’s too much air, there will be less fuel, and it will be considered as running “lean.” Conversely, if there’s too much fuel, unburned gas will expel from the exhaust, and it will be considered as running “rich.” Both lean and rich mixtures expel more pollutants, and that’s where the oxygen sensor comes in.

The oxygen sensor is positioned in the exhaust pipe, and it can detect what the AFR is at any moment. These sensors usually involve a chemical reaction that generates a certain voltage, which the computer reads as a given value for AFR. If the mixture is rich or lean, the oxygen sensor can adjust the amount of fuel entering the engine accordingly.

The following is a picture of an oxygen sensor and its attached wire harness:

The Resonator and The Muffler

The resonator can exist both inside and outside the muffler in an exhaust system. More and more cars on the road these days have additional resonators on the exhaust pipe, between the cat and the muffler. The resonator is essentially a metal chamber in which various frequencies of sound are combined to emit one certain frequency. Unlike the muffler, it’s not used to stifle the actual sound of the car, just hone it. Using the properties of sound waves, the resonator takes the multiple frequencies of sound coming from the engine and bounce them off each other to create a much more evened out frequency. The exhaust then travels to the muffler.

The muffler is the last part of the exhaust before the tailpipe, usually denoted by a large cylindrical container. Just like the headers (and the internet), the muffler is a series of tubes. This series of tubes is designed to create reflected waves that interfere with one another and cancel each other out. This creates a quieter exhaust note.

The first picture is of the muffler from a BMW 135i, and the second is of a resonator (the square box) from a Mercedes C63 AMG’s exhaust midsection, next to a midsection that contains neither cats nor resonators:

The PCV Valve

The PCV valve, in its lengthened form, is the positive crankcase ventilation valve. Its task is to take the vapors produced in the crankcase (where the crankshaft and motor oil reside) during normal operation and redirect those vapors into the intake system to be burned. The valve’s job is to carefully monitor the amount of vapor entering the intake, as they can dilute the air/fuel ratio to undesirable amounts if left unregulated. When the car is idling, AFR is critical, so very little vapor re-enters the system. At higher speeds, more vapors are allowed into the intake system.

The following picture is from a Mazda Miata, with the PCV valve still installed on the valve cover:

The EGR Valve

The exhaust gas recirculation (EGR) valve’s job is to allow a small amount of exhaust gas into the intake system. It dilutes the air/fuel ratio and lowers the combustion chamber’s temperature. Excessive combustion chamber temperature creates more major pollutants, so the EGR valve is there to minimize it. What’s interesting is that by changing the AFR, this valve is technically doing a disservice to the engine, as it is not designed to run on exhaust gas. Therefore, the EGR valve does almost nothing when the engine is cold or running at full power, in order to minimize detriment to the engine’s internals.

In BMWs that utilize the VANOS system (VANOS changes the intake and exhaust cam profiles to provide for better power and better emissions), the EGR valve doesn’t exist. The changed profile of the exhaust cam closes the exhaust valves slightly faster than normal, trapping just a small amount of exhaust gas inside the combustion chamber, essentially doing the exact same job as the EGR valve, just with one less piece that can potentially fail.

The following picture is an EGR valve from a Ford Taurus:

The Evaporative Control System

If you’ve ever spilled gasoline, you’ve noticed that gasoline evaporates relatively easily. Back in the days before strict emissions regulations, these evaporative emissions were vented into the atmosphere through vented gas caps. Nowadays, the evaporative control system (known as just the “evap” system) prohibits this from happening; the goal of the evap system is to trap and store these emissions. With all gas caps now being sealed and no longer vented, the gasoline vapors travel from the fuel tank through a charcoal canister, which traps the vapors. Once the car is started, vacuum power sucks these fumes from the charcoal and into the engine to be combusted. This whole system is controlled with a purge valve. The evap system is one of many reasons why today’s cars have a better gas mileage than the cars from the 1960’s and before.

The following is a mockup of the entire EVAP system, borrowed from aa1car.com:

The Air Pump

No internal combustion engine is truly 100% efficient. If you happen to find one that is, slap some patents on it and prepare to become a billionaire. Until then, there will always be unburned fuel in the exhaust. To eliminate this problem, they introduced an air pump into the equation. Introducing oxygen to the unburned fuel will create combustion, reducing excessive emissions. It produces no additional power, but in times where there is a greater amount of fuel in the exhaust than normal (such as deceleration), explosions that sound like popping will occur. At this point, the air pump uses a diverter valve to divert the introduced air away from the exhaust manifold, decreasing the likelihood of these popping sounds.

The following picture of an air pump is borrowed from urotuning.com:

Issues That Can Arise with Emission and Exhaust Systems

Exposure to the environment can cause every car’s best friend, rust. If this rust continues to build up over time, it can cause the exhaust piping to become brittle and start breaking apart, forming holes. These holes can divert the exhaust out into the air before it passes the muffler or the resonator, creating a very loud exhaust note that is easily noticeable. This may also increase the amount of pollutants entering the atmosphere. The same problem can happen with the headers, as well. The closer to the engine that a hole appears in the exhaust, the more noise-deadening it skips and the louder it will sound.

In regards to the catalytic converter, a dirtier exhaust can create problems. The dirtier the exhaust, the harder the cat has to work, and the more heat is created in the system. Occasionally, the cat will glow from the excessive heat. A continued exposure to heat of this degree, just like with ceramic headers, will ruin the ceramic catalysts and eventually the whole converter itself. Also, putting leaded fuel in an engine designed for unleaded will completely trash the cat. Thankfully, this is less of an issue each year, as all vehicles are designed to run on unleaded, and gas stations rarely (if ever) sell leaded gasoline anymore.

When the oxygen sensor fails, the computer can’t figure out the proper AFR, so it tries to guess, often poorly. Your car’s mileage will decrease and the car will run rich or lean, providing overall poor performance.

When the PCV valve or the system is clogged, crankcase vapors will back up through the air filter housing. It can also force the excess pressure past seals, creating engine oil leaks. Furthermore, the valve used could be the wrong one, or the system might have air leaks. If these problems occur, the engine will idle like garbage (a very rough idle), and at worst, the oil might get sucked out of the engine. A lack of oil can contribute to a lot of very bad things in the engine (parts seizing and basically putting a grenade in the engine).

When the EGR valve goes wrong, it’s usually due to too much or too little exhaust gas. The EGR valve can also stick because it’s not receiving vacuum or electric power to help it move, or because it’s dirty. A bad EGR will result in a rough idle, bad fuel economy and most likely a loss in overall power. Emissions Testing can detect a sure sign of EGR failure, the increase in nitrogen oxide-based pollutants.

There are two main issues that can arise with the evap system. The first is when the purge valve goes bad. The engine vacuum will then draw fuel directly into the intake system, causing the AFR to go very rich, which will eventually foul the spark plugs. The other problem can occur with the charcoal canisters. Most canisters have a filter that will need to be replaced periodically. A drop in fuel mileage is a good symptom of this problem.

A failing air pump, be it either the pump itself or the belt that drives it, will leave those excessive hydrocarbons in your exhaust, potentially causing you to fail an emissions test and contributing more pollutants to the atmosphere. A failing diverter valve will keep the airflow into the exhaust manifold, providing for more popping when the AFR runs richer.

When it boils down to it, most of these problems will trigger a Check Engine Light (CEL), the light on your dashboard that looks like a small mockup of an engine. Most of the aforementioned symptoms will be accompanied by the CEL. This is the one light on your dashboard that should be taken very seriously, unless of course, you’ve triggered it yourself in the process of adding aftermarket parts.

How to Address Some of These Issues

When it comes to CELs, just because there’s only one light doesn’t mean your computer isn’t telling you the specific problem. Along with the CEL, your engine’s computer gives off a code that specifically pinpoints where the problem is coming from. There are a variety of places that will scan your codes for you (for free), allowing you to figure out what the problem is before you’re forced to deal with it. More often than not, you’ll have to schedule a maintenance appointment as soon as possible after reading the CEL code. However, some codes only mean your gas mileage will drop and you’ll be polluting more, which is a problem but not terribly pressing.

Problems with the exhaust system due to environmental issues (busting holes in the piping from hitting a speed bump, rain and snow contributing to rust, etc.) are best solved by replacing the affected piece or pipe. Your car may sound “cooler” with a giant hole in your muffler, but neither emissions testers nor the police will agree with you. You may be able to get a piece welded on to cover up the hole, but it’s really only a temporary solution for a problem that won’t get any better over time. Rust increases exponentially; while it can’t metastasize and appear on other parts of your car, it will grow and continue to eat through whatever it’s already touching.

Once the problem has been pinpointed, part replacement is more often than not the answer. Replacing valves, gaskets or piping before they contribute to a larger problem is essential. Replacing the part and resetting the CEL isn’t the end to the process of fixing your emissions system, though. Read on to find out why.

The Dread of Emissions Testing

Once again, getting the CEL addressed promptly is paramount to your car’s longevity, and it’s even more important when it comes to emissions testing, as cars will automatically fail with any CEL showing on the dashboard during the testing procedure. But replacing a part and resetting the CEL will not immediately solve the problem.

After the repairs have been completed and the CEL code has been cleared, the car will run a series of self-tests during a special test drive called a Drive Cycle. In essence, the Drive Cycle is there to determine whether or not the Emissions Testing system is ready to perform its given functions, and pass emissions testing.

Most Drive Cycles are relatively simple, but occasionally time-consuming. The first test is starting the car when cold and letting it idle for about 5 minutes. Afterwards, the car will need to be driven between 25-35 mph for up to half an hour in certain cars, never passing an engine speed of 3000 rpm. After that, it’s time to hit the freeway; the vehicle needs to be driven at a steady 55-60 mph for at least four or five miles, again never crossing the 3000 rpm threshold. When the engine revs over 3000 rpm or passes 65 mph, the readiness checks will not be completed, and you’ll have to start them over. These tests don’t need to be completed all at once; some readiness checks can be complete while others remain incomplete. Most mechanics will not be able to do this for you (due to time constraints), so they’ll tell you to take the car home and drive it for a week before returning to emissions. This should give you, the driver, enough seat time to be able to complete the readiness checks without knowing them specifically and trying to consciously complete them.

Emissions testing readiness checks depend on the make of the vehicle; for example, a BMW readiness check completion can take upwards of an hour or two. On the other hand, Audis and VWs have a system through which the whole gamut of readiness checks can be completed while parked in a parking lot and holding engine speed at a certain rpm. It’s best to discuss these checks with your mechanic and see exactly what needs to be done.

In regards to emissions testing and aftermarket exhausts, there are some tricks that can be used to pass emissions, but we must stress that we don’t officially condone or suggest that you use these, as emissions is a federal matter and we’re very law-abiding. When running race headers or a test pipe in place of the catalytic converter, the oxygen sensor will need to be run with a defouler. This spaces out the distance between the sensor and the exhaust flow. Without a defouler, the oxygen sensor will notice that you have no cat and will throw out a CEL. The spacing of the oxygen sensor with a defouler will fool the sensor into believing that everything is functioning normally, and no problems will arise.

For the purpose of redundancy, we’ll say it again; don’t tamper with your emissions system with the intent of defrauding the emissions testing. It’s not legal, and it’s probably bad karma, but science hasn’t proved that second part yet.

For more information about how we can help with emissions testing check out this YouTube Video! Have you failed your state emissions exam? Fluid MotorUnion can FIX your faults – FAST and LEGAL!
If you want to set up an appointment for us to help with any emissions testing issues, give us a call at 630-305-3054 or shoot us an email at [email protected]