A Guide to Your Nissan's Emissions System

Is an emissions issue keeping you from passing Phoenix's smog test? Has an issue switched on the “Check Engine” light and is making your vehicle run poorly? While most people think of pollution control consists of just the catalytic converter, there are actually several pollution control systems in your Nissan with several components, addressing fuel storage, oil and exhaust. Here's what those systems do and what can cause them to fail.

Exhaust Gas Recirculation (EGR)

The EGR system pipes some of the exhaust gas into the intake. While the exhaust may be hot, the added thermal mass helps lower combustion temperatures. In turn, this keeps O2 and N2 from the air from breaking apart inside the combustion chamber and joining together to make nitrogen oxides.

This mix is controlled by the EGR valve. It stays closed or mostly closed when the engine is starting or idling, then opens up as engine speed increase to make sure the amount of exhaust gas matches the amount of air entering the engine. The valve is vacuum operated, letting it keep pace with the current engine conditions. If the vacuum line is blocked or broken, the valve won't move. Over time, wear and buildup of deposits from the exhaust can freeze the valve in place. If it's stuck in the closed position, increased NOx in the combustion chamber can prevent ignition, lead to knocking and incorrect engine timing. If the valve is stuck in the open position, too much exhaust will be added, making the engine harder to start. Once running, it will idle poorly and misfire.

Positive Crankcase Ventilation (PCV)

The PCV system collects vapors from the crankcase and sends them to the intake where they can be burnt off instead of letting them enter the atmosphere. These vapors are mostly the result of hot engine oil.

Most of the PCV system is internal. Problems arise when the hoses that lead to the intake split from age, or the PCV valve becomes clogged. This valve is designed to let vapors into the intake while keeping air from being pushed back into the crankcase. Over thousands of miles, a small amount of oil and sludge can make its way to the valve, cementing it in place.

Evaporative Emissions Control (EVAP)

Gasoline releases vapors that are not only harmful to the environment, but are also highly flammable and can cause serious health issues if inhaled. The EVAP system captures these vapors even when the engine is off. Once the engine is running, the vapors are sent to the intake to be burnt.

Keeping fuel vapors out of the atmosphere starts by having a good seal on the gas cap. In fact, accidentally leaving the gas cap off can be enough to trigger the “Check Engine” light. If the gas cap is broken or the seal dries out, the system can't contain the fuel vapors.

At the other end, a charcoal canister holds onto vapors gathered by the system. This canister is linked to the tank and intake by a series of hoses and metal lines. A damaged canister can allow the vapors to escape, or it can release the charcoal granules inside into the system, clogging up the lines.

Exhaust Emissions System

Once out of the engine, the exhaust is cleaned up using a catalytic converter. This device has a mesh covered in platinum, palladium, rhodium and other precious metals that act as catalysts. These catalysts react with pollutant molecules in the exhaust, turning them into less harmful materials.

For the catalyst to work correctly, the exhaust gases must be strictly controlled. One or more oxygen sensors are used to monitor the exhaust, while an air injection system may be used to add oxygen needed for some chemical reactions.

Some vehicles use multiple catalytic converters to break down as many pollutants as possible. Newer designs integrate the first catalytic converter into the exhaust manifold. This design transfers heat from the engine into the converter, warming it up faster so it can break down pollutants shortly after the engine is started. The Cummins diesel in the Titan XD also uses a DEF injection system that sprays urea into the exhaust to break down pollutants.

Under normal conditions, catalytic converters will last a very long time. However, unusual engine conditions including backfiring, extremely rich fuel mixtures and excessive blow-by can overload or shatter the catalyst. This doesn't just increase emissions, it blocks the exhaust, increasing backpressure to the point that the vehicle will be almost undriveable. If you have a failed or broken converter, the engine should be inspected to find the root cause of the problem. 

The oxygen sensors tell the Engine Control Unit (ECU) how the engine is running, allowing it to adjust the amount of fuel being added to the intake. If a sensor fails, the ECU will go into “full rich” mode. If the air/fuel mixture is too lean, combustion chamber temperatures can skyrocket and the fuel can pre-ignite (knock,) causing engine damage. To prevent this, the full-rich mode acts as a fail safe by having the injectors to spray as much fuel as possible. This keeps everything running, but will put a serious dent in power and fuel economy. Sensor issues can also cause urea injector failure as incorrect dosing leads to the buildup of urea crystals on the spray head. 

What Else Can Go Wrong with My Nissan's Emissions Systems?

If you've lived in Arizona for any length of time, you know how dry rot effects anything rubber. While suspension bushings and tires may get the most attention, there are plenty of rubber hoses used in emissions systems from the charcoal canister to the PCV valve. When trying to diagnose an issue, check these hoses first for signs of cracking. Flexing helps redistribute the oils and waxes inside the rubber, so a car that has sat for long periods is far more likely to develop dry rot than one that is subjected to the bumps and vibrations of frequent driving.

Where Can I Get Quality Emissions Parts for My Nissan?

CoulterPartsCenter.com carries Nissan's full line of OEM parts, including emissions components ranging from catalytic converters to gas caps. Our site lets you search by your vehicle's model information or VIN, or you can search by the part using part numbers and keywords like “PCV.” Have questions about a part? Call or email us to talk to our staff of factory-trained parts people.