The EGR valve is designed to cool exhaust gas by burning exhaust for a second time within the intake system.
This results in the reduction of nitrogen oxide emissions.
Recirculation of the exhaust leads to a gradual accumulation of carbon particles inside the inlet.
To recirculate exhaust back into the intake manifold, a small calibrated “leak” or passageway is created between the intake and exhaust manifolds (EGR Valve).
Intake vacuum in the intake manifold sucks exhaust back into the engine.
The amount of recirculation has to be controlled otherwise it can have an adverse effect as a huge vacuum leak.
Soot forms where the exhaust passes into the intake, and even within the valve itself, causing various problems. Fuel injectors, which are delicate and expensive components of the engine, may become partially clogged. As for the EGR valve, soot and grime may prevent it from properly opening and closing. If it becomes stuck in the closed position, it will lose all function.
The vehicle will continue to run properly, however it will emit nitrogen oxides at levels beyond the threshold of what is legally tolerated according to area regulations. On the other hand, if the EGR valve becomes stuck in an open position, the engine will become saturated at an even faster rate. Over time, the vehicle will lose acceleration power and certain symptoms will manifest, such as persistent stalling.
Common EGR Problems
- Pinging (spark knock or detonation) because the EGR system is not working, the exhaust port is plugged up with carbon.
- Rough idle or misfiring because the EGR valve is not closing and is leaking exhaust into the intake manifold. You may also find a P0300 random misfire code on OBD-II vehicles.
- Hard starting because the EGR valve is not closing and is creating a vacuum leak into the intake manifold.
An EGR valve that is stuck open:
When the EGR is stuck open, it will lead to a vacuum leak which in turn causes inefficient combustion, hesitation, rough idling and even stalling. The reason for this is that the car cannot combust on the carbon dioxide emitted from the tailpipe. An open EGR valve causes exhaust emissions to flood the combustion chamber, hence preventing it from functioning as it should. To check for a stuck open EGR valve, idle a parked car with the brake on and have someone examine the plunger shaft to see if it is stick open.
An EGR valve that is stuck closed:
This causes nitrogen oxide emissions to rise and the car might start to knock. A sharp knock occurs when the fuel in the combustion chamber ignites before the explosion in the cylinder reaches it. This leads to a disruption in engine timing. When this happens, warm up your engine and rev it, then check if the EGR valve will move.
The EGR valve has changed design many times over the years.
Most older EGR systems use a vacuum regulated EGR valve while newer vehicles tend to have an electronic EGR valve to control exhaust gas recirculation. At idle speed the EGR valve should look closed. There is no EGR flow into the manifold. The EGR valve remains closed until the engine is warm and is operating under load. As the load increases and combustion temperatures start to rise, the EGR valve opens and starts to leak exhaust back into the intake manifold. This has a quenching effect that lowers combustion temperatures and reduces the formation of NOx.
Here are some of the changes:
- Ported EGR valves (1973 to 1980s).
- Positive backpressure EGR valves (1973 & up).
- Negative backpressure EGR valves (1973 & up).
- Pulse-width modulated electronic EGR valves (early 1980s & up).
- Digital electronic EGR valves (late 1980s to 1990s).
- Linear electronic EGR valves (early 1990s & up).
Not All Engines Have A EGR
On many late model engines with Variable Valve Timing-(VVT), there is NO EGR valve because the VVT system varies the timing of the exhaust valves to provide the same effect as EGR. By changing the point at which the exhaust valves close when the engine is working hard under load, a small amount of exhaust gas can be retained in the cylinders for the next combustion cycle. This has the same effect on reducing combustion temperatures and NOx as recirculating exhaust gas from an exhaust port back into the intake manifold through an EGR valve. The big difference is that the VVT system can react to changing engine loads much more quickly and precisely than a traditional EGR valve. Using VVT for EGR also eliminates many of the problems associated with EGR valves such as carbon buildup and valve sticking or failure.
Find out what kind of EGR valve is on the vehicle so you can use the appropriate test procedure. Examine the valve or refer to a service manual. On some vehicles, you may find this information on the under hood emissions decal. Also, find out what kind of vacuum controls are used in the vacuum plumbing. Does it have a ported vacuum switch or a solenoid? Follow the vacuum connections from the valve, refer to a service manual or the under hood emissions decal for vacuum hose routing.
There are several ways to troubleshoot an EGR system. You can follow the EGR troubleshooting procedure that’s listed in a service manual for the engine. On late model computer controlled engines, there may be trouble codes that relate to the EGR system. On such an application, the first step would be to read the codes using a scan tool or code reader. You would then refer to the specific diagnostic charts in a service manual that tell you what to do next.