Exhaust gas from the combustion process inside of an engine leaves through an exhaust pipe. Before emisions, this exhaust left the engine and was vented into the atmosphere. If you have ever done any exhaust work or touched the inside of an exhaust pipe, you're sure to have noticed a black soot like substance coating it. This soot is commonly known as carbon build up.


Emisions equipped engines now have two paths for exhaust gas to flow

        1) out the exhaust pipe and

        2) up to the EGR Valve


The EGR Valve in it's simplest form, is to allow exhaust gas to enter the fresh air intake of the engine. This valve is controlled electronically through the computer. The computer tells the valve when, how much and how long the valve is to remain open. When the valve is opened, exhaust gas mixes with fresh air and then re-enters the engine.


The reason we now mix exhaust gas with fresh air is another topic all together.


Over time the soot that flows through the EGR system and into the engine starts to coat all the different components of the system.


Some of the most common EGR system components include:

- EGR Cooler

- EGR Differential Pressure sensor

- Mass air flow sensor

- Intake temp sensor

- Intake manifold sensor

- Exhaust gas pressure sensor

- Piping


These components are designed to work in perfect conditions. That being said, as soon as carbon starts building up onto these sensors and piping, we lose the perfect conitions. The sensors slowly get covered up and eventually can't do their job. When this happens sporadic codes can be produced and sometimes a CEL (check engine light) can occur. When one sensor reads incorrect data the computer changes paramaters to ensure the engine runs "efficiently". Sometimes this means opening the EGR valve more to compensate for false sensor readings. By doing this the engine might allow excess carbon into it and thus multiplying the effects on the components.


Over time the carbon build starts creating havoc throughout the entire vehicle. If left unchecked, the DPF system can start to fail, air compressors can fail and worst of all... the engine itself can sustain damage causing premature failure.


Below are a few examples of EGR system failures


Emisions Education

Above are 3 very common problems associated with the Cummins ISX engine, but all EGR equipped engines have similar components and issues. The simplest way to describe these pictures is this... Carbon Carbon Carbon. Excessive carbon build up in the EGR system is the root cause for almost all emisions related problems. The soot covers everything in the system and builds up with time.


Here's why carbon gets built up...