Rough Idle

• An EGR valve that fails to close properly can act as a vacuum leak, causing the fuel and air mixture to become too lean, resulting in an engine that sputters and misses at idle. This can also lead to engine stalling and hesitation upon acceleration.
  
  

Increased Fuel Consumption

• When an EGR valve fails to open and close at the proper rate, at the proper time, overall engine power is reduced due to decreased combustion chamber efficiency. This results in the engine using more fuel to do the same amount of work it would normally do under normal operating conditions. Modern engines also make use of computers that can read this as an incorrect fuel ratio, and instruct the fuel system to compensate by introducing more fuel into the mixture.
  
  

Overheating

• The introduction of exhaust gases into the combustion chamber by the EGR valve serves to reduce the temperature of the ignition process. If the EGR valve fails to open, combustion temperatures increase, resulting in higher overall engine temperatures, which can lead to engine overheating.
  
  

Pinging and Knocking

• Pinging and knocking commonly result from increased engine temperatures. These increased temperatures cause pinging and knocking by creating a condition where the fuel and air mixture within a cylinder ignites prematurely. If an EGR valve is opening improperly, it can increase engine temperatures, causing the pre-ignition of the combustion process before the proper timing, resulting in engine knock.
  
  

Increased Emissions

• The EGR valve's main purpose is to aid in the reduction of noxious gases produce by the engine. It does this by lowering the temperature of the combustion process through the introduction into the combustion chamber of controlled amounts of exhaust gases. This lower temperature has the effect of reducing the amount oxides of nitrogen (NOx) produced by the engine. A faulty EGR valve will fail to properly cool the combustion process allowing these gases to increase, which can lead to an abnormally high NOx reading during emissions testing.