The most common causes of insufficient EGR flow are carbon build up in the EGR valve and a malfunctioning EGR valve. The EGR valve, or the exhaust gas recirculation valve, reroutes exhaust gas back into the intake manifold where it is drawn into the engine cylinders. Without proper EGR flow, a car will not pass an emissions test because insufficient flow causes a high saturation of noxious gas.
The easiest way to determine what is causing insufficient EGR flow is to take the car to a mechanic. When a diagnostic test is run, certain codes indicate that the EGR valve needs to be replaced. If no codes result from the diagnostic test, check to see if there is excessive carbon build up in the valve. To fix this problem, take a speedometer cord and fray the ends. Insert the frayed ends into the valve. The ends of the cord attract and remove carbon from the valve.
Another common cause of restricted EGR flow is a blockage in the tubes leading from the EGR valve to the intake manifold. When a line is blocked, the tubing becomes distorted and enlarged. It is important to inspect the lines for cracks or breaks because this also causes insufficient EGR flow.