What Happens If My EGR Is Blocked Off?

by Chris Stevenson
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EGR, which stands for Exhaust Gas Recirculation, comes equipped on most new vehicles, as well as some older models. The EGR valve recirculates burned exhaust emissions back into the intake system so the emissions can be re-burned, or "recycled," out the exhaust. The system operates by the opening and closing of solenoid valves, regulated by a diaphragm. The solenoid valves open and close the EGR by the amount of manifold pressure the valves receive. When the EGR valve is blocked off or completely closes, it will show some obvious signs.

Pinging or Knocking

A clogged or closed off EGR valve will sometimes give off an audible noise resembling a "pinging" or light "knocking" coming from the engine. The trait is nearly similar to pre-ignition or detonation, both caused by abnormally high combustion chamber temperatures. A clogged EGR valve will raise the combustion temperature the same way and can create this sound.

Rough Idle

A clogged ERG valve will manifest itself in the idle circuit. When the EGR passageway becomes blocked off it keeps exhaust emissions and air from circulating in the engine, causing a suffocating condition. The engine idle will be rough, frequently stumbling and unable to hold a steady idle rpm. The rpm may be sporadic, raising and lowering randomly.


Stalling, a complete engine shut-off condition, can happen when the EGV has clogged, shutting off the flow of emission gases. This will happen most predominately at idle — and may not happen all the time.


The exhaust produced by the combustion process contains carbon dioxide, which is partly water. The carbon dioxide, along with burnt exhaust gases, enters through the EGR valve into the intake to reduce harmful NOx (nitrogen oxide) emissions, but it also cools the combustion chamber. If the EGR has clogged, it will allow a noticeable temperature rise in the engine.

Smog Test

If the EGR valve is clogged or completely blocked off it can longer re-burn harmful emissions in the combustion chamber. The NOx emissions will flow unregulated through the combustion chamber and out of the exhaust pipe. The excessive NOx emissions will show up during a smog test and cause a failure. In addition to the NOx emissions, a clogged EGR could enrichen the fuel mixture and produce excess amounts of HC (hydrocarbons) and CO (carbon monoxide).

Engine Light

A clogged or defective EGR valve will trip a "Check Engine" or "Emission Control" light on the dashboard. The owner will have to take his vehicle to a certified test and repair station that has a code scanner. The scanner will then be plugged into the vehicle's computer system, and a trouble code will be identified. If the EGR valve represents the system failure, the code number will specifically identify the EGR as the failed component.

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