Defective EGR valves can cause serious drivability issues on some vehicles, but for the most part, the symptoms that come with EGR valve failures depend on the nature of the failure. However, it is worth pointing out that on some vehicles, EGR valve failures do not produce any discernible symptoms apart from an illuminated “CHECK ENGINE” warning light and one or more stored trouble codes.
Note that not all of the symptoms listed here will always be present on all vehicles. Moreover, the severity of one or more symptoms listed here may also vary significantly between different vehicles, but at the same time, the absence of any particular symptom on any given vehicle does not translate into evidence of the absence of a fault in the EGR valve or larger EGR system.
Nonetheless, the most common symptoms of a defective, failing, or malfunctioning EGR valve are largely similar across most, if not all EGR-equipped vehicles, and could include one or more of the following-
Drivability issues can range from poor acceleration to severe alternative surging and hesitation upon acceleration when the EGR valve is stuck in the open position. These conditions are caused by the uncontrolled injection of exhaust gas into the cylinders on the one hand, and by injecting exhaust gas into the cylinders at inappropriate times, on the other.
Poor fuel economy
Fuel economy will typically not suffer when the EGR valve is stuck in the closed position, but when the EGR valve is stuck in the open position, the continuous dilution of the air/fuel mixture by exhaust gas means that a driver has to apply larger throttle openings than usual to compensate for the diluted air/fuel mixture.
While engine knocking caused by the absence of exhaust gas in the cylinders under some operating conditions can occur, this is rare. As a practical matter, knocking occurs when the air/fuel mixture ignites prematurely, but this condition is typically caused by sources of ignition other than spark plugs, such as glowing hot carbon deposits on piston crowns or in combustion chambers.
Since the purpose of EGR valves is to introduce metered amounts of exhaust gas into the cylinders to reduce combustion temperatures as a means to prevent the formation of NOx (oxides of nitrogen), as opposed to improving the combustion process, the absence of exhaust gas will very rarely be the principal cause of engine knocking.
Poor idling quality
If the EGR valve is stuck in the open position, exhaust gas will flow into the cylinders in an uncontrolled manner, so if the engine is cold the dilution of the air/fuel mixture by large amounts of exhaust gas will have a greater effect on the efficiency of the combustion process than when the engine is hot.
This is particularly noticeable when a cold engine is running at idling speed. The efficiency of the combustion process even on modern engines is so low when they are cold that ECUs have to enrich the air/fuel mixture significantly and retard the ignition timing by several degrees to achieve stable combustion until the engine coolant reaches a minimum threshold temperature.
Based on the above, it should be clear that when exhaust gas is introduced into a cold engine running at idling speed, the combination of poor combustion and a severely diluted air/fuel mixture could produce an extremely poor idling quality, if the engine idles at all.
The purpose of EGR valves is to allow exhaust gas to enter the cylinders to prevent the formation of NOx (oxides of nitrogen), which means that if the EGR valve is stuck in the closed position and exhaust gas cannot enter the cylinders, the levels of NOx in the exhaust gas will increase measurably.
However, since exhaust gas is introduced in the engine only under strictly defined operating conditions, increased emissions of NOx will only occur under operating conditions that would normally have required the introduction of exhaust gas into the engine. Therefore, under normal driving conditions, elevated NOx emissions will only occur sporadically, and then only for short periods.
As stated elsewhere, it should be noted that some, all, or none of the symptoms listed here might be present on all vehicles that have suffered an EGR valve failure. Moreover, all of the symptoms listed here could also be caused by issues that do not involve the EGR valve or the larger EGR system at all. This means that if you suspect you might be dealing with a defective EGR valve, the first step in the diagnostic procedure should always be to extract DTCs/fault codes/trouble codes from the ECU.
Since all EGR failures and/or issues will typically set fault codes because all EGR issues have the potential to increase exhaust emissions, obtaining trouble codes to verify that there is something amiss with the EGR system, as opposed to anything else, will save you a lot of diagnostic time- especially if you are experiencing severe drivability issues