FAQ #187
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EGR Valve
EGR (Exhaust Gas Recirculation)

Exhaust Gas Recirculation is a method of reducing the noxious output of the exhaust gasses by feeding some of it back into the engine to go through the combustion processes again.

This has the effect of reducing some of the unburnt gasses and chemicals in the exhaust.

They allow a portion of dirty exhaust gasses to be returned to the inlet system of an engine under certain running conditions. They are fitted to reduce Nitrogen Oxide emissions (NOx) of both petrol and diesel engines.

How do they work?

Nitrogen Oxides are formed in an engine as a result of the extreme temperature within the combustion chamber. By allowing exhaust gasses to re-enter the engine, the combustion chamber temperature is reduced and NOx emissions are therefore reduced.

Nitrogen Oxides are colourless, odourless gasses that damage our mucous membranes and prevent our blood from absorbing Oxygen. They cause acid rain and when combined with Hydrocarbons (HC) they are the main smog forming agents.

The valve can be fitted into one of two locations;

* On a pipe between the exhaust and inlet manifolds
* Directly on the cylinder head controlling the flow between internal passages.

The valves can either be vacuum or electronically (ECU) operated.

The valve is only opened during peak NOx output conditions. These are generally at times with low load, light throttle, high RPM conditions such as cruising along the motorway. Under these conditions the valve opens and exhaust gasses are drawn into the inlet system. Older systems relied on the pressure difference between the inlet and exhaust manifolds to open and close the valve. The control of the electronic valves is a little more scientific. The ECU holds details of high NOx conditions and regulates the EGR accordingly. The control of the valve is digital and is precisely controlled not just either open or closed. The majority of valves tend to be of the stepper motor type although DC servo/linear solenoids are not uncommon.

Reasons for failure:

The valve operates under extremely high temperatures and passes dirty exhaust gasses, which are the primary reasons for failure.


The valves usually fail in the open or slightly open position, which results in high CO emissions, a lumpy tick over and poor low RPM and light-throttle drive-ability. Excessive CO exhaust emissions through EGR failure often leads to catalytic converter failure.


the operation of an EGR system can be as simple as accurately measuring the emissions at tick over, with the vehicle up to normal operating temperature, then blanking off the EGR system and re-measuring the emissions. The EGR system

should be inoperative at a tick-over level so there should be no change in emissions during the test. Any changes with emissions or idle stability would indicate an EGR failure.

The externally ported EGR systems can often be blanked off by clamping the pipe that runs from the exhaust to the inlet system. With the internally ported systems a blanking plate may have to be fabricated before testing.

Late vehicles with electronically controlled EGR systems have a position sensor built in and the On Board Diagnosis (OBD) system now has the facility to self diagnose EGR failures. Some ECUs have a periodic EGR diagnosis programme where the ECU will open the EGR valve on a tick-over condition and monitor the idle stability, if the EGR is operative the idle stability will depreciate, once again a failure will be indicated by a dashboard warning light via the OBD.


The EGR system is an important part of a vehicle’s emission system and should never be permanently blanked off. Damage to other emission control devices could result, as well as a substantial increase in one of the engine’s most damaging emissions.

It is a false economy to internally clean an EGR valve, temperature is the most common cause of failure. By the time the valve has sooted up internally to a state where its operation is hampered the operating/closure spring would certainly have been weakened.
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FAQ Posted by C6Dave
Info Created: 07 October 2009
Last Updated: 18 December 2015