Below is the full text from eLearn on the Croma EGR. (Sorry for no pictures).
This is what I think will happen/be going on with EGR mods. Anybody please feel free to correct me as I'm just working out/learning based on my basic engineering skills.
Having read the info below you will see that the EGR flow is NOT measured (unmetered). The ECU calculates how much the EGR flow is by noting the drop in air flow through the main aif flow sensor. If the ECU is telling the EGR to pass 10% then the ECU will expect a 10% reduction in the measured air flow. If you blank the EGR off then there will not be a reduction and this is how/when it decides to register the fault with the engine check light.
The reason (I think) there is a loss of power when EGR valves stick open is because if the EGR is supposed to be closed and is stuck open then loads of air/exhaust gas is sucked into the inlet manifold and this air is not metered. Meanwhile the main airflow sensor is only fueling for the low quantity of air it is measuring.
With me so far?
Now to answer your question. With the EGR blanked off in the short term (at least until the engine light comes on) then if the ECU is demanding 15% EGR then it will overfuel by say 15% compared to what it would normally do for the amount of air flowing through the main airflow meter.
So in the short term the engine will be running rich. Hence the noticable incease in engine response that people have talked about/experienced. The down side of this is that more soot will be generated which means the DPF will need to be regenerated more often. Also the inlet air temperature will be reduced as no hot exhaust gasses are getting into the inlet and this will increase "charge density" thus increasing engine power a little more.
Now when the ECU clocks the EGR fault then I have no idea what it does. IF I had programmed the ECU then I would just stop the overfueling compensation an turn the engine check light on. I see no reason the do anything else, e.g. limp home mods etc. as the EGR failure when blanked off is fully compensatable but just stopping the EGR overfuelling compenation. i.e. EGR = 0% exactly the same as when the EGR circuit is shut off by the ECU in normal running.
IF the ECU did nothing and kept the overfuelling in operation as though the EGR circuit was working normally then you may not see much of a reduction in overall fuel economy as modern engines tend to run leaner for emissions reasons. This is why most performance tuning ECU mappers claim more power, torque, driveability AND better fuel consumption.
On all the older diesel engines with EGR, like Stilo, Punto etc. (prior to DPF etc) I realiably told by a number of Fiat technicians the blanking off the EGR is a common last report practice, appears to do no harm and does not affect the emissions regarding offical MOT test measurements.
Personally I would not really like to drive round with the engine check light on as this would be masking and other more serious problem the ECU is trying to tell you about. Yes you can check and reset every week but if I had to go down this route the I would buy and fit one of those engine / driving monitors the plug into the EOBD socket, or a dirt cheap code reader/reseter and leave it permanently plugged in. (ScanGuage is one such monitoring device).
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CONSTRUCTION SPECIFICATIONS
This system makes it possible to send some (5 - 15%) of the exhaust gases to the intake in certain operating conditions.
This lowers the peak temperature in the combustion chamber restricting the formation of nitrogen oxides (NOx).
The E.G.R. solenoid valve (1) operated by the injection control unit (2) carries out the task of reintroducing some of the exhaust gases taken from the exhaust manifold (4) back into the engine intake.
A heat exchanger (3) makes it possible to partly cool the exhaust gases increasing the volumetric output of the engine.
1 - EGR solenoid
2 - Injection control unit
3 - Heat exchanger
4 - Exhaust manifold
5 - Intake chamber
OPERATION
At coolant temperatures of > 20°C with the engine speed between 800 and 3000 rpm, the injection control unit operates the E.G.R. solenoid valve with a square wave signal.
The variation in this signal allows the E.G.R. coil to move a shutter, thereby regulating the flow of exhaust gases from the exhaust manifold to the intake manifold; there are two results:
less air is introduced;
the combustion temperature is reduced (on account of the presence of inert gases) resulting in a decrease in the formation of NOx (nitrogen oxides).
The injection control unit is constantly informed of the quantity of gas recirculated through the information coming from the flow meter: in effect, if there is an intake of a certain quantity of air (Qam) for a given engine speed and the value sent by the flow meter (Qar) is lower, then the difference (Qgr) is the value for the quantity of gas recirculated
Qam - Qar = Qgr
Qam - Theoretical quantity of air memorized
Qar - Actual quantity of air
Qgr - Quantity of recirculated gas
The atmospheric pressure signal is used for the operation of the E.G.R. solenoid valve to recognize the condition of driving at altitude in order to reduce the quantity of gas recirculated and to prevent engine fumes.
DESCRIPTION OF COMPONENTS
E.G.R. SOLENOID VALVE
The Pierburg E.G.R. solenoid valve, fitted on the intake manifold, has the task of modulating the flow of exhaust gases to the intake according to the command from the injection control unit.
This modulation takes place by means of the internal solenoid, with PWM operation by the control unit, which activates the control rod via the internal valve.
1 - E.G.R. valve casing
2 - Internal valve
3 - Gas intake from exhaust manifold
4 - Gas outlet to intake manifold