Exhaust gas recirculation: function and problem areas (Articles)

In part-load operation, a portion of the inducted fresh air is replaced by exhaust gases, thereby reducing the excess oxygen in the cylinders.

This leads to lower local combustion temperature peaks, resulting in less NOx in the exhaust, but simultaneously increases the amounts of soot and unburned hydrocarbons. Therefore, the AGR is essentially just a type of pollutant displacement device, which serves toallComply with the respective emission standards.
The AGR mainly consists of:
- A connecting pipe between the exhaust and intake manifold.
- a) A valve in the intake manifold that controls the intake of exhaust gases and is operated by a vacuum membrane. An electric control valve with inputs for under- and overpressure mixes, depending on the control from the engine control unit (ECU) at its output, the control pressure for the membrane at its output. Hose connections between the membrane, control valve, underpressure pump and overpressure (air filter box). OR
- b) In newer engines, a module with electrically controlled AGR valve with position feedback.
- In some engines: an infinitely adjustable throttle valve (which also performs the function of the idle valve), which creates additional underpressure depending on the angle of operation and thus supports the flow of exhaust gases in the direction of the intake manifold.
- In newer engines: an AGR cooler, which allows a higher recirculation rate through cooling of the recirculated exhaust gases.
The AGR control loop uses the LMM voltage as a feedback signal: When the AGR is open, a part of the normally aspirated fresh air is replaced by exhaust gases, which causes the LMM value to decrease accordingly. The AGR valve (and depending on the engine, also the throttle valve) is controlled so that the air mass target value is achieved according to the AGR characteristic curve depending on the operating condition of the engine (boost pressure, speed and injection quantity).
If the LMM is defective, a too low air mass flow is usually reported. For the AGR control loop, this means that the AGR is already slightly open. As a result, the AGR rate is reduced by the amount that the LMM reports too little.
The exhaust gases flow into the intake manifold due to their higher pressure when the AGR valve opens. The throttle valve supports the flow and enables a more precise control of the AGR function. Monitoring and correction of the actual AGR quantity is carried out based on the corresponding reduced air mass flow meter signal (LMM).
Rule of thumb: the AGR is active in the lower load range and inactive in the upper range.
If the AGR valve is open at high load, this can lead to a dramatic loss of power if only fractions of the air mass needed for the desired acceleration reach the cylinders. In some cases, extreme soot formation can also occur.
However, the MSG (Engine Control Unit) hardly notices all of this in older, vacuum-controlled AGRs, because there is no sensor for the position of the AGR valve and its information about air mass, boost pressure, speed and injection quantity corresponds to normal engine operation.
Almost all newer vehicles have an electrically controlled AGR module with position feedback. Unfortunately, these are also a common source of errors, as internal plastic gearboxes are often used, which suffer from wear and tear over time.
If the AGR valve is only partially blocked, it can open under partial load, while at high load it may open, close, or partially close. The result can be a different power and soot behavior of the engine with each throttle. Other parts of the AGR can cause comparable malfunctions.
A rough check of the AGR function is possible with VCDS. To do this, in the basic settings mode, select the AGR measurement block: The AGR is opened and closed in a few seconds and the current state is displayed in one of the 4 fields.
In an inactive AGR, at idle for the 1.9 l engine, a air mass value of approximately 450 mg/stroke should be displayed, while with an active AGR, it should be approximately half.
Significantly different values indicate malfunctions in the AGR / LMM area. If the higher value for the inactive AGR is not achieved, the membrane valve does not close properly and / or the LMM provides too low values; strongly fluctuating values for active and inactive AGR indicate a stuck membrane valve.
The amount of soot that is returned to the intake manifold over time can be problematic. This mixes with the normal oil vapors from the crankcase ventilation to a greasy black mass, which settles in the intake manifold and on the intake valves, significantly narrowing the intake cross-section and thus reducing engine power.
If this happens unevenly between the cylinders, "relative air shortage" can also occur in individual cylinders, which then produce more soot, which further accelerates the buildup in the intake manifold.
Some of the worst cases are shown in the appendix.
The entire system is designed in software to meet the injection quantity requirement per pedal...
a) the boost pressure is adjusted according to the characteristic curve,
b) the air mass target value is adjusted according to the AGR characteristic curve per AGR.
In the boost pressure range, seemingly excessive pressure values are present in the partial load range, which create air reserves for sudden throttle, thus improving the engine response.
Therefore, in the partial load range, the potentially available fresh air mass is increased by boost pressure and then reduced to the target value by opening the AGR.
This results (for certain speed and load ranges): the higher the boost pressure, the more the AGR opens.So, if some cars are running with slightly increased pressure in the partial load range due to tolerances in the load control (setting of the VTG lever, etc., and possibly due to a stuck VTG!), a higher AGR rate is calculated than in cars with correct or too low boost pressure, and the deposits in the intake tract grow accordingly faster and more.

In principle, the same is to be expected if the boost pressure target is also increased in the partial load range during chip tuning (and the AGR characteristic remains unchanged with regard to the emission class).This is apparently a common tactic to have more "pre-loaded" boost, which results in a more spontaneous and aggressive engine response when shifting out of partial load.


To prevent damage or mitigate it, the EGR system can be disabled in older engines. Although the intake air mass sensor (IATS) signal can no longer be adjusted according to the EGR control map, no fault code is triggered in older engines. With DPFs, disabling the system is not advisable, as it can then become difficult to quickly warm up the engine and/or achieve exhaust temperatures suitable for DPF regeneration! A complete disabling is rarely sensible!


The full-load behavior remains unchanged when the EGR is disabled, because the EGR is already closed there."Secret tips" that are repeatedly read, increased flow and performance (at full throttle) would increase without AGR, are therefore 100% nonsense - unless, in a specific case, a stuck AGR remains open despite full throttle and is only closed when stopped.


What can be minimally observed as the "Boah-ey" effect: the higher LMM value in the partial load range allows for immediately higher injection amounts when pressing the gas pedal (because you don't have to wait for the AGR to close), i.e. the engine reacts slightly more noticeably when pressing the gas pedal from a low load - but only a fraction of a second later, the engine delivers exactly the same power with AGR as without AGR. Further tuning successes through disabling a functioning AGR are technically impossible!In the partial load range, due to the higher combustion chamber temperatures, the consumption can decrease. However, the slightly higher temperatures in the combustion chamber are harmless because they are still below the full load level overall. This means there is no risk for the loader due to excessively high exhaust temperatures, etc.However, due to the changed emissions values resulting from the cessation of the AGR, the vehicle's operating permit is cancelled. Such a modified vehicle may no longer be legally driven on public roads!

Most "emissions-OBD-capable" vehicles (search terms in the vehicle registration document: EU4, OBD2 or EOBD) have the AGR shutdown also recognized and stored as an emission-relevant fault, and the warning light (MIL = Malfunction Indicator Lamp) illuminates.As long as the error has not been deleted, and the subsequent automatic self-test routines of the MSG have not yet written a "complete OK" result to the MSG, the vehicle will not pass (regular) inspection!In caseThe AGR rate will be low enough when the maximum air mass target value is reached, leaving no further error entries in the MSG - as long as no rule errors occur above the error detection threshold.


However, there is no guarantee for this: depending on the driving profile and environmental conditions, the warning light may not light up until thousands of kilometers!


In such cases, you can try to further reduce the load by extending the VTG lever: this also reduces the boost pressure in the AGR-relevant load range, reduces the actual air mass value and thus the height of the error.


Disadvantage: The boost pressure is regulated more slowly when accelerating, i.e. the engine response becomes slower. Depending on how far the VTG lever is extended, the engine no longer responds, which means a loss of power.



There are several ways to (illegally!) disable the AGR, which should only beshort-term exclusively for troubleshootingIs it really the AGR that's the problem, to be used:



-> Blocking the exhaust path, e.g., by a thin sheet between the exhaust pipe and the intake manifold.



-> Interrupting the vacuum line to the membrane box. When disconnecting the hose from the box, its opening should be closed so that no foreign objects are sucked into the vacuum system and the vacuum level does not drop. Requires a reliable closing membrane valve! If it gets stuck in the open position, the AGR will be permanently active even without vacuum.



-> Removing the connector from the AGR solenoid valve also works, but causes a constant error entry in the MSG due to the electrical monitoring of the valve.



Mechanically shutting off the AGR in engines with an AGR throttle valve causes it to continue closing as long as the air mass value is too high, until the air mass target is achieved solely by reducing the air flow. The usual proportion of exhaust gases is then missing during compression, which can cause misfires or the engine may even completely stall.



The growth rate of the oil soot depends on several factors:

1. AGR adaptation value (not possible for current engines)

2. Condition of the hardware that influences the amount of soot (nozzle spray pattern, LMM, compression ...)

3. Driving style: in the AGR operating range, the lowest air mass target is achieved with small injection amounts, which leads to the highest AGR rates.

Above approx. 2500 rpm, the air mass target is so high in most engines that the AGR is also largely closed.



Anyone who wants to keep the AGR closed as often and for as long as possible with their driving style should:arrow:often drive with the highest possible engine load, i.e. in the highest possible gear

:arrow:or keep the engine at a speed of around 2500 rpm as often as possible (but this unnecessarily consumes a lot of diesel)

:arrow:Avoid driving profiles with long periods of minimal engine load by unnecessarily high speeds below 2500 rpm (wrong: e.g. rolling at a constant 50 km/h in 3rd gear, correct: shift to 4th or even 5th gear):arrow:Easily adjust the speed while driving, as far as traffic conditions allow: briefly accelerate with approximately half throttle (at which point the AGR closes), then roll at full speed without throttle (no soot is deposited when the boost is cut off), and then accelerate again, etc.