EGR changes the fresh air the ECU can see
Bosch directs exhaust gas into the intake through a valve so that NOx falls. The recirculated flow is hard to measure directly, because it is hot and contaminated, so the control is indirect. An air-mass meter reads the fresh air. The ECU compares that reading with the engine's theoretical air requirement from data such as engine speed. The lower the fresh air is compared with that requirement, the higher the recirculated proportion. Bosch, exhaust-gas recirculation.
That fresh-air reading is also an input to how much fuel can be burned cleanly. Torque-controlled EDC lists EGR rate alongside boost as a condition the intake system has to satisfy before the calculated fuel quantity produces the torque that was asked for. Bosch, air-intake conditions for torque. A part-load hesitation, a smoky step off idle, or a flat spot that disappears at full load, where EGR is typically no longer the dominant air path, belongs in this comparison.
This page describes the control and the way it feels. It does not describe removing or defeating the system. A road vehicle has to remain within the law that applies where it is used. How NOx and particulates are formed is in petrol and diesel emissions. Where the soot then sits in the manifold and on the valves is in carbon deposits and cleaning.
Why part-load feels different from full load
At low and medium load the fresh-air target can be held down on purpose. The smoke limit, if it is following that reduced fresh air, holds fuel down with it. The driver feels a soft pedal even though the pedal map, read at a high load, looks generous. Opening the pedal further eventually asks for air the EGR valve has to give back. If the valve, the cooler or the manifold is slow or sooted, the fresh air arrives late and the fuel either waits, which feels like a hole, or arrives early, which is smoke. The steady full-load pull does not show this.
A lambda sensor can be put around the air-mass loop. Bosch describes a cascade in which air-mass control stays fast and a lambda loop tightens the EGR accuracy, and a fuel-quantity mean-value adaptation that feeds a corrected quantity into the EGR, boost and start-of-injection setpoints. Those functions are documented control methods. They are not present, under those names, on every later ECU. Bosch, lambda EGR and mean-value adaptation, printed pages 58–62. Learned corrections of this kind are in adaptations and calibration drift.
What else is in the same low-load air path
At low rpm and idle, Bosch can close one intake duct so the remaining port speeds the air up. That flap and the EGR valve are both low-load actuators. A rough idle with the flap shut and EGR open is not diagnosed by a full-load boost log. See idle speed and smooth running.
Boost control has its own desired value, from speed, injected quantity, temperatures and ambient pressure, compared with the boost sensor. EGR and boost both change the charge. Logging only manifold pressure misses the fresh-air meter that the EGR loop is actually using.
A usable part-load log
Record pedal, engine speed, injected quantity, fresh air mass, the EGR command or position if it is available, boost desired and actual, lambda if it is fitted, and coolant temperature. Repeat the same pedal step with the engine hot, because the cold calibration is a different mode, described in cold start and warm-up. If the fresh-air meter and the EGR command disagree with each other, or the command moves and the air does not, fix that path before changing a driver-wish or smoke table. Family-specific air and EGR notes start at EDC15, EDC16, EDC17 and MD1.
Sources
- Bosch Electronic Diesel Control (EDC), 2001, independent PDF mirror. EGR by fresh-air comparison, lambda cascade and fuel-quantity mean-value adaptation, intake-duct switch-off, and EGR as a condition of torque delivery. Printed pages 58–62, 73–74 and 75–77. Accessed 2026-11-11.
- Petrol and diesel emissions and carbon deposits and cleaning. Formation, aftertreatment and where the soot accumulates.