Idle is a closed loop on a small quantity of fuel or air
When the accelerator is released, Bosch's diesel idle-speed control has to hold a target speed that itself changes with the operating mode. Cold idle is usually higher than hot idle. Idle is also raised when electrical-system voltage is low, when the air conditioner is on, or when the vehicle is rolling freely. The controller keeps changing injected fuel until measured speed matches that target. The loads it has to cover include the alternator, the air-conditioner compressor, the steering pump, high-pressure fuel generation, and the engine's own friction, which depends strongly on temperature. Bosch, idle-speed control.
The target is kept low because a lot of real driving is idle, and emissions and fuel consumption are the reason given. Bosch also states the disadvantage: smooth running and pulling away get harder. A hunt, a stall as the clutch bites, or a flare when the air conditioner switches on is this loop missing the load or the friction it was calibrated for. It is not automatically a full-load smoke or torque map.
In the torque-controlled structure the idle controller is one of the internal torque demands. It acts at that stage, together with surge damping, rather than as a second pedal. Bosch, internal torque demands.
One cylinder not doing the same work
Equal injection time does not produce equal torque from every cylinder. Bosch lists cylinder-head sealing, friction and the hydraulic injection components as reasons. The speed fluctuation that follows is felt as a rough idle and, in the source, as higher emissions. Smooth-running control compares the speed after one cylinder's injection with a mean speed. If that cylinder is slow, its fuel is increased; if it is fast, its fuel is reduced. The primary aim described is comfort near idle. A related quantity-compensation function is described as extending that idea into medium speeds so the cylinders receive matching quantities. Bosch, smooth-running control.
A displayed per-cylinder correction is the controller's reaction. It does not name the fault. A correction that has grown after injector, compression or coding work is a reason to look at that cylinder before rewriting the idle target. EDC15's family page points at the same Bosch description; the correction channels are not universal.
Low speed is also an air and swirl problem
On the applications Bosch describes, a flap can close one intake duct at low rpm and at idle so the remaining duct speeds the air up and combustion improves. At higher rpm the duct opens again for filling. Exhaust-gas recirculation is also active in much of this region, so the fresh-air reading and the idle fuel are tied together. Bosch, intake-duct switch-off and EGR. A rough or smoky idle that only exists when hot and at low load belongs with EGR and part-load behaviour as well as with the idle loop.
Petrol idle uses air and ignition more than a diesel quantity loop, and the family pages differ. ME7's fuelling notes treat idle trim as a check on injector behaviour after the base calibration is coherent, not as the first place to hide a fuel error. See ME7 fuelling and MAF calibration. MG1 and Simos coordinate idle inside the torque and air structure described on their own pages. Do not copy a diesel quantity correction into those calibrations.
What a stable idle does not prove
Idle can look correct while the first touch of the pedal still shunts the drivetrain. That transition is pedal response and surge damping. Idle can also look correct when cold and then wander once friction drops and the target falls. Cold targets, glow and warm-up are in cold start and warm-up.
Log target idle speed, measured speed, the fuel or air the controller is using, coolant temperature, the air-conditioner or electrical load, and any per-cylinder correction. A stall as the clutch comes up should include clutch switch state if the ECU has one: Bosch lists the clutch signal among inputs whose plausibility is checked against vehicle speed.
Sources
- Bosch Electronic Diesel Control (EDC), 2001, independent PDF mirror. Idle-speed control, smooth-running control, intake-duct switch-off and torque-controlled idle. Printed pages 50–53, 73–74 and 75–77. Accessed 2026-11-11.
- EDC15 fuel control and ME7 fuelling. Family notes that already separate idle corrections from full-load maps.