This is brief extract from the Denso training manual

Microinjection Quantity Learning Control also called injector pilot learn on Ford

(1) Outline
• In microinjection quantity learning control, the actual injector injection quantity is estimated from the
change in engine rotational speed that accompanies a very small injection. The difference between the
estimated actual injection quantity, and the injection quantity command value at that time are learned by
the engine ECU. This difference is then used to correct the actual injection quantity.
(2) Goal
• Microinjection quantity learning control is used to accomplish the following: 1) to minimize injection
quantity deviations due to injector deterioration over time, and 2) to prevent both engine running noise
due to misfires and exhaust smoke.
(3) Control outline
• Microinjection quantity learning control is automatically performed approximately every 2000 km of
normal vehicle operation, and is completed after the vehicle has traveled approximately 500 km. Actual
learning takes place during the following processes.

Determination for learning conditions

Microinjection quantity learning control is performed when the following two engine operations are
established: 1) a reduction in vehicle speed, and 2) injection is cut off. In the determination process, the
engine ECU then judges whether or not the conditions for learning have been met. The figure below
shows the specific details for learning determinations.
Single microinjection operation

Under the single microinjection operation process, microinjection is performed once the following items
are set: 1) the cylinder for which learning will be performed, and 2) the injection quantity. The figure
below shows the specific settings and controls for a single microinjection.
Detecting the change in rotational speed via injection

In this process, the change in rotational speed can be detected using the set microinjection. The figure
below shows the processing for calculating changes in rotational speed

Correcting the injection pulse width (TQ), and actual injection quantity (Q) characteristics

In this process, the actual injection quantity is estimated from the set microinjection, and the change in
rotational speed. The actual injection quantity is then corrected such that the value equals the target
injection quantity. The figure below shows the processing for the aforementioned corrections.

[ REFERENCE ]

Learning must be performed manually when either an injector (or injectors), or the engine ECU is replaced.
As per the figure below, diagnostic tools are used to perform learning while the engine is operating.