[Lorenfb]

"When the pick-up coil is connected there is no more noise by fixing the triggering level at 0 V rather than at 0.1 V"

Not correct. Remember the pick-up coil is an inductive device (that's affected by dv/dt - noise),
and it's in a noisy environment (the HV distributor) and the green wire from the distributor
is fairly long, so having a trigger point of 0 volts versus 100mv makes the input MORE suspectible
to noise which does exist. That's why Bosch set the input to a value NOT equal to 0 volts.
Use a scope and measure it.

"For this simple test I would agree, but the instability in the triggering time caused by the 3kHz noise on a simple sine wave signal is very disturbing for the observation of subsequent signals with an oscilloscope."

So, then get a better signal generator AND use a better hookup method which will
allow signals as low as 100HZ for triggering.