Engine Misfire: Starting Issues & Troubleshooting

Hello everyone,

Yesterday, when starting the engine (PSA 2.0 8V Diesel *RHS*), a strange situation occurred. A common rail (CR) diesel engine inherently requires more revolutions during the starting process than a pump-injector (PD) engine, but due to starter wear and other factors, I always tend to release the key as soon as I hear the engine start to turn over.
However, the engine wasn't running smoothly; the tachometer wasn't moving, and there was only a rhythmic vibration that could be felt, with a frequency of about 2 Hz, although "vibration" is a rather mild way to describe it. Fortunately, there were no excessive combustion or knocking noises. The whole drama was over after just 2 seconds of frantic thinking, and I simply turned off the ignition.

After that, the engine started normally, with normal sounds, and it completed the 600 km highway journey as usual. No error codes were stored.

What could it be? Was it just a matter of reaching a certain RPM where the engine could barely overcome the resistance?

I would say it's stuck in the startup phase...

Perhaps two cylinders had already started firing, but then the starter motor assistance disengaged too early.

I've managed to do something like that before.
(not because I'm stingy, but because I somehow managed to get my finger stuck in the keychain while turning the key... )

It only ran for about 2 seconds before it turned off.