About ounce a month someone will past a similar issue.

The Injector threads into the Prechamber Retainer Ring/Collar. Under the Collar and between where the Injector seals on the Heat shield is an empty space around the Injector.

Liquid that gets into that space is going to react to the Heat and the shockway created by the burning Fuel in the Combustion Chamber. When the Engine is still cold but running put your Finger on the Injection Line and go down to the Injector. You will feel a lot of vibration.

So the Bubbles could be caused by the above or you could actually have a leak at the Heat Shelds as you suspect; or even the Prechambers.

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84 300D, 82 Volvo 244Gl Diesel