I overheated my W115 during the winter after performing a coolant flush. I left the vent plug loose and it popped out on the road test.

I'm not losing any coolant that I can tell and there's no visible coolant related smoke. But I wanted to have the coolant system checked just for peace of mind.

The local shop used a rubber stopper attached to a tube gizmo and then stuffed it into the radiator. They added a blue chemical and fired up the car.

I understand that the chemical changes color in the presence of exhaust gases. It remained blue until the coolant expanded into the tube and, even then, only the bottom changed as the chemical was forced higher.

However, there were periodic bubbles escaping the radiator and into the tube. It wasn't a constant stream of bubbles, mind you, just an occasional belch.

Anyone have a clue as to their cause? Does the test behave differently for a petrol engine vs diesel?

__________________
1974 240D "Boldie" 170K.- New timing chain/freshly rebuilt IP/replaced valve seals/injectors/upgraded stereo/new Bilsteins with Yokohamas/fresh paint and rocker panels plus lots of welds.