Coolant issue.

[JCL]

Originally Posted by WagnerX5

It wouldn't be a head gasket issue unless you saw it coming from a head gasket. Since it was coming from the cap, the cap was most likely faulty. The cap is there to give the radiator a certain "level" of pressure. This was obviously wrong, so simple cap change should solve your problem.

That is only true for a head gasket leak if it was passing from the coolant passages in the head gasket to the outside. If it was leaking from the cylinder to the coolant passages, it would pressurize the cooling system and cause a problem similar to what the OP saw, pushing coolant out, complete with bubbles in the coolant. It would be more likely to be happening when the engine was running, but it is still an area to check. White smoke would be another indicator.

My point is that the problem appears to be at the cap, but it could be a symptom and not the root cause.