Radiator cap

TrailBeast said

It could even just be a bad cap, as there is no reason for a 16lb cap to do that unless it's getting hotter than that.
(well, unless there is a compression leak in the block somewhere)

IMO, replace the 16lb cap, and if that doesn't do it then a recovery system.
Remember, a higher pressure cap will just put that much more pressure on all your hoses, radiator and heater core.

Click to expand...

I agree 20 pounds is a lot of pressure, especially for most hose clamps.

TB is correct. I have had brand new caps puke like that. I ASSumed that they had a bad run of springs or something. When you shut the engine off, the temp goes up. As much as 50 degrees. The coolant expands and it pushes past the cap, which I'm sure you know. When it can't expand it builds pressure and overcomes the cap.

I'm thinking 20 pounds is asking for a failure somewhere else.