Water pump or...? 1981 240D

[hempev]

So my 1981 240D had on occasion pushed the temp needle to the 120 mark, but would drop down in a few minutes with no other changes (is this what cavitation can cause?) Yesterday, however, something really busted: I was driving on the freeway (only doing 55-60) when suddenly a cloud of steam/smoke came out of my vents and from under the hood. Once I got it cleared out with the window down, I saw the temp gauge was in the red. I got off at a service station, and the radiator was spewing steam from under the cap - as it sat for a while, there was a click and the steam stopped (I think the cap has a 15psi threshold). Getting the cap off renewed the steam bath. After a while, I filled with water (getting a boiling reaction in the radiator at first!), and looked for visible leakage - there was none. I tried to limp home, but in a few miles, steam was coming from under the hood (no more steaming in the cabin, and cranking the heat to max gave me no hot air), so I got off again. Same thing as before - steam from the cap and then a click. I left it overnight and got it home the last few miles with care but no new incident.

I figure the water pump went out, but also wondered why the steam got into the vents - could a dead water pump do something to "blow" the heater core so water/steam leaked through? I have changed out a water pump long ago, but I may be back here asking for help if I get hung up. Let me know if anyone thinks I am looking for the wrong problem. Thanks.