No heat at idle and water pump whistling

[JehTehsus]

Yeah, I wonder about that too. I don't know the full history of the engine unfortunately, so I can only guess. My guess is that at some point something plastic got shredded up (crappy water pump impeller maybe?) and the little shards of plastic built up in that spot and melted together in the heat and pressure of the coolant over time, forming that plastic chunk. It didn't have a defined shape, but rather was formed to the inside of the pipe.

I would say the biggest concern is whether or not that was the only piece - IIRC that flows out of the back of the head, or maybe it is out of the block into the back of the head. Just makes you wonder what else might be jammed in there and where.

I think it would make sense if it was a piece of an old water pump, or possibly even a piece of an old radiator that somehow broke off. Was it definitely plastic, and not just a big glob of hardened gasket maker?

[Chebyshev]

Was it definitely plastic, and not just a big glob of hardened gasket maker?

Yeah, definitely plastic - I had to chip away at it a bit with the screwdriver to loosen it up first and plastic chunks flaked off as I did that.

I agree that there's a concern about further clogs in the system, but the car hasn't had a cooling problem beyond this heater core issue, so I'm going to put it back together and hope for the best. Ripping the engine apart looking for a possible clog with no symptoms is not a good use of time I think.