water pump fail-bolt inside engine!

[Rooster300SD]

So, we got our college son a great 240d for $900 from a customer/friend.
He put 16K into it over 15 years or so with a local indy, we have
all the records. He was getting older and wanted to pass it along to
someone who could appreciate it and we were glad to oblige

All of a sudden the car was overheating and pegging at 130 blowing out a lot of coolant.
Didn't think water pump as it was replaced about a year ago. New behr radiator, too.

We drain, citric acid flush, tighten hoses, bench test the thermostat. Nothing.
Still overheating, thinking there's an air bubble in there.

Finally, with nothing left to try, we go for the water pump.
We pull out the pump and.....the impeller stays in the housing.
Pull the impeller out and what do I see sitting inside the engine?
A small bolt! Whoever put that water pump in there left a bolt inside and
it rattled around until it finally hit the impeller and knocked it loose.

A couple of weeks before the overheating I was driving the car, not too fast
and I heard a clunk and the engine suddenly stopped. I was able to coast into
a parking lot. Looked under the hood and couldn't see anything and the car
started right back up so I just drove home. Wasn't overheating then.
Took two more weeks--probably hit the impeller again and finally knocked
it loose. The close up photo shows the impression of the bolt threads
on the impeller. It was a German water pump from MB.

We checked for a blown head gasket and all is well.

I will be calling the shop to let them know they have a sloppy mechanic on staff.

Jeff