Is the stuff blocking the hole relatively soft? If so, I suspect it is the remainder of the zinc anode that broke of from the bronze plug
. This is common. If you take the end cap off of the heat exchanger, you should be able to push the zinc into the exchanger and pull it out the end. Alternately, you can probably use a pick or something to crumble it out the hole it is stuck in.
The exchanger usually doesn't get corroded. Rather it can get a lot of salts deposits on it over time that reduce its capacity for exchanging heat with the fresh water
side. The usual symptoms are the engine
gradually running hotter than normal, or starting to overheat at higher rpms or loads.
If you don't want to remove your heat exchanger for inspection
, simply remove the remaining zinc, replace it with a new one and see if the engine runs at its rated temperature. If so, go ahead for now.
It is a good idea to visually inspect the heat exchanger at some point, though - just for peace of mind. They don't salt
up very easily if the engine has been run regularly at its normal temperatures. If it is salted, cleaning
them is fairly easy. Just remove it and soak it in a bucket of Marsolve or similar scale remover.