[FarCry]

When was the last time you replaced the two zincs in the block? If they are very worn they can fall off the stud and pieces could migrate into your cooling system. I don't have a manual in front of me but there is zinc in the front and one in the back of the engine.

What did I do before Google? Found a service manual online here http://j30.us/files/2qm15_servicemanual.pdf
Turn to page 7-8 or 100 and see where the pencil zincs are located if you do not already know.

 

[FarCry]

When I first replaced the zincs on my 2qm I couldn't believe how much crud, scale, zinc pieces etc were in there. I don't recall having too much trouble getting the large square head nut loose on either one. The one of the front of the engine may have required removal of the alternator belt for better access. It's been so long, I do not recall for sure.