I think the block heater is boiling the coolant because now as the system doesn't fully depressurize after using the heater. I haven't used the block heater or run the car for a couple days and the cooling system was still under pressure when I took the cap off from when I last used the block heater. Not good.
I think I may know what the issue is. On my engine, there is a small plug inside the large hole for some of the blocks that don't have an oil cooler. Although I removed the plug, the hole is still quite a small diameter and the block heater is a couple inches away from the water jacket so I wonder if the small hole is restricting the convective flow of warm coolant out of the block heater and into the block, causing the block heater and the coolant within it to overheat. Since convection is the only force circulating the coolant, it's path needs to be pretty unrestricted for flow to be decent.
I bet if I bored out the small inner threaded hole so it was the full size of the outer threads that would help a lot, but I can't think of a way to do that without getting a lot of crap into the block. I wish I thought to do that while I had the block torn down before I put it in the car
