I am thinking possible small head gasket leak. I did a combustion leak test and no discoloration of the test fluid was seen.
Do you think K&W Block Seal will solve this problem?
Well, first thing I would do is get a cooling system pressure test that would tell me if there was an internal (head gasket) or external leak (hoses, water pump etc). If you do not find any leaks I would check for a restriction in the radiator next.
The radiator is prone to junk/rust/calcium build up at the bottom of the radiator which you can not see or flush out with a water hose. If the coolant can not circulate throughout the system it will boil back up into the overflow bottle like you are experiencing.
I would check the temperature on both upper and lower radiator hoses when the engine is hot and make sure they are about the same temperature…they should be.
If the one hose is much cooler than the other you have a restriction, and the radiator is usually the main culprit.
Using K&W Block Sealer
If you really do still suspect you have a head gasket leaking then YES, K&W engine block sealer is the best thing on the market. You do not need to remove the thermostat, or the spark plugs….if you want to do it my tried and true method.
Just mix up the sealer in a bucket and add the contents to the radiator and then top off radiator with plain tap water then drive 500 miles total. Drain out the mix and add back the 50/50 water coolant to refill the system. The faster you put on 500 miles the better. Cross your fingers, and it if works it should be a permanent repair.