I have recently fully rebuilt my engine, minus the block and pistons. All the gaskets were replaced, valves lapped, valve lashes tuned etc. I did it because I had an overheating problem and a head gasket failure. I performed straightedge and feeler gauge tests to make sure the previous overheating and head gasket failure did not cause cylinder head warpage.
When I put the coolant back, I used one of these no-spill funnels to make sure the system is fully burped by running the engine with the funnel on, which would compensate with coolant any bubbles that get burped out. I really believe I squeezed every single little bubble out, also because I turned the heat on to make sure the heater core gets its flow. I also filled the overflow tank up to about half where the FULL notch is.
What I noticed when I first drove it more than a few miles is that there was a splatter of coolant around the overflow tank, when I lifted the hood. I though the whole rebuild was in vain and overheating was back even though the truck did not overheat at the time (but I though the splatter was an indication it was going to). However, then I drove it another 3 hours in both the city and highway and there was no more splatter or overheating. In fact, the cooling worked so well that even after 3 hours the rad cap was not too hot to be taken off and the coolant level was proper.
The question of why there was initial splatter still remained. Here is how I attempted to rationalize it:
Under normal thermal expansion, the coolant, which was cold when filled 100% into the tank, spilled over into the overflow tank. But the overflow tank also had coolant in it, which I put in when I refilled the system, so there wasn't enough room left to absorb the expanded part and it splattered out the excess via the excess valve. Once the amount of coolant in both the system proper and the overflow self-regulated by splattering the excess, there was no more splattering as there was now room to absorb thermal expansion overflow in the tank.
Is the above a tenable explanation?
Should you put no coolant in the overflow tank when refilling the system with cold coolant to leave enough room for thermal expansion ?
In general, under healthy engine circumstances, how much coolant should be getting exchanged between the cooling system proper and the overflow tank? Is it to be expected that the overflow tank fills with thermal expansion overflow when the engine is hot (not overheated) and then that gets sucked dry (or evaporated) back into the radiator when the engine cools down?
I am also an avid plumbing DIY-er. In case that you are too or at least familiar enough, does the coolant overflow tank in a vehicle perform the same function an expansion tank does for water heating systems, which is to absorb the liquid volume that expands under heated conditions? Since you don't put water in an expansion tank when installing it, by the same token should you not put coolant in your overflow tank?