Lots of great info and misconceptions about how a enigne works. Blackstone does a good job of showing stuff in the oil (if coolant makes it to the oil ) Not all head gasket failures make it to the oil. Not all HG failures show a full failure in the gasket material or Fire ring(Marco description). The bubble test is one of many tests I use to verify a HG failure. The great thing of the Bubble test is that it can Indentify a issue with little to no cost. If I see a bubble test these days I will do a coolant gas test with the Tester from Napa or equilivant(has become a great cheap tool, in the old days I used a snap on combustion gas tester, $400-$500, not a cheap tool to acquire).
so the theory, High pressure that excapes the fire ring will build up into coolant. this will create bubbles in a fully warmed up engine, pressure builds to 13-15PSI before escaping creating bubble(before that it moves fluid out of the radiator) and the Overflow bottle is partially full. It will show a stream of bubbles after the engine is fully warmed. When the Fire ring in the engine can not contain the Combustion pressure(1500+ PSI or more at a 3000 RPM no load)This is way more than a compression test can tell you by doing a compression test or Leak down test. It can escape the fire ring and warm the fluid until the cooling system can not cool the engine well enough.
About the only way an engine does get a steady stream of air when running (beside the combustion), is the water pump seal, or any place that is on the suction side of the water pump (yes, maybe a crack in the hose or loose hose clamp on suction side of the pump. Cavation can be a issue in a poorly designed water pump, would not suspect it from a Toyota design, but possible I guess. Look at Subaru on it water and oil pumps pulling air)
I have seen many head gasket leaks that show not full blowing of the material. But what I have seen is blacken fire rings,, or part of the ring black and black tracks (on head or block) leading to the coolant passage. So, if removing the head, looking at the gasket , and the block and head. If you do not see the full circle of the bright fire ring(or impression) on the cylinder head or block. Then it is not sealing. After cleaning up the block, I find the fire ring sealing area several .00x lower than the rest of the block. Toyota Specs on this is .002 give or take(there is always fudge factors, feeler gauges having their tolerances and such). Only way to get rid of these depression is milling the block. I hand sanded a block(DA sanding device) once in a remote trip to smooth out block enough to get it to what I would be happy with(Grease on pistons to catch debree)
If you have the tools,, like a bore scope, coolant system pressure testing system to pressure test the system. Remove the spark plugs
pressure test the system 18-20 PSI, and run a bore scope into the spark plug holes. If you do have a coolant leak you can not find, this is a test to find it.
Seeing leaks between the block heand an gasket is an interesting site.
We have quite a few newer cheaper tools to diagnose coolant issues. This bubble test I have used since the 1980"s working for Cummins in
Alaska. It is a good first tool for me., If I see a steady stream of bubble. I know it can only be a couple of things, But if it is a strong steady stream,
I know it will almost 99% of the time it will be a head gasket fire ring issue.
But that is not to say, I have not seen catastrophic failures of 1fz head gaskets. Steam out the side of block or tail pipes and full oil pans of coolant. Engines are engines, they fail. and they fail differently all the time.
I know of 4-5 different remakes of the Toyota head gasket for the 1FZ(I am sure there were some for a few other engines that Toyota made).
The first gen 1FZ Head Gasket had the sticky material to bond to the head, and not any other metal to the fire ring to stablilze the fire ring. When the head moved, it broke the bond to the Head gasket. Later designs bonded the HG to the Block, allowing the head to move(heads move on all engines). There was more metal that was off the fire rings to the bolt holes to stablize the fire Ring so it did not move, especially at #1 , #6 Cylinders. Then there was hole size at the back of the block, Things changed there to control the fluid move thru that part to help control the EGR Hot EGR gasses thru this area. If your engine has never been apart, that head gasket is a ticking time bomb(and I have been saying this for years, and you may still have it. really poor design.)
Good luck with it all. And this is My opinion based on my 45+ years of experience of gas and diesel. working on the 1FZ since 1998.
