I'm not arguing that it's not easy to simply remove the head on these engines THEN use your eyes to visually see where the issue is, but a test is not useless if it works.Changing d-series gasket will take less time and money than performing useless tests. Problem will be seen with bare eye when head will be pulled off either it's gasket (most likely) or cylinder crack (99% it's not).
Those tests I suggested will CONFIRM without a doubt to the owner that he needs to remove the head.
Of course through experience, the OP's symptoms are obviously that of a blown head gasket/cracked block or head. I never once thought this wasn't the case.
I was simply suggesting to him that to remove his doubt of whether or not the head needs to come off to perform the tests suggested and one of them will fail which will require removing the head. The OP seemed very reserved about this even after his tuner told him it was most likely a head gasket issue. The tests suggested should remove the doubt.
Pressurizing the cooling system and pulling the plugs out the way I suggested DOES work. I have used this as a diagnostic tool many many times to diagnose customer's vehicles with similar symptoms to the OP's. IF this test does work, occasionally it doesn't because the leak may not be large enough, it will narrow down the area of where to actually look at the block/head surface when you need to pull the head.
I am a diagnostic technician by trade covering many areas of the motive power industry, and most of the times my customers or shops ask me to VERIFY the most likely cause of a failure without spending too much up front time. The tests suggested would take me less than an hour to perform. If I can present a series of tests to them that I performed, show them which ones failed and why, this almost always sells further work to the customer. It shows confidence on why you need to dig into the guts of an engine, instead of just assuming.
I've been bit in the ass many times in the beginning of my career from just assuming, which is why now I almost always like to back up my labor with some form of diagnostic test. I only suggested tests I have used hundreds of times through the years to verify these types of symptoms. One of them will fail, and confirm the need to remove engine components.
The argument here seems to be between our styles, where your style is just "do it", pull the head and see what you got because you've seen this 100 times.
My personal style is more like: I have definitely seen these symptoms 100 times as well, but I also like to invest a little time into knowing what kinds of mechanical failure possibilities might exist. Sometimes my customers want a worse case scenario before ever tearing the engine apart. Based on the severity of what I find during my tests, I can actually recommend that worse case scenario. Most times, customers are pleasantly surprised as the failure was much less severe than expected. You can't do that for a customer without performing tests as your only other choice is investing the time involved with a tear down. You can give them a little bit of a glimpse into what "could" be instead of just saying "this is what it's gonna be, nothing else".
I like to be able to fallback on different scenarios, as I have accounted for the worst case scenario in an estimate and in talks with a customer. You never want to short sell expectations to your customers, because the minute it backfires the first thing a customer says is "Well, you told me it was gonna be this. Now you're telling me I'm gonna need all this extra stuff too??!!"
Account for most everything through logically targeted diagnostics, then there are no surprises in the end on both sides. It's just a difference in styles, neither of us are wrong. In the end, the head is going to have to come off for inspection regardless of what happens because our experience tells us this is the case.
I'm curious, would you recommend the same "pull it and see" method on a $30,000 diesel engine where pulling the head in the vehicle sometimes requires 24+ hours in order to remove? Or would you spend an hour or so running some tests to make damn sure that it was the head? Modern engines have a couple different ways of combustion gases could be getting in to the cooling system, presenting all the same symptoms as the OP, but one scenario costs $600 and the other could cost $6000. Giving the worst case scenarios through testing coupled with experience covers your ass in almost all cases.
It's all about our personal styles in the case of the OP, neither of us are wrong and I never said you were wrong either.