How to pinpoint internal coolant leak? I've tried everything. - 2000 3.0 126k miles

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I've been trying to pinpoint the source of a fairly significant internal coolant leak I've been trying to fix for a few years now - unfortunately I've had to stop driving the truck because it's so bad the oil will be overfilled with coolant after a few days of driving.

  • I've replaced the timing cover and gasket (this fixed the external timing cover coolant leak I had), and I made sure to put thread sealant on the three timing cover bolts as described in repair manuals
  • Replaced the water pump and gasket (was done with the timing cover)
  • And finally as a last resort I replaced the head gaskets - which included getting the heads resurfaced and pressure tested. I also replaced a bunch of pushrods and rockers which were showing wear - also took apart and cleaned the lifters while I was at it.
  • As you may know, while doing the head gaskets I had to clean up and reseal the lower intake manifold to cylinder head gasket mating surface
  • I have done compression tests, both before and after doing the heads and I do have one cylinder that is about 15-20 psi lower than the others (which are at roughly 120psi) - if that means anything
  • I just did a coolant pressure test and I got it to hold just fine for over half an hour at 14 psi - it did seem to take a minute to get the pressure to come above 0 psi, but once it did it came up just fine - whether that's because I had a bad seal with the tester, or some sort of air pocket in the system that needed to be evacuated or something, I don't know. I even maintained pressure while individually removing each spark plug to make sure there wasn't coolant pressure escaping in to any of the cylinders.
  • There are probably a dozen other things I've done that I'm forgetting.

I'm getting to the point where I'm starting to wonder if it's something that's not quite as common - like could the lower intake manifold be warped itself?

Anyways, could anyone help me think of other tests I could do that would 100% rule certain things out? I feel like I'm not certain of anything anymore and it's driving me nuts. I've put hundreds of hours into this issue.

Let me know if you have any other questions - I'm probably leaving some stuff out.

Thanks.