Hi. Sorry, been a bit busy. I disagree with your mech on needing to deck or 'true' the block. My dad had a rotary broach and made good money decking blocks for race cars, but ... that's what it's for. high-performance high-rpm applications where you need every cylinder to have the exact same displacement and therefore the same impulse. Step 3 after balancing and blueprinting. Your machinists would have caught it and gladly sold you that service if you needed it. You normally only get a warped block by overheating the hell out of it.
WRT pressure testing the cylinders, it's properly called a compression test. The glow plugs are removed, a gauge on a hose is threaded in, and you crank the motor. It'll generate pressure as part of the compression process, and there are reference numbers which one can use to see how good or poor the compression is on the cylinders. If you have one that's substantially off, it's a good indication that something is amiss. I would urge you to have a proper shop do this because glow plugs can swell up and if you're not careful you can brake parts off in the cylinder. That's bad. Berry Berry Bad.
Air bubbles like to park in heater cores, and if you don't purge the block correctly after you change coolant (or say...have a leak) they can stay stuck in there. And we all know what happens to a gas when you heat it. But I doubt this would cause you to get a low coolant light in an hour if anything it would push coolant back into the tank and then take it back when it cooled off.
I'm not super sold on the tests for exhaust in coolant. The DIY one I had was inconclusive at best. But that's me.
You can get on the Amsoil website and order oil testing kits. In fact, if you're running dual-bypass on an LB7 and changing the oil, you should punch yourself (IMO). Throw a Fumoto drain valve in the pan, fill with 100% synthetic and then send a sample to the lab every 7-10k and keep the oil. There are tons of studies. We do it with aircraft all over the world. The sample/test will tell you things about the motor that may have otherwise been masked. I ran my last oil to about 80k, then honestly changed it out of ....maintenance boredom. The lab said it was just fine.
So....you're losing coolant and there's a crazy amount of pressure in the system. Well, that's because the engine is pressurizing the cooling system I'd say. And for reference, I had an HG issue and never got any indication in the recovery tank. You in fact have a coolant filter on there too, so that could only help make it cleaner.
I would as a normal matter of course switch to testing the oil, and see what's in the first sample. I'd do a compression test and see which cylinder it is. Then you only have to worry about 1 side. The odds it's an injector cup but I don't know of a way to differentiate. If you want to pull and reseal all the cups, then you can skip the compression test. I myself like to know. I prefer to have good data both positive and negative. Absence of proof ain't proof of absence. Roz would be doing before and after pressure testing of the suspect cylinder.
Have you pulled the oil cap off when it's cold to see if there's any condensation up inside the cap? That's a tell. Any smoke when it starts or when it's idling?
Honestly I'm a little surprised that the place that did the HG work didn't pull/clean/reinstall/seal, the cups to start with. It's kind of a thing.
Read what the Ferm says here
Injector Cup Sealant Question - Which Loctite????
That's all I can think of. Except I didn't see the part where you put a soda bottle on the coolant overflow hose like I suggested. I musta misread it... (we need to confirm the coolant is venting overboard. This is easy, and free. In fact, you could skip the lab test for coolant in oil. But you're going to have it tested regularly anyway riiiiiiiight?)
Semper Fidelis,
Roswell
