I'm having the same issue right now. New water pump, new thermostat, new radiator, limg replaced not even 10k miles ago. No leaks from any gaskets. Oil is clean, nothing milky on the oil cap. Lower intake didn't have any cracks on it. Plugs are fine.

Wtf.

97 Olds Bravada, 149k miles

Edit: no abnormal smoke or stream from the tailpipe, only the normal condensation burning off in the morning. Other than that, nothing from the exhaust.

 

Rereading the thread, I'm wondering if the pressure test is a way to de-couple the pressure from the heat of the engine and therefore will show a leak that otherwise is not visible due to evaporation of the water/coolant.

I ask as I had the following idea pop into my head: There is a leaky, but not completely failed, LIM gasket that when is leaking while the engine is running is not letting so much water out that it cannot be evaporated by the heat of the engine. But if the system is pressurized at room-temperature, the water would leak out without being evaporated and therefore would be more visible. Also if it is the LIM leaking coolant into the lifter valley, then the oil level will go up due to water accumulating in the sump.

Finally, I read on the forum that water/coolant can mix with synthetic oil. Could that be an issue here?

 

Good thinking.

It is possible that it fails exactly that way.

My other car, the SAAB 9-5 - they are notorious for such a leak. It has started all once in a while to empty the expansion tank. Leaks water onto the exhaust and CAT. Evaporates and you will hardly ever find a small patch of water on the garage floor.

I will have to wipe all the exhaust tubing and the block really clean and maybe you can find some chalky stains after the coolant has evaporated.

 

Also check the radiator cap, a failing one will wisp coolant and not leave a trace

 

Something similar just happened to me. I was loosing coolant but I couldn't find where. Before this I had done the LIM and the heater core, replaced the hoses but still I was slowly losing coolant. I was paranoid that my case was filling with coolant an constantly checked. I finally found it after almost a year later when it did start majorly dripping... It was coming from the water pump weep hole but it only dripped when running and ran down the truck so it didn't leave a puddle. Replaced the pump and no more coolant loss. I will say this SUV has got the most failure prone coolant system of any truck I have ever had.