coolant leak

98 chevy blazer,
lost most of the coolant in a couple of day.
cant to find any leaks, replace water pump 3yrs ago, themostat 1 month ago, upper hose 2 months ago, replace radiator 2 months ago.
need help.

Pressure test the cooling system and watch for external leaks. Lower intake manifold gaskets are a very common problem if the cooling system does not receive proper maintenance. The gaskets can also leak internally allowing coolant into the oil. Pressure test when the engine is cold and watch for drop in pressure. If no external leaks are visible and pressure still drops, it's probably leaking internally and needs the lower intake gaskets replaced.

Can a vacuum test also diagnose a faulty LMG?

Can a vacuum test also diagnose a faulty LMG?

A vacuum test will not diagnose a vacuum, oil, or coolant leak. A vacuum test is good for checking the overall condition of an engine, or for diagnosing a restricted exhaust system.

If you suspect an internal vacuum leak, (underside of lower intake manifold gasket): Remove the PCV valve from the grommet in the rocker arm cover, and inject a small amount of propane through the PCV grommet while the engine is idling. If RPM increases, chances are the intake gasket is leaking vacuum. Reinstall the PCV valve before turning off the engine so the PCV system can purge any propane that may still be in the crankcase.

If you suspect an internal coolant leak, an easy check is to let the engine set for 10 to 12 hours. Coolant is thinner and heavier than oil, so if there is coolant in the oil, it will settle to the lowest point, (the drain plug), and the coolant will drain out before the oil. Very slowly loosen, don't remove, the oil drain plug. If you see coolant... most likely the lower intake gaskets are leaking.

If you look under the engine I'm willing to bet you'll see wet spots, almost gooey like spots. I'm guessing it's your intake gaskets. I lost all my coolant (not as fast as you did) and found that to be the culprit. Had a guy fix them for me. I suggest you do it privately and not at a dealer. It cost me $250 for parts and labor, the dealership wanted nearly a grand.

Pressure test the cooling system and watch for external leaks. Lower intake manifold gaskets are a very common problem if the cooling system does not receive proper maintenance. The gaskets can also leak internally allowing coolant into the oil. Pressure test when the engine is cold and watch for drop in pressure. If no external leaks are visible and pressure still drops, it's probably leaking internally and needs the lower intake gaskets replaced.
OM is correct. My 98' blazer had a lower intake leak back in 06'. I was smelling antifreeze while driving and it was coming from the rear of the engine (near the firewall) leaking onto the exhaust manifold where it was burned. It was costly to repair because I chose to have other services done while they had my engine apart.