1986 Chevrolet Monte Carlo • V8 2WD Automatic • 2,819 miles

I have 1986 Monte Carlo SS. The level of coolant is constantly going down. I have performed a combustion block leak test and the fluid stayed blue during both tests…passing. I have ran a coolant pressure test and it has held the psi. The exhaust doesn't smell like coolant and I don't see coolant in the oil. The coolant will drop to the point where it is about 50% coolant in the radiator and 50% empty (it never goes below that point no matter how much I drive it…it normally doesn't take much idle/drive time to drop to 50%), yet the car doesn't overheat and the coolant never seems to drop below the 50% point. There is engine run on when shutting the car off (about 25% of the time when shutting the car off in drive) which maybe related?
May 24, 2012.

There are a few possibilitites and it sounds like you may have more than one issue going on.
First, I am assuming the mileage is 281,300 instead of 2819.
With that kind of mileage, if you have not mainatained the anti-freeze, changed and flushed every 24 months 30,000 miles, there could be so many things wrong it might be hard to pin the button down on one issue. Corrosion could be so bad that the temperature gauge is not showing the engine overheating when it is. The sensor for the fan switch could also be corroded in the same way and keeep the fans from coming on at the right time. General corrosion will make the system inneffective.
I know you pressure teseted the system, but the radiator cap could be allowing pressure to escape and keep the cooling system from working right. In this case you might see the coolant overflow near ful after having had filled the system. Try re-placing the radiator cap.
You could have an intake manifold or possibly a freeze plug allowing anti-freeze to escape.
Otherwise a crack in the block is about the only way I can see that much fluid escaping withtout causing a lot of smoke or some other noticeable isssue.
Make sure you are bleeding the air out of the system corrextly when you re-fill it with coolant.
Turn the heater all the way on and leave thje cap offf. Then fill the system with the 50/50 mix and allow it to reachn operating temperature. If you don't see some mayjor dropp in level the thermostat mgiht be bad.
Re-fill and then relace the cap and drive around for a while. Allow the car to sit untikl cool, be careful removing cap.
See how much fluid you have lost. Fill the overflow and the radiator in the same amnner and check again. The coolant should be topped off. Most cooling ssytems hold around 1-2 galllons of coolant.
Try to determine the time it take s and mialege it take to lose a few qts and measure the amount it take sto fill.
Here are two diagnostics to help you further figure out the issue. They may help you recognize a behaviour or issue. Just use the links below.

It's 2,819 miles on the engine. It was replaced 6 years ago when the body/frame had 120,000 on it.

May 24, 2012.
Okay, so I am assuming it sat around a salvage yard.
When this happens the Anti-Freeze seldom gets changedever.
Depending on whether the motor was pulled out of the car already or still in the car might give an indication as to where the problem might lie.
Water settles out of anti-freeze, which is an alcohol, as it looses its ability to hold water in suspension with the ethylene glycol. The water then settles to the lowqest point in the motor. If it is still in the car, it is usually the lower cooling jacket in the engine block. If the motor is out of the car, it may be on either side or somewhere else depending on how it was stored. Often times it is at least close to the same place.
However, you ran a pressure test so that makes the idea of a cracked blocked a very low probability.
I am still thinking the radiator cap and thermostat and bleed the system then see what it does.