Airlift didn't fill the system all the way?

Swapped out a leaking coolant tank this weekend and ended up with over 5.5 gallons + some on the garage floor. Followed a method here on the forum where I drained the tank, drained the engine, slight amount of compressed air and removed one more hose on the other side of the engine to get all the coolant out. When it came time to use the uview system I ran the compressor up to 100 psi, pulled a 25-26wc vacuum according to the guage and it held for the required 20sec, then slowly dropped another 2-3 inwc over the next min. or so while I set up the system. When I added the fill hose to the bucket of fresh coolant it pulled 4 gallons super fast, then took a bit to get about .5 gallons more. Then it read a little above 0 and stopped. I pulled vacuum on it again for about 2 minutes, it got up to 10" WC then added some more. When I disconnected the guage from the tank I lost that extra coolant and it wouldn't take any more (and made a mess of the engine bay). Any ideas on why I'm under by a gallon to a gallon and a half? I had the car in the "on" position with the heater on "hi" as well. I guess I could just go old school, pop the burp valve and putter around till the tank drops but I'm curious why this didn't work for me like I thought it would.

You had a leak (either at where the tool connects to the tank or in the cooling system itself) at the initial pull because the vacuum should not drop at all, not even for 1 inwc, especially only after a few min. The vacuum is supposed to hold the same in 10 min or so. That explains why you couldn't get full 6 gals in...the leak was ongoing and you kept losing vacuum little by little when the coolant was pulled. Also, did you prime the intake tube with coolant completely before connecting it to the gauge head? Forget about turning the heat on, there's no water valve in our car. The coolant keeps circulating the heater core as long as the engine is running. Heated air is generated by diverting fresh air to the heater core. I don't know why the tank puked coolant after you disconnect the tool. If you are only 1.5 gal low, you could use the manual burping and refilling.

Thanks for the quick reply. I did not prime it on the initial pull, which is why I ran it again just to make sure. I guess I'll try manually burping it tomorrow. If there is air trapped somewhere in the system, wouldn't pulling vacuum on the system pull it all out after one or two extra pulls, or would that only go so far as the tank? At this point with 4.5 gallons in the car I'd assume I'm safe from any hot spots provided I take it slow when I manually burp the system. Any need to raise the rear or am I safe just to drive it around with the burp valve up till its done?

Yea, the trick of using the tool effectively is to make sure no air at the coolant intake side (hence the need to prime the intake tube) since any air there will take up the space in the tank that would have been taken up by coolant. When air is trapped inside the system with coolant, you may not be able to extract the air out. Many air pockets can be formed inside the cooling system. Imagine a u-shape tube filled with liquid except at the bottom of the U there's air trapped there. Now if you apply vacuum to both ends of the tube simultaneously no matter how hard you pull the vacuum, the air pocket won't be pulled out. IMHO, you are safe to do manual bleeding since you're less than 2 gals short. A slope will be better but a flat ground should work too. Just keep an eye on the temp gauge the whole time. Take the engine up to operating temp then rev the engine a few times. Then let the engine cool down and top off coolant. Repeat one more time then you should be able to get most coolant back to the tank. Then drive the car with the bleeder valve open for a day, then top off the next morning and close the bleeder valve.

Thanks, I'll give it a shot.

update: Just did it the "manual" way with the car on a flat level surface and got another gallon in there. I still feel like i'm short by a 1/2 gallon or more but I'll let it sit over night, check it in the morning then run it a little bit before I change the oil.

Sounds like you're doing fine.