Coolant refill recommendation using vacuum

[Matt B]

During getting familiar with my LT5 I read a lot about the challenges of filling the system with coolant. Here I have a recommendation I'd like to share with all of you as it is pretty much a sure shot. You literally can't go wrong with it, so if not already known you may want to give it a try.

What you need is a cooling system tester / filling armature and at least 40 psi shop air. The one shown on the picture is very common to get over here. As most modern BMW, Audi, etc can't be filled without it either I assume they are available wherever you are.

So, after having the cooling system drained, close the radiator petcock and hook up the cap provided with the set at the top surge tank. Now you have two options:

1) Pressurize the system to check if it holds the pressure or
2) evacuate the system with the same intention.

Option 2 also prepares for adding coolant. The system works as simple as it possibly could. What I learned is that air may get trapped around the water pump during the filling process. So air is your enemy and what you want is to eliminate it. To do so you could shoot your ZR into space - but as far as I know this is currently limited to Teslas
So better use such a testing device. These systems work like a vacuum pump, using a strong flow of shop air and the good old Venturi effect to draw all air out of your cooling system and create a vacuum. And where there is no air it can't get trapped anywhere.

So, to evacuate the system, hook up a compressor at the (in my case) left fitting, a fresh coolant supply hose at the middle and you cooling system cap at the right. From here when I speak about "valve" I mean those on the armature.

Close the valve for the supply hose, open the shop air passages and switch on the compressor. Be careful as the open tube on the left side might spill some coolant that might be caught by the air you remove.
Now you can watch the gage rise, mine typically stops at 25inHG vacuum. At this stage close all valves and let it rest. If your system holds the vacuum for several minutes you're okay. Otherwise check for leaks.

Now comes the fun part. Cramp the fresh coolant hose into a can of coolant (make sure it's enough...), switch on your shop air once more, open the shop air valve, carefully open the fresh coolant valve and watch the coolant rise until it reaches the armature. In that way you avoid that any fresh air trapped in the hose enters your cooling system jeopardizing your efforts. As soon as the hose is filled with coolant, first close the coolant valve then immediately close the shop air valve.
Now your system is fully charged and all you need to do is to open the coolant valve at the armature and watch your fresh coolant getting sucked into the cooling system. Placing the fresh coolant canister on top of the front wheel supports the process by eliminating gravity effects.

Once your vacuum has dropped to 0 and no further coolant is pulled into the system close all valves, remove the hose from the surge tank cap, remove the cap and start your engine. You should immediately see coolant circulating and maybe some air bubbles coming from the drain tube. From here follow the instructions in the shop manual.

Again, as the system (if used correctly) removes most of the air from your cooling system there is little to no chance to have air pockets somewhere. I used this in several applications with outstanding results. In my 2008 Audi coolant came even up to the bleeding screws at the very top of the system which is literally impossible when coolant is just poured into the system even with the engine running.

The one thing that's still unclear to me: how do I get as much coolant out of my system as possible? I pressurized the system with petcock open which forces another half liter of coolant out. However, I hardly get more than 6 liters squeezed out of the hoses at a time. The shop manual recommends a process that's based on repeated dilution of the coolant but this creates loads of waste water.
How do you remove the coolant? There must be a better way...

So, I hope this helps. If needed I might take a small video on that but I'm not a talented video guy...

[TX '90 ZR1]

I used this procedure a few weeks back when I swapped out my water pump and hoses.
I put a suction on the system and let it sit to verify no leaks. Then just simply let the suction pull the coolant in.
Measure the coolant needed and if it all goes in, wala!, system is full.
The procedure is somewhat similar to filling a refrigerant system.

[ImpalaDave]

When I recently did the first coolant flush on my Z I also did this, using an Airlift I borrowed from a coworker. There's a slight learning curve with keeping the fittings straight and not leaking vacuum, and making sure the refill pickup tube stays submerged, but it worked perfectly in the end and I had no issues at all with purging trapped air. If you have access to an air compressor I highly recommend using an Airlift or similar.

As for draining the coolant, I just drained out as much as I could from the radiator petcock. The coolant that was in it looked fine and tested OK, so for me that wasn't a big issue. As far as I know to get most of the coolant out you must open the thermostat housing. I'm going to hold off on that until (if) my thermostat starts acting up because I'm pretty sure I'll be installing timeserts or helicoils into the aluminum t-stat casting!