Today at Atelier Kaz - ex-Honda R&D, F1, Indy/CART engineer

Shared the following for another owner on NSX Prime so going to cut and paste here for future search.



I wrote this for other owners when asked about how I bleed the cooling system years ago and
it’s cut and paste of multiple messages from the past.

Very old info but should be still useful.


First, the basic.


The method in Workshop manual is OK but mainly written based on the theory.

The entire coolant capacity including the tank is about 16.5L for the C30A and about 16.2L for C32B.

If using aftermarket radiator, you'll need extra amount depending on the spec.

Because the hose/pipe goes up and down across the entire cooling system,
you can't drain all of the coolant even if you replace the radiator, hose and the thermostat.

You will be able to drain only about 12 - 14L of coolant depending on what was removed/replaced and also the procedure taken.

With the air bleeding method in the workshop manual,
if you study each components including the water passage, coolant flow, heater core, etc., you will understand that
it will leave several air pockets within the system unless you use dual stage vacuum pull/fill method.

At the factory, there is a big machine creating deep vacuum and that's how the cooling and the brake system are filled.

Obviously, it starts with an empty system so much easier than already partially filled system.

When you remove the cyl head, you will notice that fair amount of coolant left inside the water jacket/block due to
tilted eng installation and the position of one of the eng block drain port.

Even after opening all of the drain ports, if you have experience of replacing all the coolant hoses,
you would know that quite a lot of coolant was left inside the system.

If using single stage venturi vacuum pump like the one in your photo, the performance is very basic and
heavily depends on the temperature, amount of coolant left in the system, the compressor performance, etc.

I used to use one and I never managed to exceed more than 70% vacuum with my setup.

In fact on the first usage, I measured the amount of coolant filled and straight away,
I noticed huge amount of air left in the system.

If you squash the big coolant hose attached to the thermostat cover many times, you may be able to feel and
hear the bubbles created or if no coolant in there, you may feel/hear nothing.

That’s how much air could be left using the single stage basic vacuum pump on NSX.


It works nicely for the ordinary production cars with the short cooling system length and
much smaller coolant capacity but not enough for our NSX.

NSX has really long cooling system length and as mentioned above,
more than 16L of coolant for NA 3.0/3.2L engine.


For the clarification, it will work if other conditions were met but haven’t seen one at reasonable price tag.

Similar to what you can get for the HVAC vacuum pump between the single, dual and triple stage models.

I also use dual stage pump for the A/C Refresh as it's so much quieter and easier to get rid of any moisture left.

I kept using the single stage basic tool for a long time because I use different bleeding method from the workshop manual any way
so the vacuum fill was more of keeping the area dry and clean.

Not for getting rid of the air pockets.







Later, I switched to the dual stage type and it significantly improved the air pocket situation.

As you can see, always managed to pull deep vacuum and although I still carry out the same bleeding method described below, most likely, no longer required.

By the way, it’s always good idea to measure the amount of coolant filled into the system.

You won’t know how much fluid left in the system so there is no single figure for all cases but if you repeat the same procedure many times,
you would get some idea on the expected amount of new coolant to be filled.

If big difference, something is not right.


Any air left inside the heater core could take ages to be pushed out from there so make sure to bleed thoroughly.

When the eng is cold, the thermostat is fully closed so the coolant only flows through the eng block and the heater circuit.

There is almost no coolant flow through the radiator back to the engine at this stage.

Therefore, any air pockets trapped in this circuit won't move until the thermostat starts opening.

There is a tiny float/jiggle pin on the thermostat that helps the air bleeding process.



Above should be enough as the starter.....



Now the actual bleeding procedure.







Not a must but if you can get hold of the coolant refill funnel that you can attach at the coolant tank filler neck, it will save your time.

Basically, the gravity speeds up the process.


Depending on the model, it will help removing the eng bay cover.

I normally remove it when working around the engine any way.

No need to lift the rear of the car.

Keep the coolant tank cap removed throughout the following process.


Keep water valve fully opened.

Turn IGSW into P2 ON, set CCU at max temperature setting and wait for about 30sec.

No need to actually turn the fan/blower On.

Just keep the CCU Off and turn the temperature dial fully clockwise.

This will move the water valve lever fully open.

Turn IGSW back to P0 OFF after waiting for about 30sec.

Or, if you have disconnected the A/C CCU and can't operate the water valve, just remove the control wire and manually open it.

If you look at the valve lever, there is even an arrow with engraved text 'SHUT'.

Move it in the opposite direction, obviously.

For the LHD, the lever should be pulled towards the radiator.

For the RHD, it's towards the cabin.


After filling the coolant using the vacuum fill or whatever the method you used and before bleeding,
if it’s safe to do so, start the engine for about 30 - 60sec and stop.

This will push some of the air pockets in the heater system towards the heater core or back to the water passage.

Now top up the coolant tank up to the filler neck.


Yes, you read it right.


Fill it to the neck.


This is why I use the funnel at the tank because later, you need to top it up to the neck multiple times.

With the funnel, you can speed up the process by adding extra coolant above the tank.

Attach clear tube (about 2m length) to the 1st bleeder plug (thermostat cover) with the other end in empty container that can hold at least 2L.

You are going to rely on the syphon effect so keep the container as low as possible.

Open the bleeder and keep it open until you collect at least 1.0L of coolant in the container.

Ideally 1.5L.

Hence, you must fill the tank to the neck or above using the funnel.

During this process, keep coolant level above the MIN line of the tank.

You can go well below the MIN line before allowing the air being sucked back into the cooling system but you want big enough safety margin.

You don’t want repeating the same process again.