View original topic: New Rear Heater Core/Coolant Cleanup
Vwman55 Fri Mar 16, 2007 9:41 am

I bought a new rear heater core from the local VW dealer. Correct part number but the provision for the bleeder is plugged. Anybody have the same experience? The hole is bigger than the old one so it can't be drilled to fit. My concern is the bleeding of the system coolant.
Side question here, besides getting the air out of the system why bleed the coolant? Why not GM, Ford, etc?
91 Vanagon Westy 2.1L

tencentlife Fri Mar 16, 2007 11:08 am

It doesn't have any bleeder because it was unnecessary. Don't bother trying to put one in. Just make sure both your heater valves are wide open when you refill the cooling system. Before the thermostat begins to open, all the pumping pressure is available to the heater loops, so it has no trouble at all forcing the air out of the cores, especially the rear one that is so much lower than the front.

Why bleed the VW when you don't have to do other cars? It has to do with the unusual layout of the wbx cooling system.

The main problem is that the engine is low down, but the top of the radiator is much higher. This wouldn't cause any trouble if the top radiator hose could come all the way back to the engine at the level of the radiator top, like it does within the engine compartment of most cars, but that would have made opening the Vanagon's doors a problem, so instead it dips back down and runs under the chassis and then back up to the thermostat housing. So that leaves this very high spot in the system where air can rise and be trapped at the top of the radiator. The flow of coolant isn't going to grab onto that air and pull it back down to be purged, and any other air bubbles that come along separate from the liquid and accumulate there.

The reason air causes problems has to do with the difference between static and dynamic pumping load.

In a static pumping setup, a pump might be at the bottom of a hill, with the job to lift water to the top of the hill. Water in a column creates a force of pressure due to its weight, called head, at the rate of 1psi/2.31ft. It doesn't matter whether the pipe bends or is sloped or anything like that; the static head is strictly a function of the difference in elevation between the bottom and the top. The pump has to develop more head than is present in the bottom of the pipe in order for water to flow upward, as well as overcome the friction the water exerts against the walls of the piping.

Dynamic pumping loads are different. Picture the same pump at the bottom of the hill, but this time the water pipe goes to the top, swings a 180 and comes back downhill to supply the pump inlet. There is only water in the loop, no air. So even though there would be the same static head at the pump outlet, the same amount of head is also at the pump inlet. Any water the pump lifts is balanced exactly by the same amount of water returning at the inlet. Instead of having to develop head in excess of the weight of the water column, the pump now only has to overcome the friction of the water moving thru the loop, which takes comparatively much less work. The water moving downward helps to pull the water upward like a syphon, as long as there is no air in the loop. It no longer matters at all how high the water column is. The weights of water on both sides of the loop are balanced, just like two weights on either side of a rope pulley. If one is lifted, the weight of the other one helps pull it upward, so the only effort needed is to make the pulley wheel turn and to bend the rope.

So in the first instance, the work required to lift the water is called static head. In the second, the work required to overcome pipe friction is called dynamic head.

So an engine cooling system is modelled on the second instance. It is a sealed dynamic pumping system. Very little work is needed to make water flow around the loop, because it is sealed and forces are balanced on either side of the pump. But introduce some air and things change. The downward flowing water no longer exerts pull directly on the upward flowing water at the top of the loop. Instead, the weight of the downhill water just expands the air trapped at the top, and the syphon effect is interrupted. So the pump requires more work to lift the water all the way to the top of the loop, and consequently, the volume it is able to move over time is less.

Here I copied an explanation of what happens when you fill the wbx system I posted in this thread:

Quote: The radiator bleed opening is already the highest point in the system by far, it's true. But the coolant lines do not have a consistent upward slope, so air can be trapped there. That is why you should elevate the front of the van.

With the standard method, it is also important to raise the rpms to 2000 and fill from the rear, at the expansion tank, while the engine is cold. The higher revs allow the pump to develop better static head, as the system is open to air (an impeller pump has very little static head in an open system, but delivers good dynamic head in a closed loop, which the system becomes once it is filled and sealed).

If the thermostat is at all open, as the fluid is lifted to the radiator, filling it from the bottom, the pump suction will be pulling the fluid back to the motor down the return pipe, and the entire radiator won't get filled to the top. When the T-stat is closed, the return pipe is effectively blocked, and the pump will fill the radiator from the bottom up, fluid will pour back down into the blocked return line, also filling it from the top, displacing the air in the line, and when the return line is filled, the pump will proceed to fill the rad from the bottom to the top, when fluid will finally gush out of the air hole. That is why it is important to fill while the motor is still cold.

If you want to avoid running the engine while filling, for whatever reasons, you could fill the motor thru the expansion tank on level ground, let it settle to allow air bubbles to detach and rise, until the tank will admit no more fluid. Then seal the tank and elevate the front. You could then fill the radiator from the top air hole, but it would be a very slow process, and wouldn't guarantee getting all the air out as well as the standard method does.

Using a tall sealing funnel accomplishes what the standard method does, by creating a fluid column high enough that the static head is equal to the height of the radiator top. But to allow all the air to rise out of the long pipes without being trapped, you would still want to elevate the front, so the funnel column would need to be that much taller.

Overall, the standard method is the least work, in my opinion, and works reliably if you do it correctly. Several things are important to note:

Fill the engine first on level ground, and allow time for air to separate within the head passages. When the fluid level in the expansion tank no longer falls after a time, then cap the expansion tank and raise the front. This way, the water jackets are holding a minimum of air bubbles, and you will avoid spot boiling that will disrupt flow.

Use premixed coolant.

Fill with engine cold.

Raise rpms to 2000 by shimming the throttle stop.

Open the expansion tank as soon as the motor is running, and add coolant as fast as it will take it, never letting the tank get empty.

Make sure coolant is gushing from the radiator top in a clear stream. Seal the airhole as soon as it does. Then fill and cap the expansion tank and shut the motor off. Level the car, and run until warm, making sure there is always fluid in the expansion tank.

Finish by reconnecting the overflow tank and filling it to the Max mark. Note the changes in level here for the first week of running, keeping it between the marks.

Hope that helps it make sense.

Vwman55 Fri Mar 16, 2007 11:32 am

Yes, it does.

bucko Fri Mar 16, 2007 11:55 am

That's it. From now on, Tencentlife is known as "Master Poe" (the blind, wise dude in Kung Foo).

I don't think we will ever get the "pebbles from your hand" unless we hold you at gunpoint...

tencentlife Fri Mar 16, 2007 6:50 pm

"Take the 13mm Snap-On deep twelve-point socket with eight-inch wobble extension and swivel offset handle ratchet wrench from my hand, Grasshopper."

wbx Fri Mar 16, 2007 9:33 pm

tencentlife wrote: Hope that helps it make sense.

Ok... now you are just showing off...


Vwman55 Fri Mar 30, 2007 3:15 pm

So, I replaced the leaky heater core but a gallon of coolant must have
leaked out. I took out the carpet and sound proofing mats and ran them
down to the car wash, sprayed them high pressure water.
After 4 days of drying the sound proofing is still wet with coolant.
Anybody know how to get the coolant out of that matting material?

tencentlife Fri Mar 30, 2007 4:18 pm

I would rinse it with more fresh water, then dry, to get the coolant ickiness out.

buggnuttz Fri Mar 30, 2007 7:18 pm

A good wet vac would help

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