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Date:         Sat, 22 Sep 2001 11:35:31 -0500
Reply-To:     John Rodgers <jhrodgers@CHARTER.NET>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         John Rodgers <jhrodgers@CHARTER.NET>
Subject:      Re: head gasket problem ?
Comments: To: Malcolm.Stebbins@MSVU.CA
Content-Type: text/plain; charset=us-ascii

Malcolm Stebbins wrote:

> I DIS-agree with the statement right below (by John > Rodgers) > > > If the system is operating correctly, as the coolant > > expands when it is rising to operating temperature, > > coolant will flow from the expansion tank into the > > overflow tank. That is proper. >

Malcolm, in my experience my statement is true. Perhaps further explanation is needed.

The cooling system with it's expansion tank is a closed system with a pressure relief valve which is in the pressure cap. The overflow tank behind the access hatch to the oil filler tube is just a catch tank. It is vented to atmosphere. On my 88 GL there are three little vent holes.

The pressure relief valve is the expansion tank pressure cap.

When the system is properly filled, with the engine cold, the coolant volume should be at it's minimum.

So with coolant volume at its' minimum, but completely filling the system, applied heat is going to result in expansion. That heat comes from the engine. Expanding coolant must go somewhere. Granted there may be a little flex in the system that may accommodate some expansion, but it is not enough to satisfy the needs of the volume of expansion that will occur. When the pressure of the expanding coolant reaches that pressure setting of the pressure cap, the valve opens, allowing the expanding coolant to overflow into the overflow tank, preventing loss of

the coolant. Mind you that the pressure in the system is still high enough to prevent boiling.

If all is well, and there are no combustion leaks, there should absolutely be no gases in the expansion tank. So the system should be completely full. The radiator, the plumbing, the engine, the pump, the expansion tank. All full. In addition, if all is operating properly, the

overflow tank should have coolant with it's level somewhere between minimum and maximum, and the overflow tube will also have coolant in it.

As the coolant is cooled by air through the radiator, the thermostat opens and closes regulating the engine temperature by way of controlling

the coolant flow. When the engine is warming up, the thermostat is closed. When it is warm, it opens and allows the coolant to flow to the radiator. As long as that coolant is in normal operating range, the coolant is in the expanded state, and the system will store the overflow

for later use.

The cooling system in the ENGINE with the thermostat included, is a closed system of it's own. With the thermostat closed water continuously

circulates through the engine. On a cold start, especially on a cold day, the thermostat keeps the coolant from flowing to the radiator, until the water circulating in the engines' own circuit is warmed sufficiently. Then it will open .... maybe. Depends on how cold it is. I

lived in Alaska a very long time and learned about Vanagons and thermostats! Some days were so cold that I don't think the thermostat opened at all. Engine was a greedy little beggar for all the heat it could hold, and I was about to freeze my buns off in the seat up front. Couldn't get enough heat out of the heater to count.

But back to the operation of the system....

In normal operation, especially in moderate to cold climates the coolant

can do a lot of expanding and shrinking. It is mostly noticeable when you shut the engine down. The engine and the entire system start to give

up heat. This heat loss results in the coolant beginning to shrink. As it shrinks, if it did not have a coolant supply somewhere to add to the system, it would draw down and collapse the expansion tank or hoses, etc. or cause an air leak somewhere and draw a big air bubble into the system. However, if the system is tight, the system has extra coolant stored in the overflow tank. As the coolant in the system shrinks, the pressure in the system drops, and is now out of equilibrium with atmospheric pressure. The system pressure is lower. With the pressure in the system lower than atmospheric, back suction is created or a siphoning effect as it were,(I know, suctions don't exist - they are just pressure differentials) and coolant is drawn from the overflow tank, through the tube connection from the overflow tank to the expansion tank pressure cap, through the cap into the expansion tank, thus keeping the system filled to capacity.

If in the passage of time, combustion gases begin to accumulate, they will show up as a big bubble in the expansion tank, displacing coolant to the overflow tank. This bubble will expose the electrodes on the coolant level sensor, and the blinking light will come on. If there is a

sufficient volume of combustion gases present, it may force coolant to

completely fill the overflow tank and then from the overflow tank onto the ground.

Once a big gas bubble is present, the enormous expansion and contraction

of the gas (Boyles Laws) prevents the coolant in the overflow tank from returning properly, and thus a cycle is set up to cause more and more coolant to be lost or fail to return to the system.

Now if all is well with your engine, during your operation of it on a day to day basis, you will experience over time, some coolant loss, and this will show on your daily check of oil and coolant levels (you do at least a daily check do you not?). If all is operating ok, if the coolant is at or slightly minimum during your start for the day, add a little coolant - bring it up halfway to Max, or fill by the book.

I hope this long winded explanation is sufficient to explain my statement above.

As you said, this system is pressurized, however slightly, to prevent the coolant from boiling, so do be careful if you must remove the cap when the system is hot. Put a rag over it or something, or better yet, let it cool down before pulling the cap for service.

Regards,

John Rodgers 88 GL driver


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