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 ?
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