coolant gushing out of resevoir vent at normal operating temp.

I have a '97 jetta TD, and I thought I was overheating only at high speeds,
but it seams to be loosing coolant threw the vent on the resevoir? (
presure in cooling system?). Then it begins to overheat.....Since there is
no rad cap, does the resevoir cap regulate the presure?

Yes
And those caps sometimes go bad and/or the seal pops out of position

"jetta107" > wrote in message
lkaboutautos.com...
>I have a '97 jetta TD, and I thought I was overheating only at high speeds,
> but it seams to be loosing coolant threw the vent on the resevoir? (
> presure in cooling system?). Then it begins to overheat.....Since there is
> no rad cap, does the resevoir cap regulate the presure?
>

"jetta107" > wrote in message
lkaboutautos.com...
>I have a '97 jetta TD, and I thought I was overheating only at high speeds,
> but it seams to be loosing coolant threw the vent on the resevoir? (
> presure in cooling system?). Then it begins to overheat.....Since there is
> no rad cap, does the resevoir cap regulate the presure?
>

If the coolant is at proper mark on the reservoir when cold, the remaining
space consists of air. The air pressurizes, and the coolant expands as it
heats. If the engine is operating within the expected temperatures, no
coolant loss should occur, ever. The pressure is regulated by the heat, not
the cap on the coolant reservoir tank. All cooling systems for automobiles
are pressurized at normal operating temperature. So, I don't know what
you're implying. The facts even seem to be questions in themselves from
you.
Dave

that doesn't answer the question of why the coolant gushes out of the
resevoir vent at normal operating temp....something is causing the system
to over presurize! I'm also getting mixed feed back as to the resevoir cap
does or dosen't release some presure, as a rad cap does.

jetta107 wrote:

> that doesn't answer the question of why the coolant gushes out of the
> resevoir vent at normal operating temp....something is causing the system
> to over presurize! I'm also getting mixed feed back as to the resevoir cap
> does or dosen't release some presure, as a rad cap does.

First thing should be to put a new cap, or a known good one. As explained
in other posts, when the seal becomes weak and not holding the riht
pressure, the boiling point comes down to that of regular water. As normal
engine operating temperature is not far from the boiling point of water, a
weaker cap is less able to prevent the coolant from boiling, lowering
therefore the cooling ability of the system.
All in all, higher temperature and coolant spitting can be the result of
weaker coolant cap seal, and cap pressure control system.
Also important, too much pressure in the cooling system can be the result of
a weakend head gasket...

al.

"Dave" > wrote in message
nk.net...
> "jetta107" > wrote in message
> lkaboutautos.com...
>>I have a '97 jetta TD, and I thought I was overheating only at high
>>speeds,
>> but it seams to be loosing coolant threw the vent on the resevoir? (
>> presure in cooling system?). Then it begins to overheat.....Since there
>> is
>> no rad cap, does the resevoir cap regulate the presure?
>>
>
> If the coolant is at proper mark on the reservoir when cold, the remaining
> space consists of air. The air pressurizes, and the coolant expands as it
> heats. If the engine is operating within the expected temperatures, no
> coolant loss should occur, ever. The pressure is regulated by the heat,
> not the cap on the coolant reservoir tank. All cooling systems for
> automobiles are pressurized at normal operating temperature. So, I don't
> know what you're implying. The facts even seem to be questions in
> themselves from you.
> Dave
>

Rubbish.
The Cap Controls the Pressure. The same weather on the Rad or the reservoir
tank.

See below taken from
http://www.innerauto.com/Automotive_..._Pressure_Cap/

The radiator cap acts as more than just a "lid" for your radiator; it keeps
your engine cool by sealing and pressurizing the coolant inside it.



What makes the radiator cap special is that it is designed to hold the
coolant in your radiator under a predetermined amount of pressure. If the
coolant was not kept under pressure, it would start to boil, and soon you
would have boiled all of your coolant away.



However, the radiator (or pressure) cap prevents this from happening by
exerting enough pressure to keep the coolant from boiling. Normally, water
(coolant) boils at 212 degrees F, but if the pressure is increased, the
boiling temperature is also increased. Since the boiling point goes up when
the pressure goes up, the coolant can be safely heated to a temperature
above 212 degrees F without boiling.



What makes this important is that the higher the temperature of the coolant
is, the greater the temperature gap between it and the air temperature is.
This is the principle that causes the cooling system to work; the hotter the
coolant is, the faster the heat in it moves to the radiator and the air
passing by. So, a cooling system under pressure takes heat away from the
engine faster, which makes it more efficient.



If your cooling system is under too much pressure, it can "blow its top"! To
prevent this, the radiator cap has a pressure relief valve. The valve has a
preset rating that allows it to take just up to a certain amount of
pressure. When you turn the cap on the filler neck of the radiator, you seal
the upper and lower sealing surfaces of the filler neck. The pressure relief
valve spring is compressed against the lower seal when you lock the cap.



The radiator filler neck has an overflow tube right between the two sealing
surfaces. If the pressure in the cooling system exceeds the preset rating of
your cap, its pressure relief valve allows the lower seal to be lifted from
its seat. Then the excess pressure (coolant, air) can squish through the
overflow tube to the ground or the coolant reservoir.



Once enough pressure has been released (the caps preset rating), the
pressure relief valve is again closed by the spring.



The pressure cap can be tested with a cooling system pressure tester, using
an adapter, to make certain that it is living up to its pressure rating. It
should be replaced if it fails the test.



Note: Most radiator pressure caps are not meant to be removed. Coolant
should always be added through the expansion (overflow) tank. NEVER REMOVE
THE RADIATOR CAP FROM A HOT ENGINE. REMOVING THE PRESSURE CAN CAUSE STEAM TO
SHOOT OUT AND SERIOUSLY BURN YOU

On Wed, 25 Jul 2007 05:25:03 -0400, "jetta107"
> wrote:

>that doesn't answer the question of why the coolant gushes out of the
>resevoir vent at normal operating temp....something is causing the system
>to over presurize! I'm also getting mixed feed back as to the resevoir cap
>does or dosen't release some presure, as a rad cap does.

The coolant expansion tank cap in the VW does indeed have a pressure
rating. My 2003 repair manual lists the cap to be rated at 1.4-1.6
bar. There is an overflow tube on my expansion tank.

Do one or both of your radiator fans turn? Have you changed the
radiator fluid recently which is tricky on some cars. Air in the
system can be hard to purge. I got a lot of expansion when I changed
the water pump in my car. I left the cap off for a bit while idling
until I saw water flowing in to the expansion tank. I saw a few burps
of air go through but after that initial burp is was ok.

Failure of the fans to turn on will cause overpressure.
Failure of a thermostat will cause overpressure.
Failure of water pump will cause overpressure.
Broken water pump belt or timing belt for my car would cause
overpressure but if the timing belt is broken then nothing is running
anyway so never mind the timing belt issue.

"Baz" > wrote in message
...
>
> "Dave" > wrote in message
> nk.net...
>> "jetta107" > wrote in message
>> lkaboutautos.com...
>>>I have a '97 jetta TD, and I thought I was overheating only at high
>>>speeds,
>>> but it seams to be loosing coolant threw the vent on the resevoir? (
>>> presure in cooling system?). Then it begins to overheat.....Since there
>>> is
>>> no rad cap, does the resevoir cap regulate the presure?
>>>
>>
>> If the coolant is at proper mark on the reservoir when cold, the
>> remaining space consists of air. The air pressurizes, and the coolant
>> expands as it heats. If the engine is operating within the expected
>> temperatures, no coolant loss should occur, ever. The pressure is
>> regulated by the heat, not the cap on the coolant reservoir tank. All
>> cooling systems for automobiles are pressurized at normal operating
>> temperature. So, I don't know what you're implying. The facts even seem
>> to be questions in themselves from you.
>> Dave
>>
>
> Rubbish.
> The Cap Controls the Pressure. The same weather on the Rad or the
> reservoir tank.
>
> See below taken from
> http://www.innerauto.com/Automotive_..._Pressure_Cap/
>
> The radiator cap acts as more than just a "lid" for your radiator; it
> keeps your engine cool by sealing and pressurizing the coolant inside it.
>
>
>
> What makes the radiator cap special is that it is designed to hold the
> coolant in your radiator under a predetermined amount of pressure. If the
> coolant was not kept under pressure, it would start to boil, and soon you
> would have boiled all of your coolant away.
>
>
>
> However, the radiator (or pressure) cap prevents this from happening by
> exerting enough pressure to keep the coolant from boiling. Normally, water
> (coolant) boils at 212 degrees F, but if the pressure is increased, the
> boiling temperature is also increased. Since the boiling point goes up
> when the pressure goes up, the coolant can be safely heated to a
> temperature above 212 degrees F without boiling.
>
>
>
> What makes this important is that the higher the temperature of the
> coolant is, the greater the temperature gap between it and the air
> temperature is. This is the principle that causes the cooling system to
> work; the hotter the coolant is, the faster the heat in it moves to the
> radiator and the air passing by. So, a cooling system under pressure takes
> heat away from the engine faster, which makes it more efficient.
>
>
>
> If your cooling system is under too much pressure, it can "blow its top"!
> To prevent this, the radiator cap has a pressure relief valve. The valve
> has a preset rating that allows it to take just up to a certain amount of
> pressure. When you turn the cap on the filler neck of the radiator, you
> seal the upper and lower sealing surfaces of the filler neck. The pressure
> relief valve spring is compressed against the lower seal when you lock the
> cap.
>
>
>
> The radiator filler neck has an overflow tube right between the two
> sealing surfaces. If the pressure in the cooling system exceeds the preset
> rating of your cap, its pressure relief valve allows the lower seal to be
> lifted from its seat. Then the excess pressure (coolant, air) can squish
> through the overflow tube to the ground or the coolant reservoir.
>
>
>
> Once enough pressure has been released (the caps preset rating), the
> pressure relief valve is again closed by the spring.
>
>
>
> The pressure cap can be tested with a cooling system pressure tester,
> using an adapter, to make certain that it is living up to its pressure
> rating. It should be replaced if it fails the test.
>
>
>
> Note: Most radiator pressure caps are not meant to be removed. Coolant
> should always be added through the expansion (overflow) tank. NEVER REMOVE
> THE RADIATOR CAP FROM A HOT ENGINE. REMOVING THE PRESSURE CAN CAUSE STEAM
> TO SHOOT OUT AND SERIOUSLY BURN YOU
>

I agree. However, I've seen no evidence that this is a pressurized cap that
vents at a certain pressure. This is not a radiator cap. Have read all
replies to date on the 97 model vehicle. Still waiting...
Dave

"Dave" > wrote in message
hlink.net...

SNIP

>
> I agree. However, I've seen no evidence that this is a pressurized cap
> that vents at a certain pressure. This is not a radiator cap. Have read
> all replies to date on the 97 model vehicle. Still waiting...
> Dave

If you let your engine overheat...............guess where that coolant will
come out at! The cap will release the pressure at the bottle so the
hoses/radiator/heater core won't blow.
I still call it a radiator cap since some of these caps were either on the
radiator or on the bottle. ;-)

"dave AKA vwdoc1" > wrote in message
t...
>
> "Dave" > wrote in message
> hlink.net...
>
> SNIP
>
>>
>> I agree. However, I've seen no evidence that this is a pressurized cap
>> that vents at a certain pressure. This is not a radiator cap. Have read
>> all replies to date on the 97 model vehicle. Still waiting...
>> Dave
>
> If you let your engine overheat...............guess where that coolant
> will come out at! The cap will release the pressure at the bottle so the
> hoses/radiator/heater core won't blow.
> I still call it a radiator cap since some of these caps were either on the
> radiator or on the bottle. ;-)
>

And you, of all people, should know that VW made reservoir tanks with caps
that simply sealed. No venting... I'm driving one now. Still waiting for
evidence.
Dave