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
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.
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.
Rubbish. The Cap Controls the Pressure. The same weather on the Rad or the reservoir tank.
See below taken from
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
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.
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
SNIP
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
Have you looked at the caps and noticed a device inside of them? From the Rabbits to the 2000+ Jettas. I have not seen one VW/Audi radiator cap yet that did not have some type of pressure release system built in. I have not seen one that simply "SEALED"!
Now maybe we are talking about apples and oranges. The caps will allow air into the system and excess pressure out. When you say "VENT" what are you referring to? No, there is no hose like some vehicles that allow fluid into an overflow tank when overpressurized and will suck the fluid back in when there is a vacuum created inside of the coolant system. Is this what you are thinking about?
On my 2003 the cap fits in to a tube about 30 mm in diameter. There is an oring to seal the cap. If antifreeze is released by the inner mechanism that Dave vwdoc1 is talking about it flow to the outside of that tube to a secondary column. That column has a drain in it to the firewall side which drips down the strut tower and on to the ground.
So what VW do you believe has no vent? That would be c>Have you looked at the caps and noticed a device inside of them? From the
Boiling coolant. A wrong mixure of coolant to H2O or exhaust gasses getting into the water jacket are first thoughts. Over pressurization is common if you have either of the afore-mentioned conditions. If you find a new cap and the same thing happens, you have to look at the above. Overheating is the key. Why is this happening??? If there is a leak (cannot create a constant pressure) in the cooling system, overheating will occur. If you have the wrong mixture in the system, this will occur. If exhaust gasses are escaping into the cooling system, this will happen (such as with a blown head gasket). If timing is off, this also may occur (although this is probably more of a long-shot). Make sure w-pump and thermostat work correctly. From there, check for leaks and/or losses of pressurization. I have a 16v and have to constantly monitor these things, and I know how far- reaching these problems may be. Good luck...........
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