Pressure in system, when cold

[daverclasper]

Hi. Off to Cornwall tom in Vitesse, so just checked coolant level. It was a bit pressurised and the coolant was right at the top of the filler neck (usually a bit below).

Maybe the cap not allowing expansion into the bottle when hot?, though I don't completely understand how they work.

Any advice please.

Cheers, Dave  

[Gary Flinn]

Dave

If you have the later 2 Litre Vitesse with the seperate expansion bottle connected to the top of the Radiator filler cap I always found with mine it was best to fill the radiator and then keep the Expansion bottle half full.

My understanding is the water expands and contracts according to Temperature and pressure.

When the pressure builds the cap should release it and you will get expansion of a slight amount of water into the expansion bottle, as it cools/contracts the water is sucked back into the radiator keeping a consistent level. 

Is the spring loaded part of the radiator cap working Ok on your car? 

 

 

 

[NonMember]

The cap is designed to open when pressure in the system is below atmospheric (to draw water back from the "desertising" bottle) or above atmospheric by at least 7PSI (or 13PSI on later cars). If you've got, say 3PSI above atmosphere when cold, it should retain that.

What I'm less clear on is how you achieve that. One way is if you checked it on a warm day after a cold night. The car has got cold overnight, meaning the water contracted slightly and more was let in. Then it has warmed up, so the water has expanded and produced a bit of pressure, but not enough to open the cap.

The only problem with that explanation is that water is fairly close to incompressible, so the expansion would create quite a lot of pressure, which would open the cap. For you to notice that it was pressurised, there would have to be something compressible in there, which suggests a bit of an air lock somewhere. Probably not a bad one, though, and you're probably fine to drive it like that and just ignore the oddity.

As Gary says, keep the radiator full and the expansion bottle at half (when cold). Since the radiator cap is not the highest point in the system, leave it alone and use the level in the expansion bottle to monitor for loss.

[RogerH]

On my 4A I have the water just covering the rad tubes. This allows room for the water/air to expand  and not have the bottle constantly in use.

Always have the bottle 1/3 to 1/2 full and ensure the bottom of the tube is cut at an angle so it does not get sucked onto the bottle bottom

 

Roger 

[johny]

I think on the 4A, unlike the Vit, the heater isn't the highest point so if coolant level is a bit below the cap it's no problem while on the Vit your heater stops working😩

[johny]

1 hour ago, NonMember said:

The cap is designed to open when pressure in the system is below atmospheric (to draw water back from the "desertising" bottle) or above atmospheric by at least 7PSI (or 13PSI on later cars). If you've got, say 3PSI above atmosphere when cold, it should retain that.

What I'm less clear on is how you achieve that. One way is if you checked it on a warm day after a cold night. The car has got cold overnight, meaning the water contracted slightly and more was let in. Then it has warmed up, so the water has expanded and produced a bit of pressure, but not enough to open the cap.

The only problem with that explanation is that water is fairly close to incompressible, so the expansion would create quite a lot of pressure, which would open the cap.

I think you got the explanation there right because a rise in ambient temp can't produce enough pressure to lift the cap otherwise the system could be sitting there with 13psi in it which would make opening the cap difficult. Also don't forget the hoses expand slightly. I think I calculated my system produces an expansion of about 250cc from 0 to 100c....

[daverclasper]

Thanks chaps. It was maybe a one off/intermittent thing, as back to normal now.