I am still having some problems with coolant level in the new car.
Could I ask someone who is technically minded a question please?
The coolant system is obviously sealed so that when the coolant is heated by the engine the fluid expands and pressurises the air left in the expansion tank. Once the fluid is left overnight to cool am I correct in saying that as long as the fluid is back to the tempreature the system was filled at and effectively sealed, removing the expansion cap could not possibly effect the coolant level in any way?
The dealer is trying to suggest that the coolant may be still under pressure so releasing the cap removes pressure and allows the coolant to rise??? I just cannot logically see how that could happen but what do others think? Remember I am checking it in the morning when the coolant has been left all night to cool.
Many thanks.

 

how would it ' rise ' if under pressure ? it would rise and overflow if under pressure ! and once the pressure is relased the level would sort itself out very quickly!

 

Im a bit confused with the physics of it but after being advised to read boyles law which says that pressure x volume = constant I had assumed that if the pressure in the system increases the coolant volume reduces so if the coolant volume was say 20 litres and it was subject 1 atmosphere of pressure then by increasing the pressure to 2 atmospheres of pressure would mean that the coolant volume would decrease to 10 litres.
If I have got that right my second question is that when the engine heats the coolant, I assume it expands and the air is pressurised meaning that the air volume decreases as it is compressed by the increase in coolant volume?
Like I said I cannot see how the coolant level would rise upon release of the expansion tank cap when it had cooled down?

 

If there is a small residual pressure (psst! when you remove the cap) then the hoses might all contract a tiny amount and you'd notice the level in the bottle rise ever so slightly. Shine a light at the bottle; they're made of translucent plastic usually.

Or it may even drop on a very cold day...

What exactly is your remaining problem?

 

Just read your last.

The Atmospheric pressure may be regarded as a constant for these purposes. The volume of your coolant will expand with temperature. Hence the expansion tank.

Using the bright light, you should notice a higher level after a blat than 1st thing in the morning.

 

Charles' Law

Sorry Mole; I meant Charles' Law! I can only remember Boyle's for obvious reasons!

I was thinking Hooke's Law but that's springs...

 

Thanks for that Nick.
Basically the coolant level is dropping. I had the car back last thursday and they had filled to max mark, opc say that when i check level in morning to take the cap off as it will release pressure and level will settle to where it is supposed to. They are suggesting that it will rise slightly when the cap is removed from cold as the pressure equalises. I am trying to get a handle on the concept so that I can appear knowledgable. They have had it back twice now and the level has from last thursday gone from max to just above minimum, I checked and photographed every morning prior to start up and emailed the pics to them and told them that if it has dropped tommorow to below minimum that they can have the car back. Harsh? maybe, but im not messing around with a car that I have dumped savings into to have it go back to porsche every week for a coolant top up.
They have pressure tested it warm, hot and cold and it holds pressure so I have not got a clue why its loosing fluid unless it is at its upper most operating tempreature that they have not reached when testing.
Any technical info to quote would be great.

 

That's quite a lot of coolant loss.

I think the cap release IS just to make sure you're getting a consistent reading every time.

You can get these problems; if the pressure is holding hot and cold, I would suggest that it's probably the cooling down where it's squeezing through a HG or a porous casting or something.

If it's going into a pot, it will boil off on starting. Otherwise you'd find it in the oil somewhere. I've had steamy windscreens from blown heater matrices, but that should show up on a pressure test.

As I said before; a lot of pipes on one of those. But pressure testing ought to reveal it. Or a wet patch.

TBH, I'd give them a final (written) opportunity to fix it and then reject it, if not. I don't think you're being a tart for considering it at all. You've been patient enough. It's supposed to be a top car etc, etc... I'd hate to think what expenses might be being stored up in future at Porsche prices.

 

is the rad cap faulty?
is system over-pressurising/over-heating?

if the 2nd, the rad cap will blow out and dump some coolant when being driven, once cooled down, the level will be less since it's dumped some coolant on the road..

the fixes:
test/replace rad cap.
test pressure of system when car is running and fully warm
overheating = can be various things, but the most simpliest is wrong coolant/water mixture. too much of either and it will either boil off, or overheat.

just a few thoughts.

 

Is the level staying at the same (minimum?) point over the week if
you just leave it alone? Or dissapearing from the tank altogether?

Russ.