coolant overflowing

[Coles]

Yesterday afternoon I arrived at my lockup garage after a 25 mile drive and after pulling into the garage a flood of coolant began pouring out near the rear offside wheel. It eventually stopped but there must have been a gallon or more of coolant over the floor. My thought was that a hose had split. I arranged with my insurer for the car to be recovered to Portsmouth OPC next day. However, the recovery guy had a look around the car and topped up the system with water and asked me to run the engine. The engine got up to temperature and stayed on 80 and there was no further coolant loss. He said that the filler cap was faulty and not sealing airtight which was not allowing pressure to build as it should as the coolant temperature rose. Coolant was overflowing past the cap and down the overflow pipe. Hence the water on the boot lid which I had noticed before but thought was just minimal evaporation. The car is now in OPC for a new filler cap and flush and refill with coolant. The lesson I have learned is to make sure the filler cap is screwed tight, to look out for condensation on the inside of the boot lid above the filler and to replace the filler cap if it is likely to be faulty.

But I will see what the OPC say on Monday.

 

[Glyn]

Coles, for every pound of pressure the cap is rated at it raises the boiling point of water by 2 degrees (at sea level), if the cap seal has an issue then the system will not pressurise and water will start to boil - which expands and creates the overflow.
Whilst I appreciate the overall water temp is about 80 degrees and therefore under its boiling point localised spots within the engine are much hotter so water temperature gets much higher especially around the cylinder bores so the cap must prevent localised boiling by increasing the boiling point.

The condensation on the boot lid is a clear sign moisture is present and that the cap or expansion tank may have a problem.

 

[Coles]

Thanks Glyn
That does explain why the temp gauge can show 80 while the temp around the cylinders is much higher. I can understand that the cap must be letting by or the reservoir may be cracked and therefore not allowing higher pressure to build nor raising the boiling point of the coolant.

I am not clear about where the coolant overflow pipe is connected to the system. Is there a pressure release valve in the overflow which only opens when the pressure rises suddenly because coolant is boiling? or is the overflow simply an open pipe in the floor of the boot?

This coolant overflow started when I stopped the car just before driving into the garage but I didn't notice it until after I drove the car into the garage. At that point it seemed to be pouring out in a torrent. That seems to suggest that the overflow is connected directly to the coolant system.

Grateful for any explanation of how the overflow works.

Brian

 

[Glyn]

On the Boxster its just a rubber hose connected to a drain hole underneath the expansion tank.
If you remove the oil cap and the water cap you can lift out the false floor its just a plastic plate that slides over the two fillers, you will also see the bleed valve for the coolant system - a lift up type valve that allows the air to bleed out.
As the water pressurises and expands the cap holds pressure up to 15lbs - so boiling point is raised to 130 degrees (sea level - water boils at a lower temperature the higher the altitude) - If the temp exceeds the 130 then the cap allows the pressure to vent and with it the expanded water - it simply runs from the overflow and dumps to the floor.

 

[Coles]

Thanks again Glyn. Portsmouth OPC did a pressure check on the system and found that the gasket on the pressure release valve was letting by, so that gasket will be replaced. The filler cap has suffix 02 which I understand is the improved design so hopefully this will not let by. It looks in good nick but I guess appearances can be deceptive. They will run further checks tomorrow and make sure all is well. I am gradually learning bits and pieces about the car, which is useful and interesting. Had a chance to have a look at he underside of the car while up on the lift and the exhaust clamps and bolts look badly rusted so I will need to get that done soon.

Brian

 

[Coles]

Sorry, I should have said "bleed valve" not "pressure release valve" (quite different things).

 

[Glyn]

With ownership you will come across the foibles of the Boxster, there are a few but usually just irritating issues.
If you want to learn a great deal about the Boxster have a look at Boxa.net - another Porsche forum, I think most of us are on a number of forums