Coolant coming from reservoir

[Covy]

I'm scared!!!!!!

Can anyone tell me why, after bringing my 2.5td bongo up to temp on a short run, the coolant wants to bubble and gurgle out if the reservoir when I unscrew the lid

The temp gauge sits at the usual place, just below half and the aftermarket tenp sensor doesn't sound. E dry thing actually appears fine but as I say the back pressure in the reservoir is worrying me.

[Bob]

Welcome to the Forum, If you unscrew the filler cap on any vehicle cooling system while hot coolant will fly out as it is under pressure.

You can easily be scalded by hot coolant.

[g8dhe]

The system is pressurised and at its working pressure the coolant will be above 100°C so if you release the pressure it will boil!!

Never open the cap when the coolant is hot, it is very likely to do you serious harm!

[Simon Jones]

The coolant level will naturally rise when hot and should not noticeably bubble when the cap is on. If you remove the cap it reduces the boiling point such that it will bubble up. As long as the cap is on and you are not loosing any coolant from the overflow pipe then you shouldn't have anything to worry about. If however, the level rises and and comes out the overflow under pressure, it suggests either a faulty seal on the cap or air is present in the system. This could be due to a hole in the pipework which is allowing coolant to escape / air to be drawn in, or combustion gases are getting into the cooling system either from a failed head gasket or a crack in the head.

Have you had the vehicle long? Any history of problems? Has the behaviour changed in any way?

[Diplomat]

Years ago I had a Hillman Imp which did something very similar.

It had all the appearance of boiling but the temperature gauge disagreed.

Turned out to be combustion gases getting into the water system, i.e. head gasket trouble.

If it only happens when you unscrew it, then the above posters have hopefully solved your dilemma.


Frank

[oxb1l]

My passat was similar it had a blown gasket very little loss on short journeys but the bubbles were rising into the header tank on longer ones and quickly drained
it never got hot so the head was fine and only required a gasket