My 2010 mk2 157k 1.9 pd Octavia has started blowing it's coolant out of the expansion tank, through the little plastic moulded in thing that goes from the top near to the cap, down to the ridge that joins the two halves of the tank together. There is no sludge in the oil filler cap, no sign of oil in the water, no water in the oil, and if you drive it at less than 2000rpm it doesn't lose any coolant at all. It has never overheated, every time it has happened I have stopped and filled it up. To test it, I warmed up the car, took off the tank cap (it was holding pressure) and revved it a good few times up to about 4k, then water came bubbling out of the open tank. Am I right assuming that the HG has failed? Or is there a common thing like a blocked coolant hose which could cause this? Thanks.

It's going to overheat if you take the cap off when hot. The system is under pressure and coolant can be 120 degrees so as soon as you take the cap off it will boil at normal atmospheric pressure.

Check thermostat isn't stuck closed and might be worth getting a new coolant cap. Is it tuned? They can suffer from head lift under higher boost.

I had something similar on a mk3 golf TDi only when using the car on the motorway did coolant expel from the expansion vessel leaving the tell tale white/pink residue in the area. Once cooled the coolant was low and the low warning came on. New HG resolved issue. I can’t remember what mileage it happened but it had 189k on it when I sold it. 

2 hours ago, Blue8793841 said:

It's going to overheat if you take the cap off when hot. The system is under pressure and coolant can be 120 degrees so as soon as you take the cap off it will boil at normal atmospheric pressure.

Check thermostat isn't stuck closed and might be worth getting a new coolant cap. Is it tuned? They can suffer from head lift under higher boost.

To tell the truth, I've always been suspicious that there isn't a thermostat in it at all, it takes ages to warm up in winter. It's not tuned, just a standard 105bhp 1.9pd. I've never seen the temp gauge go above half way. I've never heard of a vw diesel engine suffering from head gasket failure either.  Until now obvs.

2 hours ago, gav_is_con said:

I had something similar on a mk3 golf TDi only when using the car on the motorway did coolant expel from the expansion vessel leaving the tell tale white/pink residue in the area. Once cooled the coolant was low and the low warning came on. New HG resolved issue. I can’t remember what mileage it happened but it had 189k on it when I sold it. 

Did you do the HG yourself? If not, how much did it cost to get it done? I suppose that while the head is off a new Cam belt and tensioner need to be done too!

On my previous car it was blowing from cylinder 1 into the water jacket, never overheated, just had strange coolant pressure issues.

 

Eventually stuck a pressure gauge on the expansion tank and went for a spin, as soon as you hit a hill and the boost kicked in harder you could watch the pressure rise sharply. It was going over 20psi in the cooling system. That was a 2007 2L pd

8 hours ago, Billy Balthorpe said:

Did you do the HG yourself? If not, how much did it cost to get it done? I suppose that while the head is off a new Cam belt and tensioner need to be done too!

No it was done by an Indi. It was a very long time ago pre 2008. I think about £300. I suppose cambelt kit would be an idea at the same time, yes 

9 hours ago, Billy Balthorpe said:

I suppose that while the head is off a new Cam belt and tensioner need to be done too!

Not "need to be done", but it would be silly not to since the parts are less than the labour for a replacement and they're out.

15 hours ago, Billy Balthorpe said:

it takes ages to warm up in winter. It's not tuned, just a standard 105bhp 1.9pd. I've never seen the temp gauge go above half way.

 

They do take a long time because they are so thermally efficient, I would get moderate heat from the heater after 4-5 miles in freezing conditions but the temperature gauge would never reach the 90°c normal indication unless I went up a relatively steep hill with a wider throttle opening.

 

The temp gauge will spend its entire life stuck rigidly at 90°c even though the true engine temp could be anything between 70°c and 110°c, if ever you do see it go above then you have a problem.

 

The next generation of TDi's with the sliding sleeve water pump reach operating temperature in winter much faster but bring problems of their own.

It sounds like the coolant system is getting pressurised and leaking out the expansion bottle overflow. It does sound like a HG cause as likely. You could replace the coolant bottle cap if it is a design that has a pressure relief valve in it to see if it resolves..  

Dont they all have the PRV?

 

An inexpensive combustion gas "sniff tester" will confirm a head gasket or EGR cooler failure.

Edited 29 August, 20241 yr by J.R.