jonny81

The coolant in my header tank has started bubbling, as far as i can tell it only happens after the engine has been switched off. There are no other symptoms of head gasket failure & the car doesnt overheat & runs great.

When the engine is running i can see water returning to the header tank so the water pump must be working but a few seconds after i switch it off it will start bubbling & its got me beat!

This is on a sapphire cosworth by the way, any help would be great, thanks

AzzDSM

hi mate. i had the same prob on my scoob and it turned out to be a leaking bottom hose. or the pressure cap might be leaking.

good luck

jonny81

Im hoping its something as simple as that mate, havnt noticed any leaks yet so i think i'll try a new pressure cap. Thanks

Dee NI

Is the heater matrix leaking? Air getting into the system could be the problem if not the HG. Sometimes you don't always get the other hg failure symptoms when it goes. My old 1.6 GTi used to run on 3 when started in the morning cause of a crack in the hg into one of the bores. It was fine when it cleared.

Damien

jonny81

Heater matrix isnt laking mate, but thanks for the suggestion, i think the turbo on these is oil & water cooled maybe it could be something to do with the water thats cooling the turbo getting hot & turning to steam after i switch the engine off?

Dee NI

The water in the coolant system shouldn't turn to steam unless it's meeting air. The coolant system is presurised and hence 'closed'

Damien

jonny81

The other day i topped the coolant up & put too much in & when it bubbled up some came out through the cap. This would point to a dodgy seal on the presssure cap then?

Dee NI

Was talking to a mate who who's a big Ford fan. His cousins did the same even after a new hg. Gave no problems and they still never found what it was. Try a different cap though as you say. Under extreme pressure the spring in the cap will be pushed up and coolant can flow out the little pressure release pipe thingy but not directly from the cap.

Give it a go anyhow.

Also check passionford.com it's to ford what scoobynet is to scoobs. Loads of info, little crap.

Damien


Damien

ZEN Performance

It's normal. The turbo will boil coolant, it rises to the header tank and cools/condenses, expect it to last about 15mins. The coolant will find it's own level, usually about 1cm below the neck of the header tank, so if very full it will oftet blurt a little coolant out into the overflow. Provided you're not constantly losing coolant it should be fine, it is designed to lose a little, and draw it back in upon cooling. If it's constantly chucking it out, and overflowing the coolant expansion tank (which is much bigger than it appears as it's mostly hidden) then you have a problem.

jonny81

cheers for putting my mind at rest lads, i did post on PF but didnt get much of a response.

Does that mean that it should use some water then as its turning to steam?

AzzDSM

i never noticed any leaks on mine until a garage did a pressure test on the coolant system.
most garages wont charge you for it so give it ago mate.

StickyMicky

is that advice for a cosworth?

AzzDSM

probably not because cosworths dont have a coolant overflow. as far as iam aware (i could be wrong)

jonny81

Is it?

ZEN Performance

Good point. It should hold true assuming the car has a water cooled turbo. Boiling is normal, but excessive overflowing out the header tank isn't.

AzzDSM

it shouldnt boil if the system is pressurised and no airlocks...

ZEN Performance

Not true.

It shouldn't boil when the engine is running, and the system is pressurised, but raising the pressure to 0.8 bar doesn't stop boiling all together, it raises the boiling point.

When you switch off the car, the coolant stops flowing and the heat in the turbo WILL boil the water, as the core of the turbo soaks up heat from the exhaust housing which will be around 350-400ēC after some idling, or nearer 800ēC if you've been thrashing. Since the water isn't circulating it just sits in the turbo, and increases in temperature beyond the boiling point, which with pressure and a dilution of antifreeze will be up at around 130ēC.

On a Subaru, you can hear the coolant boiling in the turbo, and this also thermosyphons water through it, cooling it after the engine has been stopped.

AzzDSM

good info mate but should it be that noticable? ive had loads of turbo`d cars and never heard any bubbling. only on my scoob and that was because of a leaking bottom hose.

jonny81

Just to add closure to this thread for any one else with the same problem, it turned out to be the pressure cap on the expansion tank!