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MG MGF Technical - Water boiling in expansion bottle

Had a 48,000 mile service. Two months on and I'm occasionally hearing boiling water bubbling away behind me in the expansion bottle. The cooling fan comes on ok, so what does this symptom indicate?
Dimo

Get yourself a new cap for the expansion tank IMMEDIATELY.......!

I had this happen recently and proved to be a duff cap.

Boiling coolant in the tank is bad!
tim woolcott

This happen to me last night, after driving 25 miles in very hot weather. My fan at the side did not come on and the water bubbling away. I opened the boot and let the engine run for a while. My husband has wired up a switch for me so I can put the fan on whenever I think its getting too hot i.e when stuck in traffic. Your lucky my fan has never worked at the rear. My engine blew up last year in very hot weather just like we are experiencing now.
Jeanne Dyson

Jeanne,

I must recommend a new cap for you too, 3.50 from MGR
tim woolcott

Boiling water in the expansion bottle does not indicate overheating. The coolant temperature is frquently above 100ºC - the reason that it doesn't boil under these circumstances is that the coolant system is held under pressure (remember your chemistry classes - see figure of the 'triple point' - http://local.phys.strath.ac.uk/12-142/lectures/HeatCapacityPhase_files/image023.gif)

So if the coolant system looses pressure, the boiling temperature of the coolant drops. And this is probably what has happened here Dimo - and why Tim says to replace the coolant expansion tank cap :o)

The cap contains a pressure valve. If it becomes faulty, it will fail to maintain the required 15psi pressure and the coolant will boil in the expansion tank (and you'll loose coolant).

Of course the coolant system can loose pressure in other ways, but the cap is the commonest problem, and is the easiest and cheapest to sort (costs under a fiver).

Rob Bell

Nice graph Rob.
tim woolcott

Rob,

Surely its the sensor must be at fault, this would explain why my fan does not come on. Engine compartment very very hot. Does anyone has a wiring diagramm, not in manuel. We only get problems on very hot days. You open the boot lid and the heat hits you.
My engine blew up last year, and the dealership said the fan was working. I have never heard it in the four years we have owned this car.
Jeanne Dyson

Jeanne,

VVC or MPi? The engine bay fan comes on at a different temperature - higher for the VVC. The engine fan rarely comes on with my '98 VVC, but the engine bay always seems hot when you open the bootlid!

Dave
Dave

Jeanne, you may soon get mail with leccy diag jpg
John Ponting

>The coolant temperature is frquently above 100�C

Rob I think it's 150 with antifreeze, but otherwise correct ;-)
Will Munns

Jeanne, the engine compartment temperature is not directly related to water temperature - so you could get boiling (overheating) of coolant without operation of the engine bay fan.

If the coolant pressure is inadequate, and the boiling point drops below that required to trigger the radiator fan, then you wouldn't necessarily expect the operation of the rad fan either with boiling fluid. All this would be completely independent of sensor function.

Sensor dysfunction would be indicated by a high coolant temperature (dashboard gauge) and boiling...

Rob Bell

Hi Guys, just an alternative option to consider:

I had evidence of boiling in my expansion tank too (white anti-freeze 'salt' round the cap). Mike Satur diagnosed the radiator was knackered (bottom 3 rows of fins missing for a start). Had the rad (and a few other bits) replaced, not had the problem since with the same expansion cap.

Leigh
Leigh

On my '98 VVC, I never remember the fan ever coming on.

Yet on my Trophy 160, the fan is always going on, even on relatively cool days.

Were some changes made to the fan or temperature start point for the Trophy?
Chris

This no doubt sounds stupid but what does boilng coolant look like. I ask as at idle mine is calm. If you rev say to 3000 rpm then there is evidence of some bubbling. It has always done this so I see no need to worry.
S Laithwaite

Stephen, this may sound even more stupid but when you rev is it just vibration causing the surface to 'bubble'.

Coolant boiling should look just like water boiling in an open pan or kettle with the lid off.
John Ponting

Probably. Give it a try.
S Laithwaite

I also never heard my engine bay fan run on my VVC.
Had the boiling noise after a service - I pulled over and the MG Dealer sent out a driver to find me and replaced my cap. Nothing since
Paul
Paul Lane

>>> This no doubt sounds stupid but what does boilng coolant look like. I ask as at idle mine is calm. If you rev say to 3000 rpm then there is evidence of some bubbling. <<<

IMO, it is just caused by water entering the bottle while the waterpump is working.
Erik

mmm replacing the cap seems to be a good move, wish I'd remembered this morning after I topped it up with water (currently awaiting yet another appointment with dealer for repair/replacement of some leaky component near the front). Anyhow, drove about 9 miles to work, no problem, opened the boot to check the level before driving home F@@@ wheres the cap........aah just where I left it..........feeling a bit dopey at the moment, but I hadnt lost any more coolant
Colin Ashworth

This thread was discussed between 10/07/2003 and 23/07/2003

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