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MG MGF Technical - Hissing of coolant expansion tank

Last night when taking my bag out of the boot, I heard a hissing or whistling sound. It seemed to come from the expansion tank for the coolant. I had only driven around 5 miles from the station at speeds of up to 70mph...just enough time for the oil temp gauge to start moving. I'm pretty dure that the coolant level is constant and the e-tank cap has a white dot. There does appear to be a smidgin of green fluid on the seam of the coolant expansion tank.

Has anyone experienced this before? Can expansion tanks spring little leaks? Could the cap need replacing?



Very normal.

Don't be tempted to over tighten the pressure cap.... if in doubt you could buy a new cap for a couple of quid from either Halfords or MGR. The up-rated cap does have a white dot on it.

A little bit of coolant on the rim is not un-usual, since buying my car 6.5years ago I have always had this. I have found that keeping the coolant level (when cold) halfway between the min and max mark reduces the amount which gets spat out.

If you have real concerns take the car to a garage to have a pressure test of the system..

tim woolcott

>Could the cap need replacing?
sounds like the job- I wouldn't expect the system to be very pressured after just 5 miles (unless you were really ragging it) esp given the cold weather. Not to worry, but definatly worth a few quid for piece of mind
Will Munns

Thanks gents. When you say don't over-tighten the cap, Tim, how do you judge it? Last night I certainly gave the cap a twist and it turned say an extra 3-5 mm.

Also, I have no min/max marking on the tank, just the seam, which seems to be used as the target by the garage.

Any pointers would be welcome...



I could see the min and max for several years.... Look in the top of the tank and towards the boot side of the tank, there you will see two plastic steps, one in min the other is Max.

Re Tightening, don't use two hands, firm finger tight is good.

tim woolcott

"Hissing of coolant expansion tank"

Wasn't that a Joni Mitchell album?
David Bainbridge

I agree with you Tim, be suspicious about the expansion cap in the first instance.

However, if hissing is a persistant problem, and coolant levels are dropping, then consider other causes of reduced coolant pressure - such as corroded under-body coolant pipes, failed unions, or even damaged radiators.

Coolant level in expansion tank can be seen on

Rob Bell

Excellent advice all and nice diagrams on the site Rob. Seems that my MGR garage rather like to keep the coolant level about 5-10mm above the max mark. Check at night showed coolant at the max mark after 5 mile run, so I'll check again when cold.

David: great song and great album, non?

Are you certain the hissing was from the coolant header tank. It could have been from the evaporative canister which could be powered for several minutes after switch off?

Bruce Caldwell

Hello Bruce...what and where is the evaporative canister? I'm fairly sure it was the expansion tank, but... This weekend I cleaned the area thoroughly, topped up with 50/50 mix of Unipart's green stuff and will check carefully for a while until confidence is restored.



I think Bruce reffered to the active charcoal can left hand of the expansion tank.

Anyway. Due to the noise I would ...
- check whether you have a expansion tank cap with white dot
( this seams to having better quality pressed metal parts inside)
- check the coolant level when the engine is hot just after a longer ride.
If the level increased nearly up to the top of the tank... then the cap is broken and doesn't provide correct pressure build up.

The hissing could be caused as well while de-pressurising and cooling down of the system, which would be normal.

Dieter K.

The cap was a white dot one. I changed it this weekend in case and note that the new caps don't come with the white dot - may be I should paint one on? Thank's for the tip about checking when hot - I'll do that Wednesday after the long drive I have planned into London (just before congestion charging starts!).



>- check the coolant level when the engine is hot just after a longer ride.

Errm, don't open the tank to do this! if you do then the coolant will boil in the engine block possiably leading to airlocks and finally HGF
Will Munns

Now I'm getting a little worried and hope it is just paranoia. I went for a 15 minute drive last night on my way home, enough to get the oil temp up to about 100 deg (speeds aronud 40-60mph and revs around 3000 generally, so not a thrash). I wanted to check on Dieter's comment before driving all the way to work tomorrow.

I found that the coolant level was about 2/3rds up the upper half of the expansion tank (outside inspection only, Will). I had topped up the tank to exactly the max mark on the weekend and changed the cap for a new one. Is this usual? Is this too close to the top of the expansion tank as Dieter suggests?

Incidentally, does anyone know whether the top step is level with or above the seam on the tank? I finally found the min and max markings on the outside of the tank (at about the most invisible point possible) and looking at the diagrams on Rob's site and the pictures on Dieter's want to be sure of the max level.

Lastly, do people experience any gradual loss of coolant? Mine has been rock steady at the level set by the garage in Jan 2002 (a few mill above the seam and probably at the max mark) until last week, when it was just revealing the top step when cold.

Yours, in an oh so very slightly hysterical tone...



The top step is level with the seam of the tank..

The coolant rising to 2/3 I would say is pretty normal if you filled up to the seam (when cold). This is actually the reason I generally fill to the minimum mark (step).. it then only rises to the seam when hot.

On the last point coolant level should be pretty constant when cold... you could get a pressure check at a local garage to waylay your fears.

You don't say how old your car is...!


tim woolcott

Yep, the system pressure incresses (water expands like most things when hot)
and this expanded water has to go somewhere, some is taken in the pipes which expand under the pressure. the rest squashes the air (much easier to squash a gas) and shows up as a higher level in the tank. This is why you don't fill the tank, the air is there for squashing!
Hope this rests your mind.

Will Munns

Mine's the 275th car off the line (VIN...000525) registered Nov 95 as a Dutton Forshaw demonstrator, now a fwe owners later and 65,000 miles to the good. Flame red with, ehmm, vibrant red cloth and a little chrome (MS's chrome gear knob also reducing the height of the shift perfectly). Not quite as old as Scarlet Fever, but...


I have an early vintage car too No 4007 I had to have all the hose clips checked/tightened/replaced last year... also do you know if the inlet manifold gasket was changed..... Early black items were prone to perishing as mine did last year..... it can be checked, there is a good diagram on Rob's site

Again, a pressure test would show any leaks

tim woolcott

>Lastly, do people experience any gradual loss of coolant? Mine has been rock steady at the
>level set by the garage in Jan 2002 (a few mill above the seam and probably at the
>max mark) until last week, when it was just revealing the top step when cold.

It was very cold last week, same explaniation, but in reverse, I wouldn't worry, keep an eye etc.

Tim, stop scareing him ;-)
Will Munns

Chaz, monitor the fluid levels for a little longer.

If the level falls again over the next couple of days, then Tim's suggestion of a pressure test might be the best way forward.

Hey, your car is even older than mine! LOL Mine's about 2,200th off the line...

*IF* there is a leak, then I'd recommend that you check the radiator and coolant pipes under the car. My rad expired about 14 months ago...

Other 'weak' areas include the inlet manifold gasket that Tim refered to, as well as other 'rubber' tubes that can be prone to perish over time.

As Douglas Adams wrote: "DON'T PANIC!"
Rob Bell

I'm sure the expression is "DON'T PANIC MR MAINWARING!"

I've got my towel with me, though I could not claim to be a dude that knows where his frood is.

Thanks for all the comfort guys. The inlet manifold is definitely the new type, the head gasket failed a year ago (with spectacular results as contributed to Dieter/Mike's list) and was replaced with latest MGR type and steel dowels etc. I am a little sensitive to coolant levels as result, so I'll monitor as before and keep my fingers crossed (though not when driving).

Thanks as always for the great advice.


This thread was discussed between 05/02/2003 and 11/02/2003

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