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MG MGB Technical - Unusual noise

Hi. For the last week, my car (72 BGT) has started making a loud 'doing' noise about 30 secs after the engine is switched off after running until warm - I assume just as it is starting to cool? It sounds a bit like a metal expansion/contraction sound - but the car has just started doing it.

Every time - one doing (very loud!) - and it sounds to be coming from the radiator area but I cannot be sure. when it did it the first time I thought my head had just cracked.

i replaced the rad cap which was looking pretty old wondering if it was to blame as it was not fitting correctly - but it's still happening.

any ideas please?

regards

Graham

Graham Moore

Graham - Nest time you go out, loosen the filler cap for your fuel tank. The vent in the cap maybe clogged and the fuel tank is oil canning as the fuel is pulled out. Cheers - Dave
David DuBois

Interesting. I will check that as I have just put a new locking cap on. I will report back. thanks.
Graham Moore

Ah-Ha. A new locking cap! Elementary my dear Graham. Pound to a penny it has no vent. Take it back and demand a vented one. There is no other way for air to get in the tank to replace the petrol used. It's amazing that the pump can draw enough vacuum to partially collapse the tank, but it definitely can. Eventually if you drive far enough the pump will no longer be able to draw fuel and the engine will falter and die. Releasing the cap at that point will be accompanied by a whoosh of air rushing into the tank.
Mike Howlett

Well chaps, it is not the fuel cap. And it is coming from the rad area. Also weird is this - engine off, 30 seconds - "doing". Never 20 secs, never 40, not even 25 or 35 - always 30!
Graham Moore

Maybe a thermostat going funky?

Charley
C R Huff

exhaust or fittings?

what poundage cooling pressure cap do you have?

if it's 30 seconds could you, pull the bonnet release, get out of the car, lift the bonnet and listen (and possibly record) the noise whilst standing near the rad area

I'm not sure what a doing noise is
Nigel Atkins

How loud is the 'doing'? Mine makes a 'dink' a few seconds after switch off, rad area, one of a series over several minutes, and that is the low-pressure valve in the rad cap opening to let air back into the system as it cools. Try removing the cap - carefully and slowly, with a thick cloth over it - as soon as you switch off and see if you get the noise immediately, still after 30 secs, or not at all.

In the first case it could be the rad or something flexing back as it cools and pressure reduces, perhaps making the same noise as it warms up but you don't hear that. In that case it will happen as soon as you release the pressure. If it happens at the same time with the cap off, then it is some other component cooling and contracting. If it doesn't happen at all it could well be the cap as in my case.

As to why mine does it is another matter. When it is running it is continually hissing air very slightly out of the over flow. It's always done it, and I've never really taken any notice. However a few months ago I fitted a pressure gauge and ran the engine up to see what happened. It only got up to about 3 psi after a run, but that didn't surprise me as under similar conditions the V8 is the same. So I blanked off the rad to get the temperature higher than normal, and it still only got up to 4 psi, which did seem a bit low. Switched off, and very quickly it dropped to zero, then started sucking air back in. So the rad cap seems not to be holding pressure - it did come to me with the car 25 years ago after all, so I bought a new one, only to find it does the same hissing during running and cooling. As it's never caused a problem, it can carry on.
Paul Hunt

It could be a 'dink' if you imagine central heating pipes warming up - it's one of those, or whack a spanner on an empty oil drum. In fact, today I was driving and I noticed it did it whilst the car was moving (whilst warming up) at about 30 mph - so I could hear it over the volume of everything else (the radio was on too).

when i heard it for the first time, I though I had just experienced a live head crack. as it's cracked already I thought i would open the bonnet and see it in two pieces.

i just renewed the rad cap thinking the old one was doing it as i couldn't imagine what else might be responsible. I think the MGOC said it was a 10psi

taking the cap off when hot is a bit unpredictable as to which direction boiling water will spurt but i think that might be the only action that will help to eliminate it. i'll dig out my heavy duty gloves and motorbike gear and give it a go. or even just leave the cap off, warm up then switch off. I'll lose some coolant but I would with previous approach..i'll report back.
Graham Moore

10lbs is correct for your car, it'll be printed on the cap

obviously you'll need to be very careful releasing the pressure cap when it's hot but if you let the pressure off really slowly, especially at the start, it might not be quite as bad as you imagine

if you have to top up the coolant then do so when the coolant is fully cooled and follow the instructions in the Driver's Handbook so as not to introduce 'hotspots' to add to your issues

I always prefer to add the coolant premixed with water or better still a commercial premixed coolant and I think it's better to add slightly more than slightly less as the excess will get blown out and the coolant system will find its own happy level, better than going through the whole top up procedure twice

the noise could be expansion and contraction of something that's not quite held right by a fitting perhaps
Nigel Atkins

With your central heating analogy, the noise is not the pipe but the pipe moving against something. So in the car scenario the points at which the rad and pipes are secured. Rad to shroud;metal heater pipe to rocker cover;heater pipe to heater in it's housing, and out again. Are any of the pipes close enough to chafe if they move when hot? The rad cap test should indicate if it is pressure related or heat related
Michael Beswick

"warm up then switch off. I'll lose some coolant"

You shouldn't, I wouldn't expect it to boil at idle, that would represent more than half-way from N to H. I know that the coolant is passing surfaces that are hotter than that, and what you see on the gauge is an average of that with parts of the coolant that are passing through cooler areas, but neither of mine do it - having just changed a thermostat and run it up at a fast idle a couple of times with the cap off to keep an eye on coolant level.

Incidentally, the new stat had no jiggle-valve, bleed hole or notch to prevent air-locks, and it only took two litres to fill the rad after having the bottom hose off - i.e. a massive air-lock under the stat. Of course the level in the rad dropped like a stone as soon as the stat started to open, and needed half a kettle of hot water to bring it back up again, and another half litre or so at the second warm up from cold.
Paul Hunt

This thread was discussed between 20/05/2014 and 25/05/2014

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