Bongo Fury

Can anyone tell me in simple terms??
Under what circumstances coolant would "fill up" the expansion tank and blow out of the overflow.
It would be a simple matter (we think) to add a "high" level to the "low Coolant Alarm" Before we research into this, it would be good to know when this would be useful and if it would not be a case of "shutting the door after the horse has buggered off"

Pinhole leak, vacuum created on engine cooling, sucking in air which then expands/air locks when engine running - maybe?

Blown head gasket causing exhaust gases to mix with the coolant under pressure causing coolant to push into the expansion tank and out of the overflow.

haydn callow wrote: Under what circumstances coolant would "fill up" the expansion tank and blow out of the overflow.

If the return hose to the radiator got blocked and the pump was still functioning would coolant be forced up into the chamber from the hose by the thermostat?

Veg_Ian....wouldn't this be a case of the horse having bolted ?? The damage has already happened for whatever reason. The "high" alarm would only be saying "your heads gone" it wouldn't be preventing anything.

return hose to the rad is the "top hose" this is a big hose and I don't see that getting blocked.

It's a question of degree. If an alarm tells you the coolant is overflowing at the point of HG failure and you switch off immediately after, then the damage may be just a torn gasket. If you didn't realise you had a boil on until 3 miles further down the motorway, the damage incurred could be seriously worse.

Using your analogy, the horse might be part way through the door but if lucky you might just catch it!

Trapped air....I can see this "exploding" into the tank and perhaps causing bubbles/spray etc..But then surely the coolant level woud quickly drop to replace the ejected air and the Low level alarm would go active

Veg-Ian....good point...thats the sort of feedback I'm looking for.

If I argue against any suggestions please don't anyone take offence....argue back and we will come to a conclusion. The above is a example of this .

If the trapped air isn't actually escaping but caught in an air lock, it's going to push up the level in the header & keep it there/cause overflow & coolant loss which will then be picked up when the engine cools down by the low level alarm, but possible damage - localised or otherwise already caused?

Veg-Ian....good point...thats the sort of feedback I'm looking for.

If I argue against any suggestions please don't anyone take offence....argue back and we will come to a conclusion. The above is a example of this .

Argue? On this forum? Surely not

You're very welcome.

Hmmmm!!! yes, I can understand that to a point....How does the air get in in the first place ? Via a pinprick hole on cooling down ?? then surely the pinprick hole would have been squirting out hot coolant and lowering the level.. which comes first ?? the loss or the gain ?? Gooid point though....over !!

Thinking about that a bit more Kirsty....I would think that if you had a pinprick hole and a little coolant squirted out when hot and under pressure...rather than air being sucked in upon cooling the coolant level would drop on cooling and next time you switch on the alarm would sound.

missfixit70 wrote:If the trapped air isn't actually escaping but caught in an air lock, it's going to push up the level in the header & keep it there/cause overflow & coolant loss which will then be picked up when the engine cools down by the low level alarm, but possible damage - localised or otherwise already caused?

I did notice that whilst the coolant is warming up and the expansion cap is off the level increased more noticably than with the cap on (pressurised system holds it back?). So if the cap isn't sealing correctly the level could rise in normal operating conditions. I'd imagine it only being a problem if the system was overfilled to start with though. How much does the coolant expand when warm?

about 1 cm from cold to hot. Also if the pressure cap was not sealing you would in fact be constantly loseing coolant when running and the level would drop.