Expansion bottle cap

[lamb125]

Can one tell me if the plastic cap has a valve of sorts built into it? I removed mine today and it appeared a bit gummed up. Is there a non-return valve in it as I could only blow through it from one direction...that being from the header tank.

[Mocki]

it has got a valve in it, and it should only allow you to blow one way. It would seem from your description that yours is working correctly.

the "gumming" is indication that you should maybe be looking at changing your coolant sooner rather than later.

[lamb125]

thanks for the prompt reply, must admit it doesnt blow through that freely, or should it be like that? Coolant was changed about 4000 miles ago, but a bottle of rad weld was added so that might account for it being difficult to blow through. Liken it to a lime scaled shower head if that makes sense?

[Mocki]

RAD weld????? OMG!
imagine what the tiny little pipes in your heater rad and main rad are like now! They are a 10th of the diameter of the hole in the valve in the darlek cap.

thats your sludge identified then, anyway!

[lamb125]

Luckily the heater blows out nice warm air so no blockages thankfully in that department, but I take your point...

Back to the science bit for a mo, I assume then that when the expansion bottle becomes low a vacuum is created thus drawing coolant from the header tank. For weeks I'd noticed hardly any change to the level in the header tank which would account for the one-way valve in the darlek cap being clogged and unable to draw through coolant.

[Hacksawbob]

The expansion should not get low it should always be full. The coolant expands as it gets hot and coolant goes out the cap into the top up bottle (if there was some air in the expansion this should escape into the open to atmosphere top up bottle too). When engine cools it sucks it back from the bottom of the top up bottle, keeping the expansion bottle full.

[lamb125]

ah ok, so coolant can flow through the cap in both directions then? I'm a bit confused...sorry

[Ian Hulley]

ah ok, so coolant can flow through the cap in both directions then?

Only under pressure out to the top-up tank and under suction back to the header tank as the system cools. You'll not be able to blow or suck (or perhaps more accurately seal ) enough to open this valve manually.

[Hacksawbob]

apologies, I always thought it was one way valve, out to top up and vacuum sucked back in through bottom pipe... I'll have to have another butchers.

[Ian Hulley]

apologies, I always thought it was one way valve, out to top up and vacuum sucked back in through bottom pipe... I'll have to have another butchers.

Nope, what you said is dead right Bob

[lamb125]

Wondering if its just worth replacing mine for peace of mind..........

[Mocki]

lets just remove the confusion here......

the header tank is the one with the darlek cap on, inside the engine bay
the top up tank is behind the (infront of) the number plate flap.

the header tank should always be brim full
the top up tank should be between the marks depending on the temp when you check it.It has vent holes in the top of it, so if it gets over full it vents out the top

the coolant is pushed out of the headertank into the top up tank, and sucked back ( through the same pipe and cap) when the engine cools and the pressure turns to vacume, in basic terms.

so if the top up tank is empty it has nothing to suck, but will still blow into there.

it is a one way valve as such, pressure forces the coolant past it , but it allows vacume to draw it back.

anyhow, buy a new one, its worth a try!

[Ian Hulley]

anyhow, buy a new one, its worth a try!

http://www.brickwerks.co.uk/shop?page=s ... ory_id=118

[lamb125]

Thanks for the details on how it works, such a simple little thing, yet can make so much difference!

[lamb125]

I just flushed out the gunge in the bottom of the header tank and discovered what looks like a piece of roof tile!! WT.....