Header tank losing coolant, expansion tank not...

[timjwharton]

Any pointers would be much appreciated. The flashing light of doom came on today. I stopped and checked the expansion tank behind the number plate and it was filled to MAX. I checked the header tank in the engine bay and that was a few inches down. Once cool, I topped it up with 50-50 coolant/water.

I'm confused. I thought that the idea is that, working properly, it is the expansion tank that occasionally needs refilling, and not the header.

I'm also presuming something is wrong. Any ideas?

Thanks.

[dave friday]

Have a look at the"Wiki".

[timjwharton]

Already did.

[timjwharton]

Got it now Dave. Thanks. Pipe between the two split, yes?

[dave friday]

Ok so...the darlek cap is faulty or the pipe going to the top up tank is blocked or you've got a leak,heater on/off water tap?
Oh! jolly good!!

[dave friday]

Split...that will suck in air instead of water.
That reminds me mines looking a bit dodgy.
Good luck.

[timjwharton]

Admirably clear. Thanks so much.

[timjwharton]

Thanks again Dave. Much appreciated.

[datgap]

Ok so...the darlek cap is faulty or the pipe going to the top up tank is blocked or you've got a leak,heater on/off water tap?
Oh! jolly good!!

First time I have used this site so don't know if responding in the right place.
I have the same problem air in the expansion tank but and coolant staying in the top-up tank
you mention about a leak in the heater on/off water tap is this easy to check?