low coolant warning light.

[boxer]

OK the light comes on and I get everything out of the van and top up the header but what is the purpose of the reservoir behind the number plate? Water never gets pulled out of it. Or do I need an new cap on the header tank? It is pressurised when the engine is warm.

[kevtherev]

The header tank must remain always full.
As the coolant expands it transfers to the top up tank.
As it cools the coolant transfers back to the engine.

If this does not happen you either have a duff cap.. or you have a leak somewhere when the engine is hot and the system is under pressure.
The top up tank is not under pressure.

[boxer]

Thanks Kev
First stop new cap

[kit]

Get a proper one from Brickwerks as the GSF ones don't pass the quack test.

[MidLifeCrisis]

Get a proper one from Brickwerks as the GSF ones don't pass the quack test.

The first rule of the 'quack test' is you must never explain the 'quack test'!!!

[Ant-t]

Get a proper one from Brickwerks as the GSF ones don't pass the quack test.

The first rule of the 'quack test' is you must never explain the 'quack test'!!!

[CovKid]

Equally I've come across dalek caps that don't 'quack' yet work perfectly well. Its not a definitive test of a working valve. Been caught out like that a few times. Quality matters though for sure.

[boxer]

Thanks all. I tend to go brickwerks everytime. Never steered me wrong yet.

[boxer]

Latest:

I've changed the cap and it is still not pulling water from the reservoir but is still coming up with air in the header every so often. Could it be that there is air in the system and its moving around to the highest point or do I have a leak? None visible and the oil looks good. Did get the system flushed and refilled by my local garage when I had the MOT done last year. Also, is a litre of oil in 3.5K miles reasonable consumption?

[Bubble Meister]

This coolant system isn't self bleeding. To expel all the air the radiator bleed screw must be operated. With the engine at operating temperature is the top part of the radiator cool yet the bottom part hot? If so open the bleed screw. Ensure the cabin heater is selected to hot so that any air in that pipe line is expelled.
If it's fully bled and there's no water leaks its most likely to be false indication.

[MidLifeCrisis]

The hose between the two tanks isn't blocked is it?

[clift_d]

I wouldn't have thought you would get air just appearing in the coolant system.

If I was to hazard a guess it would be that you have a head gasket issue that is allowing combustion gases to leak into the coolant system and pressurise your header tank. My suggestion would be to get a friendly garage to do a sniff test on your coolant system, or get a tester and do it yourself, to check for the products of combustion.

[boxer]

The engine itself has just done 3.5K miles around France and Spain , electric fans kicked in maybe twice , both at the top of big climbs, I do have one of them point and read thermometer things and nothing appears to be getting too hot. The pressure in the system doesn't seem to be excessive. water is nice and clear with no oil film or funny smells. It is weird because I can top up the header screw everything back together drive five miles and have to top up again , or drive 50 miles and not lose any fluid.

Can't check the hose at the moment cos I've lent it to a couple of mates for their camping trip this weekend. They are texting progress reports from the M62 as I type and all is well they must be over the Pennines by now.

[Oldiebut goodie]

Maybe the clue is in the fact that a garage did a coolant change last year - did they bleed it correctly, did they rev the bo**ocks off it and replace the cap whilst holding the revs high. If you don't do this you will find air still in there. Proved that to myself by not doing it, air still present, then doing it, no more air. You may need to do it a couple of times after running a bit. You can't just refill to the top of the tank and then drive without air still appearing.
I thought that everyone would be aware of the way to do it. Simon Baxter's treatise on doing it has been posted so many times.(The wiki has the info)

[Oldiebut goodie]

For those short on clicking powers I have reproduced the relevant part here.


By Simon Baxter:
Bleeding the system.


Lets presume you have a totally empty cooling system, say, you have changed the front to rear coolant pipes.
The T3 cooling system takes somewhere in the region of 16 litres of coolant, so for a 50/50 mix you need 8 litres of antifreeze.

First off, if you are filling a completely empty system, undo the bleed screw on the radiator, like we said earlier water finds it's own level, if you don't the job will take longer and you will use more fuel!
make sure heater controls for front and rear heater if fitted are in the hot position.
So, pour in neat antifreeze first until you can't get anymore in.
Close the bleed screw.
Fit header tank cap.
Start engine.
Remove cap, now pour as much coolant in as you can.
Once you can't get anymore in, rev the engine to 2000rpm ish.
The coolant level will drop, add more coolant till you can't get anymore in.
Keep revving, screw the cap back on, if you let go of the revs without the cap on and the level will swiftly rise and you will have just wasted a "pooh" load of new antifreeze onto the floor, well done!
Okay, Got all your antifreeze in yet? You need to get that in first before you put any water in.
Go to the cab and give it a right good rev!, like a right good up to the redline rev!
Go to the radiator, undo the bleed screw and let some air out.
Go to the back, undo the header tank cap and add some more coolant, if the level has dropped a load, rev it to 2000 rpm again and get a load in till no more will go, cap on.
Go to cab, give it a rev, go to rad, let some air out.
Once you get to a piont where it feels like you can't get much more coolant in, then go to the cab and have a break, sit down for a minute and keep it revving at 2000 rpm to get it to warm up quicker.
To make it look like you really know what you are doing, flick the heater on and see if the heaters are starting to blow warm, for a real manly "I'm a mechanic me" look, set the heaters to demist and hold your hand above the vents, works great and if the missus is watching you may even get a brew out of that one.
If the heaters aren't even tepid yet, rev the nads off it again.
The heater circuit isn't controlled by the thermostat, so the heaters work regardless of thermostat position.
Once the temp gauge starts getting towards 1/2 way best let some more air out so let it idle.
Go check water level.
Top up as needed.
About this time I like to fill the expansion tank up right to the brim and attach the pipe from the header tank to the expansion tank cap.
Give it a rev, let some air out.

Now, listen to this bit, it's kinda fundamental as to how the system works.
The cooling system is meant to run at about 1 bar of pressure, thats about 15PSI in old money, that means that the coolant won't boil until about 115°c
So, the header tank is full to the brim, the cooling system reaches 1 bar and a valve opens in the cap and lets the excess pressurised coolant out into the expansion tank, the level in the one behind the flap rises.
once the engine cools, the coolant cools and contracts, if the system were to be totally sealed you would get rubber hoses collapsing, so inside the header tank cap is a second valve, a vacuum valve, this opens at some pressure or other (dunno that) and lets coolant from the expansion tank back into the header tank.
This keeps the header tank constantly topped up with coolant and no air in the system.
Right...
back to bleeding..

What we are doing here is warming the cooling system to increase it's pressure, we are using this pressure to expell the air trapped at the highest point by letting it out of the bleed screw.
easy really when you think about it.
When did you ever see your plumber with a acro prop under your house jacking it up so he could bleed the radiators?
You didn't thats why, so why should you do it on your van?

Right.
Where is the temp gauge? as we mentioned before, the thermostat and fan do a really good job of keeping the engine temps stable, from new the gauge should come up to half and stay there more or less, just over half fan should come on, just below half and the thermostat will start to close and bring the temp back up.
If at any point the needle goes over 3/4's you may have a problem.

Always run the engine up to test the cooling fan works.

With the engine running, cap on and pip to the expansion tank connected, i find it helpful to unscrew the cap with the pipe connected to let the last bits of air out.

Go for a good run

Come back.

Let some more air out of the rad, once it runs warm water with no bubbles it's done.

let it cool, check level when clap cold in the expansion tank.

If this doesn't work you are either stupid and havn't listened to the above or you have problems, possibly head gasket problems.

okay, pick that lot to bits then!!