expansion tank cap

Can anyone tell me if there is a way of checking the cap on the expansion tank is working ok, after a run out the water level drops in the bottle and when I go to unscrew the cap there seems to be too much pressure in the bottle. The engine has never overheated and it seems to be fine running with the cap off as ive had it running for an hour on the drive with the cap off incase it had an air lock (heater is blowing hot). Ive had the head gasket replaced and the head scimmed about a year ago and ive probably only done 1500 miles since. Can anyone give me a few ideas?

Prospective HGF. Draws water into combustion chamber(s) when cooling down. normally pressurises coolant system when running.

The water level drops when its running then ill switch the engine off and crack the cap open a touch the water comes gushing back up It does'nt seem to be loosing any water unless I open the cap fully then it spills over (only when hot) but then the cap does say dont open when hot. If I leave it to cool down fully the water level returns to normal. I just carnt understand why the level drops so much when its been for a run sometimes the bottle looks almost empty.

disco graham said:

Can anyone tell me if there is a way of checking the cap on the expansion tank is working ok, after a run out the water level drops in the bottle and when I go to unscrew the cap there seems to be too much pressure in the bottle. The engine has never overheated and it seems to be fine running with the cap off as ive had it running for an hour on the drive with the cap off incase it had an air lock (heater is blowing hot). Ive had the head gasket replaced and the head scimmed about a year ago and ive probably only done 1500 miles since. Can anyone give me a few ideas?

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Dont do that...there must be 1.4 bar in the system which is controlled by tha caps pressure release valve... if you run without the cap you'll lose coolant and it might start to boil if it's warm outside, that cap is a very important part of the cooling system even if it doesnt seem to, it could happen that it's pressure release valve is blocked and then the pressure might grow...if you feel that's more than 1.4 bar there when you release tha cap you better buy a new one.

"The expansion tank is fitted with a sealed pressure cap. The cap contains a pressure relief valve which opens to allow excessive pressure and coolant to vent through the overflow pipe. The relief valve is open at a pressure of 1.4 bar (20 lbf.in 2 ) and above." (RAVE)

at normal pressure(without the cap fitted) the coolant's boiling poit is at 100*C which is not unusual for a Td5 engine in very hot days or at forced usage... at 1.4 bar this boiling point rises to aprox. 123*C ...if you see what i mean

Thanks for your help it does seem more then 1.4bar ill try a new cap.

disco graham said:

The water level drops when its running then ill switch the engine off and crack the cap open a touch the water comes gushing back up It does'nt seem to be loosing any water unless I open the cap fully then it spills over (only when hot) but then the cap does say dont open when hot. If I leave it to cool down fully the water level returns to normal. I just carnt understand why the level drops so much when its been for a run sometimes the bottle looks almost empty.

Click to expand...

due to the pressure in the tank which pushes the coolant down into a dilated/distended system, when it cools down the pressure drops aswell cos the coolant volume's constant ...it's pure thermodynamic (Pressure x Volume) : Temp = Contant (all the time) so as the T rises the other two components must adjust as the proportion to remain the same

let the tank opened over night if u ran it without the cap ...ther might be small bubbles accumulated in the system as the coolant was close to it's boiling point... refill the system if needed then put a new cap on and tighten it well ...

You shouldn't remove the cap when hot anyway as the release of pressure could cause it to boil up,as Sierra says 1.4 bar is quite a lot of pressure(around 22psi) so that will cause the water level to gush back into the bottle when released

Thanks for all your help, can anyone tell me if its worth getting a genuine landrover cap or are the after market ones just as good? Ive been looking and it seems the genuine one is twice the price

make sure that the aftermarket part is OE(original equipment) not "replacement part iso ...." and it will be fine, if you dont find a cheaper OE my advice is to go for genuine cos this bit is too important

Ok thanks again