i just driven 20 miles at speed without expansion bottle top

[ChrisBarns]

with apparantly no ill effects! (325i) Left it off after topping-up. Muppet.

Got an interview in the middle of know where an hours drive away. Temp gauge already dodgy. I'm going to arrive terrified of breaking down, if I arrive!

[Chase007]

Hell there u may of just got away with it u shall soon find out as u say

[ChrisBarns]

hasn't used any water, will go for test tonight. What should I look out for?

[ShepsEvo3]

I would say you have a cooling system which is in tip top condition if you have lost no water, head and head gasket 100% and radiator foing a top job. Wouldnt worry about anything ;)

[Tommo32]

usually if the system is not pressurised then the water just boils and goes everywhere. it mustnt have got hot enough.

[nickso]

usually if the system is not pressurised then the water just boils and goes everywhere. it mustnt have got hot enough.

thats what happened to mine when i did exactly the same thing. drove at full pelt for 16 miles with no cap on, got to my dads place in a cloud of steam

[mash320i]

i left mine of for a good week till i realised, lost very little water, and has been ok since, so i wouldnt worry!

[murran]

the car should be fine without the cap on! as been said, if youve lost no water and not overheated then that means everything about it is in tip top order........
think about it. the system isnt pressurising so head gasket is fine. not overheated so rad and pump are working well. not boiled over so thermostat opens at the right time.
the cooling system should be able to run without the cap on with no ill effects!!
the pressurised system is just to stop emissions, smells and what not.
pressurised water also has a higher boiling point, as does antifreeze btw. so when the cooling system is up to pressure with antifreeze in, the temp can go up to 115 without it boiling.
the pressurised system can hide unforseen problems/provide a fail safe.
if you do have probs without the cap on. then all is not well.

infact my top tip is when you have a coolant leak either rad or split pipe. run without the cap, means it cant pressurise itself so just drips out insted of squirting out. and stop to top up when the check lamp comes on, or when the heater runs cold if the car has no check lamp system. completed many a journey like this. including a 40 mile run for home in a mk2 golf gti with a popped top hose, stopped to top up 4 times if i remember rightly.

[harry_p]

should be alright really. when i refilled my coolant i left it running till the rad fan kicked in with no coolant cap . the cap is just there for a little bit of extra head room really, although i think a lot of systems are designed to pressurise to help prevent cavitation at the waterpump at high revs.

[HairyScreech]

mines been used without a top cap on a couple of times, nothing to worry about really, all that will happen is the coolant will dissapear over a few thousand miles.