Pressure in the cooling system

[pink_floyd]

My car is 1998 4ltr with separate swirl and expansion tanks. I managed to loose the blue expansion cap, bought one from Halfrauds for £3.49, looks like the old one. When I got home lastnight the engine temp was high, fans kicked in. I carefully unscrewed the blue cap expecting a hiss as the pressure was released, but nothing. I removed the cap and the coolant was halfway up the expansion tank.
Is this what I should expect, or is there a problem with the cap? or the system?

[andy-w]

I would have expected a hiss as well.

Was the stat open....as in was there a fast flow of water comming through?
I would have thought water only being half way up the swirl pot was a bit on the low side?

[GreenV8S]

Sounds like it isn't holding pressure. You should be able to feel whether there is any pressure without removing the pressure cap, by squeezing the top hose.

[pink_floyd]

The swirl pot was full, it was the expansion tank that was half full. Sounds like I might have a leak somewhere.

[grumbledoak]

I wouldn't worry about the absolute level in the tank - each car is slightly different and they will find their own level by spitting some out if you overfill.

But yes, if the temp is anywhere near 100 the system should be under pressure. Sounds like you have a leak, so that level should be dropping each time you drive her. I'm not sure what a pressure test kit costs, but they can't be much more complex than a bicycle pump...


Can I ask how you lost the old cap (just in case the story is funny !) ?

[Nick P]

is it a problem not holding the pressure?

Mine runs fine, doesn't lose water, but it hisses slightly out of the top passenger side of the radiator where there is a join.

[Cider Andy]

Nick P said:

is it a problem not holding the pressure?

Not in itself, no. But it's a closed system and if there's no pressure it implies there's a leak, most likely where you hear the hiss from. I'd be very surprised if it wasn't losing some water.

[grumbledoak]

[redacted]

[Nick P]

i carry water etc already, but she's honestly not losing any liquid...thats the odd thing...i've seen where the hiss is coming from and its on top of the rad. oh well...i'll just keep an eye on it

[snorky]

my expansion tank level is about 2" full, fill it up any higher and it just spits it out....

[f1fx]

pink_floyd said:

My car is 1998 4ltr with separate swirl and expansion tanks. I managed to loose the blue expansion cap, bought one from Halfrauds for £3.49, looks like the old one. When I got home lastnight the engine temp was high, fans kicked in. I carefully unscrewed the blue cap expecting a hiss as the pressure was released, but nothing. I removed the cap and the coolant was halfway up the expansion tank.
Is this what I should expect, or is there a problem with the cap? or the system?

IT WOULD SEEM TO ME THAT THIS WOULD BE A CAP PROBLEM.. AND LETS NOT HOPE ITS THE HEAD GASKET. BLEED THE RAD TOO !

[GreenV8S]

Nick P said:

is it a problem not holding the pressure?

Mine runs fine, doesn't lose water, but it hisses slightly out of the top passenger side of the radiator where there is a join.

Yes it's a problem. Without the system pressurised, you will get localised boiling in the heads when the engine gets hot, and it will overheat at a lower temperature.

[Nick P]

i'm putting the old girl in for a service next week, and the guys have said they will repair the radiator as apparently its a common fault and quite easy to fix....while its there they are also doing my blowing exhaust gasket

[APMAUTO]

pink_floyd said:

My car is 1998 4ltr with separate swirl and expansion tanks. I managed to loose the blue expansion cap, bought one from Halfrauds for £3.49, looks like the old one. When I got home lastnight the engine temp was high, fans kicked in. I carefully unscrewed the blue cap expecting a hiss as the pressure was released, but nothing. I removed the cap and the coolant was halfway up the expansion tank.
Is this what I should expect, or is there a problem with the cap? or the system?

make sure the hose from the top of the expansion tank to the swirl pot is not blocked or pinched by the clip holding it to the swirl pot had this a few times

andy (APM)