Radiator cap Spring squeezes just few mm in depth=broken? +POPPING Noise when opened

Danny_G

DoaM wrote: »

How long after the engine has been running do you release the cap?

The next morning (aver it has cooled / switched off all night)

see my original posts here, which I have quoted here:

the water level gets low after long drives (2-4 hour drives)

checking water level the next morning, the water level is less
(about 0.5 pints can be refilled of lost water)

so 0.5 pints has gone, hence asking about the cap

DoaM

Once a system is pressurised it never goes all the way back to atmosphere - even when cold - so there would always be a pop.

However if the level is falling then you must have a leak somewhere. (And as was mentioned earlier, the pop could actually be air being pulled IN rather than escaping OUT - i.e. the system is under vacuum rather than pressure).

chrisw

Could still be the cap. Coolant escapes from the cap when it's hot, then a vacuum forms as it cools down.

Either try replacing the cap or get the system pressure tested - not very expensive.

Danny_G

Could the fact that it is popping, indicate that there is also air in the system?

so needs to be bled out?

Joe_Horner

Jackmydad wrote: »

You are filling the tank to the correct marked level and not right up to the top?

Probably this.

If you over-fill it then, as the engine warms up, it will blow the excess coolant out of the overflow.

When the engine cools down the coolant will contract again and - because there's less of it than there was before - you'll ed up with a partial vacuum in the system. That will cause a pop when you remove the cap and air enters.

Note that if there was a leak (from the cap or anywhere else), then any vacuum or pressure would have disappeared by the next morning thanks to air or coolant leaking in / out. So the fact you get a noise several hours later pretty well rules out any leaks and suggests over-filling of a well sealed system.