G'day thrillseekers, up until the other day, I was having massive problems with pressure in the cooling system. Basically, the cooling system was developing enough pressure to blow the rad. hoses off the radiator!! And to make the level in the expansion tank drop by at least 10mm. Also when I removed the cap, coolant would spray everywhere, the level would rise, with a massive release of pressure. This happened almost as soon as I bought a new radiator and radiator cap the significance of which you will see soon

I was concerned that I had a blown head gasket into a cylinder, but there was no loss of coolant (other than when I removed the cap) , no bubbles in the coolant, and when the engine was rev'd with the cap off, no rising of the coolant in the tank.

Anyway...I put up with this for about 2 months, bought new radiator caps, and actually had to put two clamps on the rad. hoses to hold them in place. (Radiator end only tho')

I then got hold of an original radiator cap from a car that was being dismantled.

Lo and behold...all my problems have disappeared!!!!! Normal pressure in the system!! I can remove the cap when hot without scalding myself!! Coolant level stays constant, and I have taken one of the two hose clamps off the rad inlet/outlets.

From what I can see, the main difference between the original cap, and the replacement cap is the thickness of the rubber at the base. The aftermarket on seems thicker, and I am wondering if the slight drop in coolant when the engine is running caused a vacuum to form, and the thicker rubber couldn't release.

Maybe someone has a different explanation, but, afaiac, I will not be buying any more aftermarket caps, and I will be going to a wreckers to find a couple more original ones I can stash away

That's all

the cooling system is supposed to be under pressure when hot, the seal on the cap just stops water and pressure escaping past it. when excess pressure exceeds what is stamped on the rad. cap, it forces the seat up so that pressure can escape

Hey mate i have had the same issue and it happens randomly too. the coolant will decide to piss out the overflow after the car has stopped on the odd occasion (and not afta a floggin either) top it up and it wont lose a drop for 2 weeks. Sounds like a good idea will have to head down to the wreckers and pick me up a radiator cap, Cheers.

Although mine didn't overflow, simply because I think the cap wouldn't let it!! It wasn't random either, it was always highly pressured

I got an aftermarket cap a few years ago from supercheap and have never had a problem. I would say you got the wrong one twice over..

Also if you can now remove your reservoir cap when hot and the coolant doesn't rise and spray out. You have serious issues!!!!!!!!!!!!! It means the cap isn't even working and holding pressure.

But never before have I had such pressure that forces off the rad hoses to such an extent that I have had to put two extra clamps on each hose. Even when I released the pressure, I could hear the rad. end tanks "creaking" with the pressure relief!! Even the Ford clamp on the thermostat housing bypass wouldn.t hold, and that has never happened to me before.

I do now get a hiss with the removal of the cap when hot, but it is a "normal" hiss, if you understand what I mean. Prior to this, I could quite easily empty the coolant reservoir if I wasn't very careful!! Even after leaving the car overnight, the pressure would still be there.

Sounds like the head is pressurising the system, Still possible to have done a headgasket. Needing clamps to keep the hoses on means there is excessive pressure. Air lock?