15 replies to this topic

[Rubbershorts]

Big run out in the sun yesterday, with a some extended idling in traffic.

 

The temperature gauge usually sits nicely dead centre but in some instances yesterday crept up ever so slightly in the traffic, but not much extra, so not a concern. Every time It did this though I could smell something hot. On inspection when I got home there was evidence of the coolant escaping from the radiator cap. It appears to come from the pressure relief valve in the centre. Unfortunately the picture of the cap is after I'd cleaned it. A small trace is still on there. I think this has been going on for a while as I've had to clean it up previously. In those instances I thought it was something to do with the leaking temperature sensor, which is now fixed, so never suspected the cap. 

My understanding was the valve opened to let the expanding coolant out of the overflow pipe into the catch tank under the wing (I've not checked the coolant level in that yet as I need to remove the wing liner first, but probably do that tonight.) then it gets sucked back in when cooling down. Does it have another function where if the pressure is too high it just lets the expanding coolant out of the top of the cap? Like I say, the temperature didn't get anywhere near maximum.

 

The cap is only a few months old, could it be defective? 

 

 This is the one that's fitted (GRC207 0.064 8708299000 RAD CAP '92-'96 FOR RADIATORS WITH SENSOR SWITCH,15lb) and I chose this after I'd posted regarding my expansion tank here:

 

https://www.theminif...expansion-tank/

 

Is it possible I have the wrong one? I currently have an order in with Minispares so may just add this one to try.

 

https://www.minispar...-expansion-tank

 

I know that GRC126 actually has expansion tank in the description, but the GRC207 is the correct year, It's for rads with a sensor switch, which I have, and the blurb accompanying it suggests it is for an expansion tank set up also.

 

Any help thoughts greatly appreciated.

 

Cheers everyone, Daz. 

 

 

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[Earwax]

Hard to say , but could the leak be from the soldering?  

[68+86auto]

I have found many failed caps which will leak at very low pressures. I have had them fall apart too. If you want to be sure it is working correctly and that the combustion process isn't pressurising the system, there are cooling system testers available cheaply.

Edited by 68+86auto, 11 May 2026 - 10:59 PM.

[Spider]

Given the Expansion Tank you have is a Plastic type and while it may have been rated for normal cooling system pressure, not pressurising it will definitely make it last way longer, so I wouldn't suggest using the GRC126 Cap.

 

What is needed though is a Recovery type Cap on the Radiator. These have an upper and lower neck seal. I'm not certain that the cap you have is that type ?

[Rubbershorts]

Hard to say , but could the leak be from the soldering?

From the picture it does look as if that's possible. If so, then it's coming out in two places as it was definitely coming out of the top of the cap. I'll add it to the list of things I need to investigate further. Thanks Earwax.

[Rubbershorts]

Given the Expansion Tank you have is a Plastic type and while it may have been rated for normal cooling system pressure, not pressurising it will definitely make it last way longer, so I wouldn't suggest using the GRC126 Cap.
 
What is needed though is a Recovery type Cap on the Radiator. These have an upper and lower neck seal. I'm not certain that the cap you have is that type ?

Thanks Spyder. Would it be this one I need?

https://ebay.us/m/JD3tjF

[Rubbershorts]

This is the cap I'm currently using. It seems to have the double seal. GRC207 from Minispares. The option pictured on eBay seems to have a bigger seal on the top of the cap, but is stamped G1050, as is the one I'm running. Daz

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Edited by Rubbershorts, 12 May 2026 - 06:09 AM.

[Spider]

That's the right cap. They usually have a sweated pin in the centre of them, ie, something that might be rivets but also soldered to seal them, though I'm sure some are just pressed over and hope for the best.

[Earwax]

Picking up from a point Spider noted.  If the plastic tank has a split ( above the coolant level) it can suck air causing the system to be non pressurised and potentially lower the boiling point  ( same as cap not working)

Check hoses as well.

[Rubbershorts]

I've ordered another double seal cap that appears to have a larger upper seal. In the meantime I've discovered this with the leaking cap I have.

https://youtube.com/...AudYM8ecZYRpMdI

Should the pin have that much movement, if any at all. Also, I was holding the fitted cap and moving it back and forth to investigate the suggestion that the neck of the tank may be leaking, and it doesn't seem to sit very solidly on there. I'm not very confident it seals fully every time. I'll see what the replacement is like. Cheers guys, Daz.

[68+86auto]

I've ordered another double seal cap that appears to have a larger upper seal. In the meantime I've discovered this with the leaking cap I have.

https://youtube.com/...AudYM8ecZYRpMdI

Should the pin have that much movement, if any at all. Also, I was holding the fitted cap and moving it back and forth to investigate the suggestion that the neck of the tank may be leaking, and it doesn't seem to sit very solidly on there. I'm not very confident it seals fully every time. I'll see what the replacement is like. Cheers guys, Daz.

 

That is 100% defective. I had one even fall apart recently.

[Steam]

The re overy caps only work on the short neck rads, if you hve a long neck rad then it will simlpy seal at the top.

[Rubbershorts]

The re overy caps only work on the short neck rads, if you hve a long neck rad then it will simlpy seal at the top.

Cheers Dave. How would I know if mines either of those? Any idea of what that would measure? Daz.

[Steam]

From memory the shorter neck is 20mm and the long neck is 25mm measured from the top to the neck base. And the recovery caps for the long necks are either not available or rare as rocking horse $#!T . But you can use just a blank sealing cap, or fit a recovery cap despite it only working as blank sealing cap

Edited by Steam, 14 May 2026 - 07:00 AM.

[Spider]

 

The re overy caps only work on the short neck rads, if you hve a long neck rad then it will simlpy seal at the top.

 

Cheers Dave. How would I know if mines either of those? Any idea of what that would measure? Daz.

 

 

Daz, you'll find some info on caps here ;-

https://www.theminif...stralian-mokes/