3 replies to this topic

[sparkymae]

Hi everyone. One quick question. I have a 998cc 1990 Mini , completely stock with a 13 pound radiator cap on it. When doing my normal maintenance, I checked the fluid and it was a bit low. Added about a cup to top it off. The engine was cold. Decided to do a pressure test. Pumped it up to 13 pounds and it slowly drops to 7 pounds and holds. Did this 3X same thing. No leak anywhere that I can see. Head gasket seems fine. Can someone tell me why it might drop to 7# when I put 13# of pressure in it? Thanks to anyone who might be able to help me out here.        Gary

[Spider]

There could be a bigger issue, but the first thing that spring to my mind is the Cap itself.

It could be leaking or in fact the wrong type. While there's a few different pressures at which they will relieve pressure, there's also many different reaches of caps. If it's too short in reach for the neck that on the radiator, then it will vent off before it's stated pressure.

 

 

The Left and Centre caps here are both 13 lb Caps but with different reaches.

 

[GraemeC]

The pressure tester would’ve been in place of the cap though wouldn’t it?

[Ethel]

Could be normal...

 

Once the cap lifts off its seat there's extra surface area for the pressure to push up on, against the spring. If it'll hold a steady 12.99 lbs of pressure before it releases then it's good.

 

The point of the cap is to raise the boiling point throughout the system: 13lbs = about 120^oC. 

 

The cylinder head could reach 3 or 4 times that without adequate cooling, so it's also down to a combination of water circulation & airflow as well as system pressure that will cause the cap to blow. That said, there isn't much expansion space in the top of a Mini rad and the water, alone, will have expanded by about 3% when it gets hot enough to open the thermostat.