So here is the question. How is the reservoir cap supposed to work. Like a normal radiator cap, meaning after some pressure is reached, it releases air/coolant?

And how do they fail. Two possibilities. Releases pressure at a higher than correct pressure, or releases pressure at too low a pressure. Or both can happen.. Anyone know?

 

It keeps the system pressure at what it is supposed to be.

 

What pressure ratings are for what particular resevoir caps? For example, I know of a black, grey, and green cap for resevoirs. What's within limits? what's within "warranty spec"?, if any apply.

 

Good question. In my own case, my Volvo when purchased used, came with a green cap, but the Volvo dealer hauled out a black one, as the "correct" replacement. And he said the green one was not the same, although they appear identical other than the color, even with close inspection of the underside of the cap.

Pressure could be the difference of course. It could be an internal spring or whatever is responding to pressure, inside the cap, much as you can not tell the difference in a regular radiator cap except by the labeling present on the cap.

Apparently the color is meant to be the method of designation, which leads back to your question. Anyone know?

 

I will have to see if I can find out anything.
As far as I know the systems are 16 PSI systems.

 

Since I'm one of the guys that is trying to sort this out, I did find out a bit, checking Kragen online, Stant etc. They sell the after market parts. What they show is that the "green" cap is 22lbs whereas theblack is 11 lbs. I checked over a number of years and engines on Stant's site, they consistantly show the same.

Could not find any models that take a "gray" cap. Anyone use that, let me know as I'd like to include that cap for future reference.

But at this point, I'm sure that "green" is a 22lb system. And black is 11lbs. Volvo sold me the wrong part. Which is why it blows out on my vehicle.

I have two green ones, one works correctly, and one does not. I am going to boil it in vinegar to see if I can get it to work as does the other.

Anyway, in summary:

Green is 22lbs.
Black is 11 lbs.
Grey or Gray... . what models?