Coolant overflow line restrictor. Why???
 

Mazda placed a restrictor in the coolant line going from the AST to the overflow tank. Its a plastic joint that is spliced into the line where it goes across the rail to the overflow tank. The restrictor narrows the diameter of the line to a much smaller size. Obviously, it is there to restrict flow. My question is why. Why would you want to restrict the flow?

It seems to me that changes in coolant temperature are not instantaneous, and that the system should easily equalize itself without the need for restriction.

From what I recall (I believe from KevinK2), it's because the AST basically bypasses the cooling of the radiator. So the restrictor is to limit the amount of coolant which doesn't get properly cooled.

If it's the little plastic thing in the line to the overflow tank, I've always thought it was simply a quick disconnect for that line. The overflow tank is pretty low volume in flow, coolant goes pretty slowly to and from it, so a restrictor won't do much at all.

I believe the stock AST itself has a restrictor in one of the inlet nipples. It can't be that useful, as the Mazdaspeed AST doesn't have a restrictor.

I bought a new overflow tank for my project car, and it came with the short piece of hose and the plastic hose connector. I think it's just for faster installation at the factory.

Dale

I agree with Dale. I think it's just a quick way to connect and disconnect at the factory. Thats all.

Have any of you taken it out to see if it is actually a restrictor with a small opening, or just a connector? I have not.

Now that I think about it, I removed my overflow tank and ran some water thru the AST, to clear the line of anything that may have been it it. It seemed to flow at a normal, non restricted rate.

That is correct, but that restriction is in the nipple to the line from the AST to the bottom of the radiator. I put one the same size - 0.090" diameter - in the line from my Pettit AST to the rad bottom to avoid bypassing any more coolant than necessary.

However, he was asking about the plastic coupler in the line from the AST to the OF tank.

Dave