I disagree, i orginally had the stock cap on the filler neck and the rubber underneath went bad. Solution: went to Autozone and bought the only 1.2 bar cap that they had, which had a spring (pressure relief). Result: nothing... still loosing crazy amount of coolant from overflow tank. End Result: Bought a factory replacement cap (without the spring) from Mazda and it worked fine.

Lets see if it holds up though.

 

Are you guys sure that only one cap has springs, I checked both caps today. And they are completely identical except for the one on the ast, is different in that the circular piece that dips into the coolant is of a bigger diameter. Other than that they are the same width and length, and both are spring loaded, in that they can both be pressed. But there is no visible spring on the outside. Can someone please clarify this for me the information above does not really state the differences between the two caps clearly enough.

Lorenzo

 

The oem configuration is a flat cap on the filler neck (no need for a pressure relief cap, but should be harmless) and a .9 bar pressure relief cap on the AST.

tookwik, glad your problem seems resolved, but the cap on the filler neck plays absolutely no role in the passage of coolant to the overflow bottle (assuming you still have your AST).

 

I'd better let my FD know that since it kept spilling coolant over the overflow tank but stopped once I replaced the filler neck cap.

In all seriousness it's all a pressurized system and I don't see how having a 'leak' on either cap is not going to affect the overflow tank.

 

My point is that the "valve" that controls movement of coolant to and from the overflow bottle is in the AST, and there is no such valve in the filler neck. And tookwik's problem was not specifically due to the fact that he used a pressure relief cap rather than a flat cap.

Certainly if any cap doesn't seal properly you won't hold pressure/vacuum in the system and you can lose coolant from the overflow as a result.

 

I figured it was related anyhow because when I tried a pressure relief cap from autozone it didn't help at all and still spilled coolant out the overflow tank just like with the messed up (no rubber) stock one.

The filler neck cap can be pressed down when turned and it will feel as though it has a spring in it but thats simply just the rubber/metal in which its mounted to flexing. It has no "spring" at all any where on it. Now the AST cap is different, on mine atleast, it actually has a spring in it which (when the pressure in your system reaches the pressure the cap can hold) lets the coolant spill out to the overflow tank VIA the tube at the top of the AST.

 

Well the problem seemed to fix itself for awhile then the light started coming on again this week twice. So I put more water in the filler neck to shut it up and it started putting it all back into the overflow container again then on the ground from the container. Looks like its time to replace the caps on both the filler neck and ast. Is the ast cap for an aftermarket ast the same as stock? If so I have the one from my stock ast that I can use. Looks like its time for me to pay the 24 for the mazda cap. Hope this solves the problem as I really hope its not a coolant seal on a engine that has only around 8k miles on it.

Laterz
Jeremy

 

I happend to lift on the red tab on the AST and i heard a rush of bubbling coolant go to the overflow tank and come out the small hole on the top of the ovewrflow, but gust right after the car has been run.. Also have all the same DAM deal with missing coolant and slight leaking out of the overflow tank. plane to burb the car and get new caps, and lines. Also gust the last few days I have seen a small amount of black **** flotin in the filler neck after i add some coolant..

 

You don't have to have white smoke to be sure it's coolant seals. There are other tests to determine this.

 

And these other test are?