There are two pressure caps on the coolant system for my fiance's car. What is the purpose of the second one? Some people I have talked to said they have seen it before, but are not sure either. One guy thinks that it is to relieve air in the coolant system, but is that not what the plastic screw in the radiator is for? Just slightly confused.

EDIT: Second cap is barely visable in lower-left corner near overflow bottle.

 

only one is a pressure cap. the other is a solid cap (doesn't open with pressure)

I think some years had 2. either that or someone swaped in a rad from another year so now you have both.

 

Only the one on the radiator is supposed to be a pressure relief cap. The one on the filler neck should be a plain cap.

If there is pressure relief cap on the filler neck you should replace it with a plain cap.

 

Maybe this is a little better.

 

What happens if you have a pressure cap on the radiator and a pressure cap on the filler neck?

 

Dude- I just bought a set of mazda caps- and both are 13lb pressure caps- ^^ so ditto question^^

school us, NZ!!

 

What is weird is that my base is the same year of my fiance's and it only has one pressure cap - on the filler neck.

 

The pressure relief cap is there to relieve pressure into the overflow bottle. Only one of the necks has a hose connection, so you only need one pressure relief cap.

If you have two pressure relief caps, when the one on the neck without a hose opens you can have a coolant leak.

Some FC's only have one cap, but if it has two, the one on the neck without a hose should only have a plain cap.

 

Ah yes, I see the wisdom of it, NZ-san.

Domo!

 

when i bought my car, i had 3 caps.....i took one of them off. it was something a PO put on there as an attempt to tell himself the motor wasnt blown from being overheated.

Daniel

 

If you guys check out the picture that is only the second time that ive seen a setup like that....is that common? the radiator had a n### C on it and it looked brand new in a car that had over 200k on the clock... Also that radiator cap near the bumper had some kind of lock on it where you could not take it off without taking 2 screws out. I was just like WTF when i saw it..

Most 2 cap setups i see have one on the filler neck and the second on the radiator (i have this) with the overflow tube coming from a neck on the radiator...in the 86-87 models it came from the filler neck so thats why i thought it was so weird

 

Wait a minute - I see 3 caps in that setup.



Maybe some high altitude feature?

 

No, I do not remember there being three caps. I only remember two. I think that is just a leaf that blew in or a piece of plastic.

 

That is what this setup is. You cannot remove the cap without unscrewing two small screws.