So my overheating problem wasn't really a prolbem in that I never saw dangerously high temperatures. But I'd began to notice that my temps were begining to creep above 87C and that got my attention; I have a Greddy Temp guage. When the temps started to routinely hit 90C I began to check things out. Nothing obvious jumped out at first. But, I began checking the coolant level every morning before starting the car.... at first I was adding only an ounce or two... soon it was 5 ounces... and then 12 !! Where the hell was all this coolant going? Well, after I satisfied myself that it wasn't a coolant seal... and all the hoses checked out to be Ichi Ban Joto number 1 I began really scratching my head! Hmmm... No leaks, no blown seals ... I then checked out the radiator cap on my AST tank... I have the metal upgrade.... The damn cap was bad!.... Mazda Specs call for a 13PSI cap. I've replaced the cap and my temps are back to normal...

Now, Does anyone know weather the cap for the AST is supposed to be vented ? or no? I've not seen anything on this.... If you know the answer.... post here or send me a PM....

BTW, I have a 180 degree thermostadt, fluidyne radiator and am running 20% antifreeze... so, before my cap took a **** my temps never went above 84C.

 

i have the pettit ast and it is a vented GM cap i believe stant

 

whats the normal PSI supposed to be on the AST?

 

13

that's awesome, I never see past 83C but I always run the fans on high. Unless im going to race or something then I turn them off because most times my car is at 178F with them on and it never warms up all the way.

 

thanks

 

Depends.. the higher the pressure the less ability the system has to deal with heat soak! Thus the boiling pointis lowered as pressure increases... so less pressure is more capable temperature resiliance..

 

Where did you learn physics, in the Ukraine with F*ckin' David ?

As pressure increases boiling point temp increases......

Dad, the AST purges the system of excess gas and maintains a constant psi......the cap is supposed to be like that. Your silly Subarus and '66 L88 427 Vette just didn't have ASTs

 

I disagree and hold my point... and it was serbia!

 

This is simply not true. Boiling point of any fluid will raise with an increase in pressure. The converse is also true; boiling point will fall with a decrease in pressure.

You can boil a fluid at room temperature if you put it in a vacuum chamber.

 

Want2race, then I should run less coolant in my RX7 so that it can handle better heat???!!!
Since the overheated coolant will start to boil and the pressure in the system will increase, then the temperature will decrease due to the pressure increase in the system???

Are you for real???

Do you really believe this???

Question for you, why do think pressure cookers work faster at cooking food???

1 is it because they produce less heat due to increased pressure

2 is it because they produce more heat from the increased pressure



answer: 2

PS Einstein, Newton, Boyle......are turning in their graves now

 

Man,

I got into similar topic couple of weeks ago.. Maybe I should find that thread..

The problem is not just the heat.. bigger issue is the boiling.. As something "boils" it release gas.. in turn build higher pressure.. So, say in your system, you are running pure water and some reason your coolant(water) boils.. where do you think that sudden pressure will go?? Its not going to travel all the way to Ast and bubble out.. The sudden gas pressure will most likely blow your coolant seals.. Thats the easiest way to blow your coolant seal.. This is one of the reason for using coolant.. It lowers the BP and also most coolant will help corrosions and etc..

Also,... For instance the rad cap rated at 13 will hold 13 and than release extra pressure by relieving the volume, ie, reserve tank..
Also, this is one of the reasons why when you get your coolant system checked, you don't check for 100psi.. you check for specific psi... otherwise, you will also blow out your coolant seals.. which is the reason why you don't want to take your car to some idiots who don't know this and when they check it, they pressurized the system higher then recommended..



i hope this clearifies couple of things..
And as always, I could be wrong about this.. and if so, please post why..

 

Just get NPG+ and run 0 psi

 

You guys will never be in danger of being mistaken for a bunch of rocket scientists... Houston, I think we have a problem...

First of all.... The Mazda cooling system was designed by Mazda to safely tolerate 13 PSI. At this pressure no damage or breach of the hoses and fittings of the cooling system will occur due to pressure increases... hence the 13 pound cap.

Second... While putting any gas under sufficient pressure will in fact change it to a liquid state; dry ice is a good example, likewise pressurizing the liquid coolant of a an automobiles coolant will raise its boiling point above 212 Fahrenheit or 100 Centigrade for you metric types by keeping any gases in suspension. Again, the purpose here is to prevent gas from coming out of solution and causing cavitations, which is destructive to the water pump impeller. Also, keeping the coolant in a liquid state improves its ability to exchange BTU's of heat.

Third... When calculating the index number of any mediums ability to absorb and give off heat, water is used as the base line. It has an index number of "1". Anything that is added to the water will decrease this efficiency index number. Any medium used in a heat exchange application has its efficiency measured against water for the purpose of predicting its capacity potential.

Forth... Ethylene Glycol, or anti-freeze is added to the cooling system to:
a. help raise the boiling point of water
b. lubricate the water pump
c. inhibit metal oxidation (rust)
d. lower the liquids surface tension and thereby minimize cavitations

Fifth... Cavitations or air bubbles caused by the ships prop or that of a water pumps impeller crashing through a liquid creates bubbles. These bubbles have a "ph" that is below 7 making it acidic. This acid is known as carbolic acid, which is very bad. Carbolic acid attacks and degrades the metal of an impeller blade or prop. If you look at an old water pump impeller you're likely to see pitting and thinning, especially at the blade tips.


So... Getting back to the original question... Does anyone out there know if the radiator cap for my aftermarket AST tank should be vented? Or, Not Vented?

 

Actually the original fittment on the RX7 was a 1.3 bar cap and the system was designed to take 1.3 bar of pressure. The engineers were wrong, real world problems became evident and the 1.3 caps were recalled and replaced with the .9 bar cap.

As to the Q "to vent or not to vent"----logic would suggest that a vented cap would reduce the burping required post refill but would also increase the number of possible failure points from 1 (the large gasket) to 2 (the large gasket and the one way gasket). Seems like a judgment call to me.

 

But as least we know what kind of cap to use Just to make sure I follow you, when you say "vented" you mean a hole that vents to atmosphere? If so, you do NOT want a vented cap. The pressure cap on the AST will relieve excess pressure via the hose on the AST neck into the overflow bottle which IS vented to atmosphere. If the AST cap were also vented to atmosphere over time you would lose coolant since it wouldn't be recovered properly when the car cooled. In fact, some of it would be gone!

 

Ooops. Ignore the "judgment call" part of my last post. Of course Damon is correct. The overflow bottle will vent uneeded gasses.

 

The AST "vent hole" is supposed to go to the overflow tank (which is at atmospheric pressure and is vented at the cap), so the AST is "vented". The hose goes to the bottom of the tank. This allows expanding coolant (as it warms up) to flow into the overflow tank. It also vents any gases trapped or introduced into the cooling system. When the system cools off, it creates a vacuum, which then sucks back coolant from the overflow tank. One problem many owners have had is this can fail if -one- of the two caps in the system develops a -vacuum- leak. The system cannot suck back the coolant, instead sucking in air. The owner notices lower coolant in the engine and tops it up. Next heat cycle this coolant is also trapped in the overflow tank. Soon the overflow tank overflows, and the owner panics (my "O" rings are gone). It's just a bad cap. Replace them both and you're good to go.
This happened to me shortly after I got my car. I also replaced the small AST hoses with proper pressure hose, as they can kink and also stop the recycling of coolant. The pressure hoses are too strong to kink. I had to re-route them as they were too stiff to bend the way the stock ones were installed. Of course I also replaced the plastic AST with a metal one (the most important reliability mode I've done).

The best mod. I've done in terms of improving the car was putting in the Apexii Power FC - no contest here!!!

 

i had the same problem with the damn cap. I just replaced mine again. This is my 3rd cap in a year and a half. I think i always mess mine up buy opening and closing it all the time. This time i just replaced it and i'm not touching it again.

 

I went and bought high pressure fuel line and ran that from the AST to the overflow. It's cheap, flexible and will take a real beating.

 

fuel injection hose is great

 

For those of you who have to get their vehicles periodically inspected as per State DMV's, the machine used to check the pressure integrity or your fuel system and gas cap can also check your radiator cap...

According to a friend of mine, who's a certified inspector, he'd routinely check the radiator caps of customers cars. He said that of the raditor caps he tested, fully half failed!

So, I now regard the $7 to $10 for a replacement cap about the same as an oil filter. It's just an expendable but necessary item.