Radiator Cap

[theKnifeArtist]

so not to long ago i thought i had head gasket failure because my overflow resivour was full, althought i was not getting any other HG symptoms. after doin tests, i came to the conclusion that the HG is fine. then i thought maybe it was a piece of the coolant hose coupler that exploded not too long ago and got loged in some line somewhere causing high pressure in the system.

well when i took it to school for some tests and stuff and my teacher said that the reason my overflow is overflowing is because of the radiator cap. i thought this was impossible because i've been using this cap for like 4 months now and it was fine. he told me that its not broken, its the wrong cap. He wouldn't even let me explain why i thought it wasn't it, he said he was 100% sure and it could be nothing else. this kind of pissed me off because i still thought he was wrong, but he seemed so sure of it.

when i put my cap on, i dont need to put any pressure on it to turn it to the locked position. i am told that that is not supposed to be like that and thats why coolant is escaping through the overflow tube because im not compressing the spring inside. the radiator i have was replaced when it was cracked but this was when my dad owned the car..he says the mechanic told him it was an oem one, but then again im not sure of that. im thinkin...if its not oem, thats why i have the wrong cap..because it was looked up for an 88 supra radiator, not an aftermarket radiator. i was lookin for some numbers or anything on the radiator but i couldnt find anything.

how do i order a cap for this radiator if i don't know what kind of radiator it is? any ideas or thoughts or anything...please do tell.

 

[CRE]

Just take the cap to an automotive parts store. It shouldn't take more than a couple minutes to find the right one and they're only a few bucks.

EDIT: Also, does the cap have the PSI rating on it?

 

[theKnifeArtist]

13psi
or
.9bar

 

[CRE]

Yeah, you should have more pressure. Some parts stores can test the cap to see what pressure it's holding to. It's not really accurate when the cap's cold, but if it's completely shot (tests to 13psi or less) it's fine. Test it or just replace it.

What makes you think it's not a stock rad? The MKIII had two different types of rad cap, they're not cross compatible. So you may not have the type specified for the year of your car, but the other model cap may work.

 

[theKnifeArtist]

i'm gonna go to the parts store tommorrow to try to find a better cap.
i took some pictures maybe to help show what i'm saying. i don't know if you see anything there, but that's kind of what i'm showing..seated and with pressure, you don't see anything happen, im not putting any spring under pressure.

 

[jetjock]

In my experience many people don't appreciate how much engineering actually goes into a closed reservoir system cap or how they even work. IIRC the standard cap is 32mm ID and designed for a 16mm deep filler neck. It's known in the cooling biz as a mini cap (versus the micro cap that appeared later). It's rated at 13 pis but should be tested to 16, which is the max relief pressure allowed. The vacuum valve can be either the spring loaded or weighted type. OEM caps are spring vacuum valved but either type will work.

 

[mkIIIman089]

JJ, the new OEM radiator I bought does not have a spring vacuum valve. That being said, the original did; but at some point they moved to an atmospheric pressure cap.

 

[theKnifeArtist]

yeah i've learning that theres allot more to these caps then i thought. so what exactly are you saying? you think this is a "weighted" cap?

 

[jetjock]

Thank you Colin. I was unaware of that. Call me cheap but the cap on my engine is the same one that came with it. It gets tested twice a year and as long as it checks out I've refused to replace it. Zero problems thus far.

KA: if the vacuum valve jiggles (loose) it's the weighted type. They allow the system to run at atmosphere until pressure builds whereas the spring types seal the system from the git-go. I prefer spring valved because junk can inhibit the weighted kind but both types have their pluses and minuses.

 

[theKnifeArtist]

if a weighted one has an air gap until it gets to temperature, wouldn't that allow coolant to flow past it and throught the overflow tube when the car is turned off and its cooling down and still has pressure in the system?

 

[jetjock]

Pressure guy, not temp. Let's back up a bit:

Your car has what is called a pressurized closed reservoir system. In days of yore systems were open and operated at atmospheric pressure. There was not enough heat load generated by the engine to make the coolant boil so pressurization wasn't required.

Then along came pressurized open systems. Those systems operated under pressure but were filled to less than full, say a few inches under the radiator neck. The space was needed to allow the coolant to expand so it wouldn't lift the pressure relief valve in the cap. Air was drawn back into the system through a vacuum valve to prevent hose collapses, ect as the system cooled. In effect the cooling system needed to breathe. If over pressure occurred coolant was dumped out the overflow hose to the ground. I still see people fill open system radiators to the brim and wonder why their car takes a piss. It's because they don't know they shouldn't be filling it all the way up.

Then came the closed pressurized reservoir system like we use. They're designed to be filled to the radiator brim. No air is allowed. On these systems it's a common misconception the radiator cap never opens unless there's a problem. On the contrary. As the coolant expands the cap's pressure valve is lifted and excess coolant is dumped into the overflow tank. As the engine cools coolant is drawn back into the system. This is why you need to be sure the overflow hose and snorkel in the tank are airtight, otherwise air will be drawn into the cooling system every time it cools off. I can't tell you how many cars I've seen with leaky overflow hoses and snorkels, ect. And people wonder why their cooling systems need burping.

It's the vacuum valve we're talking about here. A weighted valve will hang down and allow the system to run at atmospheric pressure as long as it can. When pressure builds or rises sharply the valve will shut and the system goes into the pressurized state. On the other hand a spring loaded vacuum valve seals the system from cold onward and is only open when the system is cooling ie; when it needs to "breathe" coolant back into the radiator.

A couple of points about closed pressurized reservoir systems. First, although many people do it the radiator cap is not supposed to be routinely removed. Coolant is supposed to be added only to the overflow tank. Second, in such systems the coolant in the radiator should always be filled to the brim. Anytime you look at it, it should be 100% full. The level in the overflow tank will rise when the engine is hot and fall when it's cold. If you look at it when cold the level should always be the same from one day to the next. It should never change. If it does coolant is being lost.

 

[theKnifeArtist]

how do i check to make sure that snorkel in the overflow tank is recirculating correctly? and how does this recirculation work? its the vacuum that draws the coolant from the resivour back to the radiator?

 

[jetjock]

Sigh...another example of the continuing death spiral of science education in America. I just told you, the coolant will rise and fall but should always be the same level in the tank when cold. If not you're losing coolant somewhere. What do you mean how does it work? When things are heated they expand, when cold they contract. Coolant is no different. If you fill any volume to the brim with a liquid then seal and apply heat something has to give. In this case it's the pressure cap. When the coolant cools it contracts, the vacuum valve opens, and coolant is drawn back into the system. Thus the radiator remains full at all times.

 

[theKnifeArtist]

so tell me, when you stick your radiator cap on, do you compress the spring inside when you put it on? or does it just fall in place and you apply very minimal pressure to close it up.
because i see at the bottom of the spring of the cap where the gasket is, is not making contact with the the bottom of the neck and sealing the radiator. if i dont have contact, that means coolant will flow past it and into the overflow tube. seems logical right? just weird to me that one..i've have this cap for like 4 months and it just started happening and two..i sized out the old cap i had on this radiator (the original cap that came with the radiator when it was put in by the mechanic some time ago) and the spring height is exactly the same. whats up here?
what could be causing my overflow to build up with coolant and overflow?