As the subject matter asks, which is the proper design to use with our radiator system??? I've seen several responses in search mode, one even stating the STAT lists the wrong "vented" cap, it should be non-vented??? Which leads to: can a non-vented cap still draw fluid from the over-flow bottle upon engine cool down vacuum???

Does anyone know for certain (it's not called out in GM Shop Manual), which one it's supposed to have, and why (if you know)?

It's a standard replacement cap. The spring load allows it to vent into the tube which connects to the overflow tank.

Arn

When you say "Standard" does that imply "non vented"? If what you say is true, then how does the coolant get sucked back into the radiator when the engine cools down, the pressure is off the spring closing the passage for coolant to flow?

A vented cap will not suck coolant back into the radiator from the overflow tank. For proper operation the Fiero wants a sealed system

quote Originally posted by Phil: A vented cap will not suck coolant back into the radiator from the overflow tank. For proper operation the Fiero wants a sealed system

Well, all cooling systems are "sealed". The radiator cap holds up to 250lbs of pressure. Pressure on the cooling system greatly lowers the boiling point, pressure also helps the system flow. The cooling system has to draw coolant back into the system from the overflow, it has no choice, the hoses would all collapse if it didn't.

[This message has been edited by FieroBrad87 (edited 08-20-2008).]

Standard non vented cap. More information on that and the Stant part number confusion in theogres cave.

Read the article in Ogre's cave (link at top of page). He gives a very good explanation of the whole vented vs non vented cap thing

Thanks, article in Cave is a great write-up. I needed that confirmation to be sure what I did was right.

Shop had replaced my cap (without asking me) and put on a 16 lb vented cap, which is probably part of my engine hot running problem. Shop believes I'm wrong about the non vented cap, I replaced it with a 18 lb non-vented cap, but didn't have time to test it out before alternator decided to take a dump last weekend.

The small stuff, out of my control, has been killing me lately.

quote Originally posted by FieroBrad87: The radiator cap holds up to 250lbs of pressure.

Try 15 psi

quote Originally posted by Phil: A vented cap will not suck coolant back into the radiator from the overflow tank. For proper operation the Fiero wants a sealed system

No offense Phil but I think that you are mistaken. There is a check valve in the cap which allows the cap to suck fluid back into the system from the overflow tank but not allow fluid past the cap unless 15 psi of pressure rating of the cap is exceded. The non-vented cap has an additional spring in the check valve where the vented one relies upon fluid flow. Vented vs. Non-Vented is a bit of a misnomer as it really doesn't indicate a sealed vs. non-sealed system.

[This message has been edited by Jefrysuko (edited 08-20-2008).]

quote Originally posted by Jefrysuko: No offense Phil but I think that you are mistaken. There is a check valve in the cap which allows the cap to suck fluid back into the system from the overflow tank but not allow fluid past the cap unless 15 psi of pressure rating of the cap is exceded. The non-vented cap has an additional spring in the check valve where the vented one relies upon fluid flow. Vented vs. Non-Vented is a bit of a misnomer as it really doesn't indicate a sealed vs. non-sealed system.

I know how it works but sometimes it's easier to oversimplify things. This vented/nonvented thing is like trying to explain the difference between heat and temperature,

quote Originally posted by Phil: I know how it works but sometimes it's easier to oversimplify things. This vented/nonvented thing is like trying to explain the difference between heat and temperature,

Sorry, if you couldn't tell I have been accused more than once of over complicating things

quote Originally posted by FieroBrad87: Pressure on the cooling system greatly lowers the boiling point........

It actually raises the boiling point, which is a good thing. When coolant flows past head surfaces that cool the combustion chamber (much hotter than 212 degF) you want the coolant to stay in liquid form and not vaporize in localized areas around hotspots.
Liquid coolant carries much more heat out of the head than does vaporized coolant. The pressure on the system helps the coolant do its job by keeping it in liquid phase to a higher temperature.

"Water wetter" also helps keep liquid contact with cooling passages, but by a different mechanism.

Just for a little bit more information, I have been using Evans NAPG (read non-aqueous propylene glycol) coolant in all my vehicles for several years now. You can use a "0" pressure cap, no water is needed, much higher boiling point and lower freezing point, better thermal conductivity from the heads (no possible water steam pockets near chambers), usually eliminates any chance of leaks in radiator and heaterr core, helps hoses last longer, and is ECO-friendly.

I guess you can tell I like this stuff. It's a bit more expensive than regular coolant, but I feel well worth the cost.

quote Originally posted by rjblaze: Just for a little bit more information, I have been using Evans NAPG (read non-aqueous propylene glycol) coolant in all my vehicles for several years now. You can use a "0" pressure cap, no water is needed, much higher boiling point and lower freezing point, better thermal conductivity from the heads (no possible water steam pockets near chambers), usually eliminates any chance of leaks in radiator and heaterr core, helps hoses last longer, and is ECO-friendly. I guess you can tell I like this stuff. It's a bit more expensive than regular coolant, but I feel well worth the cost.

That sounds interesting......could you explain this part:
>>>>>>>>>>>usually eliminates any chance of leaks in radiator and heaterr core<<<<<<<<<<

Yeah, I'm an idiot. Raises boiling point, that's what I meant. Not 250lbs, I had edited my statement and I didn't back up far enough and I had two statements run over eachother. Sorry, put me on full ignore.

DEFINETLLLY NON VENTED. ADVANCE AUTO STORES COMPUTER SHOWS VENTED CAP. EMAILED CORP ABOUT THEIR ERROR AND HOPE THEY CORRECT THEIR MISTAKE. USE A VENTED CAP ONLY IF YOU LIKE TO OVERHEAT/

The cap must be the "Non Vented" type no matter what the catalog books/systems indicate. All Stant numbers ending in 31 ARE WRONG. They must end 30. If the center disk hangs loose, it is the wrong cap no matter what brand it is.

The radiator cap is a bigger factor in setting the boiling point than the coolant package. It is why you have a boiling point of around 265*F with a 15PSI cap. With coolant only and no pressure you'd only raise the boiling point a few degrees at most. (Like about 5 if I remember right. It's not much.)

Running a higher pressure cap than called for should be avoided. Especially on an old system. That and it's not needed anyway. If you're boiling with a 15-16 PSI cap then you've got other major problems with the vehicle.

12-13 PSI caps are available for many applications. I think they are mainly for high altitude use but I'm not entirely sure. If you think the system is weak, running one of these might get you by in a pinch but you'll drop the boiling point considerably and boil over that much easier. Running low pressure caps is generally almost as bad as no pressure at all.

Worse... low pressure increases the chance of spot boils and hot spots in the engine. Remember that coolant and engine temperatures ARE NOT uniform thru the system. It's normal that some areas in there could be running 220-240 when the reported temperature is 190-200. If you don't have the right cap the coolant can boil in those hot spots. This sort of thing is why some vehicles have higher pressure systems even tho the coolant temperatures reported are similar. Putting a 15 PSI cap on a vehicle that calls for 18+ is just like putting a 13 on a Fiero. Not a good plan.

[This message has been edited by theogre (edited 08-23-2008).]