STANT Radiator Cap: Warning!!

[prince402]

Beware of the Stant Radiator Cap for the EX. My brand new one does not relieve the pressure when the engine heats up as witnessed by a constant level in my coolant reservoir.

A few days ago I replaced my radiator cap on the recommendation of my some-time mechanic. I got the basic Sant model from AdvanceAutoParts for my EX, double checking the exact model at the counter on their computer screen. I'm as sure as possible that I got the proper one. It states "16 lb" which should be right. Over too many years now this brand has been OK for me.

To me, my old cap was fine but I went with the advise. As a new owner, I monitor things pretty closely and was pleased to see the up and down of the coolant level in the reservoir with temperature of the engine (an inch or two or more with my original cap, not a STANT). But with the new cap the level is constant. To me this is very, very bad and could do some major damage to gaskets and seals!!!

There are four user reviews on the AdvanceAuto site (they have not yet posted my review at this time of writing) and one specifically mentions this problem. He is a qualified reviewer and I still ignored him. My mistake. See the review by Lee (scroll down) that I regretfully ignored: http://shop.advanceautoparts.com/we...nt_17110018-P_116_R|GRPCOOLAMS____#fragment-3

 

[Anime]

The stock replacement Motorcraft cap is ~$5, even at a dealer.

No point in buying a more expensive aftermarket replacement anyway.

 

[kert0307]

Your coolant system is supposed to be pressurized. It keeps you from boiling coolant if you are working it hard...

 

[Jason94sport]

If pressure builds up it releases from the cap not the coolant tank. The coolant level in the tank only goes up or down depending when the tstat opens up. It normally is supposed to stay at a set cold level or set hot level based on your cooling needs & how much coolant is allowed to flow through.

 

[prince402]

Good to know about the Motorcraft part. Had the local parts store had it I would have gotten it originally. Now I know to go the extra mile for it.

When the coolant heats up, it increases in pressure. If the cap is working properly, at 16 psi it releases some coolant to the reservoir, maintaining the 16 psi in the radiator. The connecting hose enters the radiater between the two radiator cap rubber gaskets, enabeling the transfer between the reservoir and the radiator. The hot coolant takes up more volume than the cold coolant and the difference is stored in the reservoir.

When it cools, it contracts and the airtight seal causes the coolant to be sucked back into the radiator. There are two rubber gaskets in the cap that make this happen and the spring controls the release pressure. If all is working properly with proper reservoir level, after a few of these cycles there will be no air space in the radiator. This is a good check.

Since the reservoir, when using the STANT cap, did not rise in level, it indicates that the pressure was not being relieved, but building up in excess of the 16 psi rating for the cooling system. The high pressure excursions could cause severe damage to seals and gaskets.

The thermostat controls coolant flow between the radiator and the engine. When the engine is cool, the thermostat is closed. As it heats up the thermostat releases coolant to maintain Normal running temperature. It is not directly involved with the reservoir.

The pressure maintained by the cap at 16 psi raises the boiling point above the normal operating temperature so it does indeed mitigate boiling.

Off the top of my head: pure water at normal atmospheric pressure (1 atm or or zero psi gage pressure) boils at 100 C. Water under 15 lb (2 atm or 15 psi gauge) pressure boils at 121 C (21 degrees C higher). A similar effect would seen with coolant.

The coolant mixed 50:50 with water also raises the boiling point and the actual amount is usually printed on the coolant container.

For years in Florida (no freezing) I have not used coolant (antifreeze), just distilled water and anti-corrosion-water-pump-lubricant. This is non-corrosive and the rad. cap pressure is enough to prevent boiling. On a marginal vehicle coolant could be added to raise the boiling point even more. Tap water and spring water contains corrosive minerals, so I use distilled.

BTW, normal household pressure cookers operate at 15 psi gauge (2 atm), so the steam inside is 121 C. It is this extra 21 C that causes the food in the cooker to cook three times faster, i.e. one third normal cook time. It is not the pressure that makes it cook faster; it is the elevated boiling temperature.

 

[Jason94sport]

Gee want to come over & do my taxes............

It is not directly involved with the reservoir.

On a 1st gen X it is. Maybe you got a bad cap then. I never had issues.