12-20-2010, 12:38 AM
Join Date: Aug 2008
It won't change a thing boiling it in a pot of water on the stove, not a valid test for that, but good to do to make sure it opens when it should.
Without the little bleed hole hot water stays in the engine, trapped by the T-stat till it opens, there's 0 circulation - hot water on one side, cool on the other. With the bleed hole a little hot water can flow into the radiator - slight circulation, allowing cool water to come in through the bottom hose, gently letting the engine warm up.
If the bleed hole isn't there, nothing circulates till it reaches "open" temp, then all of a sudden it open, hot water flows out, cold water fills the engine. It's a shock to the engine. Kinda like a nice hot shower then someone turns on the cold water. You wouldn't spray cold water on a hot engine would you? Same idea.
If you watch the temp gauge you'll see it reach "open" temp, then it suddenly drops, closes again, then opens again.
And, if and when any air gets in the system 'cause you had it open or there's a slight airleak, that bleed hole allows the air to get to the radiator - then out - before the T-stat opens.
When the engine cools down, the system creates a suction - that's how the overflow catch bottle works to keep the system full.
A small airleak, like a hose not fitting tight etc leaks air into the system during that suction phase. That bleed hole gets rid of it before it causes troubles. A small airleak may not leak fluid, since air is so much thinner than liquids.
A pressure test may or may not show an airleak, since the pressure usually seals it. But a vacuum test shows it.
I have no idea why aftermarlet makers don't put the bleed hole anymore - Jeep must think it's important otherwise they wouldn't use them. It's possible because some of the newer cars have a small bypass in or around the housing anyway, Stant and the rest don't bother with it - saving steps in the manufacturing.
My Astrovan and my Suburban both have bypasses built into the housings, no bleed hole needed.
Older Fords had one just above the water pump.
Not 100% sure without opening it up, but I think our 2011 Sonata has a bypass too.
It's usually good practice not to re-engineer what the factory spent millions on developing - unless you have a solid basis for changing things.