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Old 12-19-2010, 01:52 PM   #1
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Replacing Thermostat

I'm trying to replace my thermostat and the FSM states that I should insert the new thermo with the arrow and air bleed hole pointing up. Problem is that there is no arrow or air bleed hole on the new thermo nor was there on the old thermo I just pulled out. Any advice?

Thanks.

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Old 12-19-2010, 02:36 PM   #2
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The factory thermostat does have a bleeder. Most aftermarket ones do not so it does not matter which way is up.

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Old 12-19-2010, 03:28 PM   #3
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Simply drill a 1/8" hole so it has one like the one you took out.
It helps bleed the air out when you fill it, and it helps with the warm-up.

Without the hole it will wait till the engine is very hot, then it suddenly opens up and circulates cold water through it. That kind of a quick temperature change is rough on the engine. You can see that on your gauge - it gets warm, then suddenly gets cold again. The little hole lets it open slower and smoother.
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Old 12-19-2010, 09:28 PM   #4
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Thanks for the info. I already installed the new thermo and did not read this in time. I wonder why they would make an after market thermo without a bleed hole, especially if it's hard on the engine.
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Old 12-19-2010, 11:42 PM   #5
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Mine never "snaps" open that I have seen at least it gradually gets warmer till it's at operating temp. I have boiled a few thermostats before and what I saw was the spring gradually retracting as the water got hotter but it never snapped open all at once. Just what I have seen.
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Old 12-20-2010, 12:12 AM   #6
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Did your last thermo come with the bleed hole?
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Old 12-20-2010, 12:38 AM   #7
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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.
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Old 12-20-2010, 12:48 AM   #8
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Point taken!! I'll yank that sucker back out and drill a 1/8" hole in the flange. Thanks for the advice.
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Old 12-20-2010, 04:29 AM   #9
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Naw, I wouldn't now that it's already in. There are thousands running around without the little hole. Chances are yours will be fine.

But if you have trouble getting the air out, or it overheats, or the heater acts strange - cold - hot - cold hot - etc., or you have it out again, drill the hole.
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Old 12-20-2010, 10:22 AM   #10
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Will do. Thanks.

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