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Old 12-08-2012, 10:25 PM   #1
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Driving without a thermostat

today i was changing out all the coolant out in my tj and i took the thermostat out so i could run the flush through to clean everything out easier and faster, then my father said on his car, up in northern michigan, in summer he took the thermostat out and his engine ran cooler. im going on a trip and will be driving about 2.5 hours away next month so he said when i drive up i should leave it out to let the engine run cooler. Does anyone know if this has any negative effects, or if it will even do this??

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Old 12-08-2012, 10:27 PM   #2
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Leave it in, your engine is designed to run at a certain temperature.

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Old 12-08-2012, 10:28 PM   #3
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The purpose of a thermostat is to let the car get up to operating temperature and then open up to not let it get any hotter. It is the optimal temperature for the car. If you run without a thermostat it would not let the car get up to operating temperature for a long while. There is a reason w it is not on the car. You know how you should not really drive e car hard before it is warmed up. Kind of the same idea. You need to let it warm up.
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Old 12-08-2012, 10:45 PM   #4
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It's not a good idea with modern computer controlled engines.
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Old 12-08-2012, 11:34 PM   #5
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today i was changing out all the coolant out in my tj and i took the thermostat out so i could run the flush through to clean everything out easier and faster...
There is no need to remove the thermostat. The heater hose passage bypasses the thermostat completely; just flush from it.

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...so he said when i drive up i should leave it out to let the engine run cooler.
Cooler than optimum is not optimal. If the thermostat was intended to be a decoration, it wouldn't be hidden from view.
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Old 12-08-2012, 11:45 PM   #6
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It might also trick the computer into making it run a little richer so it can try to warm up to the normal operating temp. There is usually two temp senders one for the gauge and the other is for the computer it might not seem to do much but I think it might decrease the gas mileage some.
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Old 12-08-2012, 11:56 PM   #7
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It might also trick the computer into making it run a little richer so it can try to warm up to the normal operating temp.
That's a great theory. Except for the fact that overly rich mixtures cause engines to run cooler, not hotter.
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Old 12-08-2012, 11:59 PM   #8
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That's a great theory. Except for the fact that overly rich mixtures cause engines to run cooler, not hotter.
Sorry more of a diesel mechanic.
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Old 12-09-2012, 12:23 AM   #9
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Depending on the exact application (never tried to run a 4.0 without a thermostat) the engine can actually run hotter without it. The coolant flows too fast and doesn't stay in the radiator long enough to cool off.
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Old 12-09-2012, 01:04 AM   #10
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Depending on the exact application (never tried to run a 4.0 without a thermostat) the engine can actually run hotter without it.
Try it and you will sing a different song. You will be hard-pressed to break 160 degrees.
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Old 12-09-2012, 01:14 AM   #11
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I had a thermostat get stuck closed on a trip in the summer. I had the tools so I removed the thermostat. Yes it ran a little cooler, but gas mileage was horrible. The computer keeps dumping fuel to get it up to temp, but it never gets there especially in northern Michigan this time of year. Keep it in and make sure it is a 195 one.
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Old 12-09-2012, 05:04 AM   #12
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In real old vehicles, you could get away with running without a thermostat, but it is totally dumb to do that now a days!
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Old 12-09-2012, 06:32 AM   #13
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You could do the "no thermostat" trick strictly as an emergency procedure in an old engine with a carburetor. However in a modern computer-controlled engine with fuel injection it is a REALLY BAD IDEA.

The engine has a temperature sensor that is part of the engine management system. When the temperature is below the thermostat setting, the engine computer runs it rich because it thinks it is still warming up. This does in fact cause the peak cylinder temperature to run cooler - and with truly terrible gas mileage. However, the rich mixture also causes the catalytic converter and the pre-cats in the exhaust manifold to run much hotter than normal. The longer this rich condition persists, the more damage is done to the pre-cats (including quite often a cracked exhaust manifold) and the main catalytic converter. Replacing these along with the damaged parts of the exhaust is over $1000 in parts alone if you buy new. But you have to have these converters in working condition for the oxygen sensors that are another part of the fuel injection to manage mixture without a smog-related CEL (Check Engine Light).

It is also possible to get the converter so hot that you melt the carpeting above it and set the Jeep on fire. That would be the worst case, I think.
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Old 12-09-2012, 09:41 AM   #14
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Try it and you will sing a different song. You will be hard-pressed to break 160 degrees.
I SAID, in SOME applications. I also said I've never tried it on a 4.0.
Pay attention before you post.
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Old 12-09-2012, 10:47 AM   #15
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I SAID, in SOME applications. I also said I've never tried it on a 4.0.
Pay attention before you post.
I am recommending that you do try it, so that you will know what you are talking about before you post.

Maybe you weren't paying attention when I said, "Try it."
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Old 12-09-2012, 10:53 AM   #16
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I had a thermostat get stuck closed on a trip in the summer. I had the tools so I removed the thermostat. Yes it ran a little cooler, but gas mileage was horrible. The computer keeps dumping fuel to get it up to temp...
The computer does not dump fuel "to get it up to temp." The poor fuel economy results because the temp fails to reach the threshold for closed loop operation.

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The engine has a temperature sensor that is part of the engine management system. When the temperature is below the thermostat setting, the engine computer runs it rich because it thinks it is still warming up.
If you observe engine operation on a monitor, I think you will find that closed loop operation occurs at a temp somewhat below "the thermostat setting."
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Old 12-09-2012, 11:22 AM   #17
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The bottom line is that a modern computer controlled engine like is in our Wranglers will not run properly without the proper thermostat being installed. For temporary emergency use? No problem, I had to do that last year... but install the factory specified 195 degree thermostat as soon as you can to keep the computer happy and insure the engine runs properly.
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Old 12-10-2012, 08:52 AM   #18
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It will set an engine code saying your engine is not coming up to temp fast enough. Can't remember the number.

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