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Old 12-15-2016, 10:34 AM
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Thermostat or Not

Is there anything that says, you need the thermostat or not? I am going to replace with a new one but was wondering if it's needed and runs well without it. I have a lead so that's why. 4.2L MT with 185K miles.

In addition, any thought on replacing the stock distributor with a HEI DISTRIBUTOR. Some were saying, it will mess up the cam gears?

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Old 12-15-2016, 11:29 AM   #2
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Our Jeeps were designed to run most efficiently at 195*. A thermostat will stay closed until engine temperature reaches that pre-set level and then the thermostat will open to allow coolant to circulate.

In answer to your first question, yes, your Jeep will run without a thermostat. However, it will run better and more efficiently with a good (Mopar) thermostat. Make sure that the weep hole is at the top and use a new gasket. Many posters here have had issues with Fail Safe thermostats and don't recommend them.

I don't know of any benefit to not running a thermostat. What does "Having a lead" mean?

In answer to your second question, a HEI distributor is a great mod to make if it won't interfere with your Jeep passing emissions, assuming that your area has testing.
The issue with the cam gears can be avoided by buying your HEI from a company like Summit Racing. They get a little more for their HEIs than some of the internet sellers but they claim that their lower gear will work and mesh with the cam gears. If you go with the cheaper internet HEI, most posters recommend that you switch the lower gear that comes with the HEI with the gear from your original distributor.
I bought my HEI from Summit and the gears had different internal shaft diameters so I used the gear that came from summit and have had no problems.

Good Luck, L.M.

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Old 12-15-2016, 12:19 PM   #3
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I second Luckymac. The engines were designed to run at 195. Running too cool will also change the oil viscosity and can reduce operating efficiency and lead to oil starvation, albeit probably minor unless you live in a cold climate. The effect on how the ECM determines air/fuel ratio would be affected as well. Another thing that can happen is overheating at highway speeds. The coolant won't stay in the radiator long enough to dissipate enough heat. I've only seen this in high temperature climates (110+), but it can happen.
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Old 12-16-2016, 08:56 AM
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Sorry for the typo. It was I have a leak! thanks for the great information It does help.
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Old 12-16-2016, 09:15 AM
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Do you have a part number from Summit Racing?
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Old 12-16-2016, 10:24 AM   #6
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My rule of thumb is, if the thermostat is more than two years old, and the cooling system is drained, replace it. Some recommend MOPAR, but that's a $30 thermostat. I'm comfortable putting in the best thermostat at the parts store, since it's gravy work job, and I don't leave it in there for too long.
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Old 12-16-2016, 02:03 PM   #7
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Your t-stat is a flow regulator, and if it's not there, then you can actually run hot, and overheat. Coolant moving too quickly thru your radiator will not have enough time to lose heat. Big diferance in the size of the tstat housing, and what can flo thru an open tstat.
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Old 12-17-2016, 10:58 PM   #8
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Quote:
Originally Posted by Ov yj View Post
Your t-stat is a flow regulator, and if it's not there, then you can actually run hot, and overheat. Coolant moving too quickly thru your radiator will not have enough time to lose heat. Big diferance in the size of the tstat housing, and what can flo thru an open tstat.
THIS!!!

And, just to add, that the restriction provided by the thermostat helps to force the coolant all the way to the rear cylinders. Without the restriction, the flow just cools the front cylinders.

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