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A little back story, this is a 2015 Jeep Wrangler. Since 2017 I've kept an eye on coolant levels because sometimes I can smell burning coolant when I'm in front of the vehicle (not in the vehicle). However, prior to last week I've never added any. I never needed to.

Last week I got a check engine light, with no drivability symptoms. I pulled the code and I got "ECT low". After a bit of research, I learned that my Jeep has a 203F degree thermostat yet it was running at 181F (steady-state). I verified this reading with a scan tool while driving because my first thought was that if it was low on coolant, air pockets could make the sensor read erratically, but the output in the vehicle information may just be an average. I also checked the coolant, I maybe added a pint. Considering this was the first time I added coolant in 5 years, I don't think it's low. When the engine is cold it's right at the max fill mark, when hot, the overflow bottle is just about empty.

I thought about replacing the thermostat housing, but I cannot surmise a reasonable explanation for how a mechanical thermostat could be the cause of a lowered operating temp. Usually, they are stuck open or closed.

My other thought is the battery. I bought a Optima red-top in 2017, but the date on it was 2016, so it's over 4 years old. I was debating if an aging battery could contribute to a bad reference signal.

Any other thoughts, or troubleshooting suggestions? I'm running out of ideas.

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  • Buy a thermometer (for example) capable of measuring the given temperature range and wire it up as a secondary check of the coolant temp being read. It's a lot easier than pulling the thermostat out only to find that the stat is fine and the gauge is wrong. I concur with paulster2, that your mechanical stat may be sticking open (the design aim is to fail open rather than closed; less disastrous for the engine that way) but first, verify the actual coolant temp
    – Caius Jard
    Nov 1, 2020 at 5:16

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I believe your thinking on how a thermostat works is misguided. It can be fully open or fully closed, or anywhere in between. When the engine is cold, most thermostats are completely closed and stay that way until the engine coolant gets to operating temperature, then open the needed amount to allow coolant to flow until it gets back down to the operating range. Unless the engine is under a very heavy constant load, the thermostat has no need to be completely open, because the cooling system should have more than enough capacity to keep it in the operating range without issue.

I'm going to bet your thermostat is stuck partially open or isn't closing all the way when the engine coolant is at operating temperature. This would allow the coolant to flow and the engine to operate at a cooler temperature. It wouldn't have to be open much for this to happen. A constant flow of coolant past the thermostat could cause this. The thermostat itself is most likely a fairly inexpensive part. They can wear out over time. If you've not touched your coolant in five years, it's probably time for that to be changed as well. Not that changing your coolant has any benefit to your underheating problem, but it is good routine maintenance.

As for the battery, it would have absolutely nothing to do with your issue.

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  • I might as well try replacing the thermostat. Cheaper than the battery. As for the coolant, the maintenance schedule calls out 100k miles
    – mreff555
    Oct 31, 2020 at 16:35
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    @mreff555 - There should also be a time frame as well as mileage on the maintenance. Even so, it won't hurt to change it early. Coolant, along with brake fluid, are two areas which usually suffer late changes.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Oct 31, 2020 at 16:50
  • Yeah it might have been about 5 years. Defiantly due. I’ll do a flush when I change it.
    – mreff555
    Oct 31, 2020 at 19:51
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I flushed the coolant and inspected the housing. It Didn’t appear to have any defects. Regardless I replaced it but I also replaced the ECT as the consistent low temperature seemed more like a sensor/reference issue. One way or another, the problem was fixed. Temperature is right at 203.

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