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Engine temp drops at highway speeds

MeanGreen86

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I flushed the system over summer when I installed a new fan pulley and have had zero engine temperature issues until this past Friday. I was on the highway to meet my dad for dinner when I noticed the engine temp drop to around 130 from the normal 200-210. It almost seemed to be about the time it initially reached normal engine temp. Could the thermostat be acting up? Once I got off the highway the temperature when back to normal operating range for the rest of the drive to the restaurant (+/- 1 mile). It repeated the same behavior on the way home. I'm stumped on this one
 

freedom_in_4low

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is it a failsafe thermostat? Their design principle is that if it gets too hot (and therefore too far open), it locks open and won't close. But they're known for nuisance-locking, when they shouldn't. Could be what's going on here.
 
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Steel City 06

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I would say the most likely culprit is the thermostat. Is your heater as powerful as normal when this happens? If not, the coolant temperature probably is dropping because the thermostat is stuck open.

There could be other causes as well. It could be a faulty temperature sensor, though you would probably notice that the heater is just as powerful as normal.

It could also be an air bubble in the system, or even low coolant. Peek inside the radiator to see if it appears filled almost completely. Even if there is fluid in the bottle, if there is a leak, an air bubble can develop in the cooling system.

It could even be a bad water pump. If there isn’t enough circulation, you can develop hot and cold spots. This is probably the least likely of the four scenarios I listed (chances are you would have other issues too), but is possible.
 

pagrey

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It almost seemed to be about the time it initially reached normal engine temp.
If you have air in the system it'll do this as the thermostat opens. A sudden drop right at operating temp is normally a sign of this. Replacing the thermostat is a great idea. How is your radiator cap? If it has a broken center spring it wont pull coolant from the reservoir, wont hold pressure and you wont get rid of that air easily.
 

Jerry Bransford

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My experience with air in the cooling system is the temperature gauge shows HOT when the superhot air in the bubble hits the coolant temperature sensor adjacent to the thermostat.
 
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MeanGreen86

MeanGreen86

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Thermostat is likely factory and i have 105,XXX miles on the LJ so its not a bad idea. I don't think its air because I haven't touched the coolant system since June/July. I don't think its a failsafe thermostat either since I haven't seen a cooling issue like this since I've owned the Jeep.
 

macleanflood

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A torch and a infrared thermometer are a great way of testing thermostats...if you want to know you replaced the right thing...

-Mac
 
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MeanGreen86

MeanGreen86

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I'm seeing mixed comments about people adding (drilling) an air bleed hole into the thermostat....
 

Jerry Bransford

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I'm seeing mixed comments about people adding (drilling) an air bleed hole into the thermostat....
It's a good idea to drill a small bleed hole into the flange, away from its edge, if there is no bleed hole with a jiggler valve like my Stant Superstat has. It helps get trapped air out of the cooling system faster and more easily. Just make sure the bleed hole gets positioned at the very top when you install it.

Thermostat drilled hole.jpg
Thermostat Factory Hole.jpg
 
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MeanGreen86

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I wanted to update this incase anyone else has a similar issue. I replaced the thermostat and the temp sensor at the same time and the problem is gone. definitely one of the weirder engine issues i've seen.
 
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Jerry Bransford

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I agree but for it to only happen at highway speeds seemed strange to me. Driving around town the temp sensor stayed around 210 but when I hit the highway it would read 130-150
More air flowing through the radiator cooling it more when at highway speeds.
 

williambmac

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I agree but for it to only happen at highway speeds seemed strange to me. Driving around town the temp sensor stayed around 210 but when I hit the highway it would read 130-150
Thats because when you are driving around town, you are not forcing a huge amount of air thru the radiator.
Once you are on the highway the air forced thru the radiator will cause the coolant temp to drop dramatically.
 

sab

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Understanding a cooling system is key to effective diagnosis of system issues. The purpose of the thermostat is to allow the engine operating temperature to be more controlled. With today's emissions requirements, coupled with tight tolerances that must be maintained to avoid friction-induced damage to key engine components, the temperature range for an engine is pretty narrow.

The thermostat is a device that alters the flow rate of coolant through the radiator, depending upon the temperature of the coolant. It opens at a certain temperature (195°F for the stock TJ thermostat). This means that if your Jeep has cooled down to ambient temperature, the thermostat is initially closed. When it is closed, the coolant does not flow through the radiator so that the engine can get up to operating temperature as quickly as possible.

Once the engine gets up to the thermostat opening temperature, it opens and allows coolant to start flowing through the radiator. If ambient temperature is low (cold winter day), the thermostat may immediately start to close because the radiator is very efficient in the winter. This opening and closing may happen quite frequently in cold weather, but if it's summer, the thermostat will open initially, and stay open until you shut the Jeep off and it cools down.

Now, if the thermostat sticks open, and it's a slightly cold day, at low vehicle speeds, the radiator may not cool enough to drop the temperature. It depends upon the ambient temperature and the cooling efficiency of your system. However, at higher speeds, the efficiency increases, and now the radiator cools too well, dropping the temperature of the coolant. Since the thermostat is stuck open, the coolant temperature stays low until you slow down, reducing the radiator's ability to cool and bringing the coolant temperature back up.

In really cold weather, a thermostat stuck open will actually prevent the engine from ever getting up to operating temperature. And in extremely cold temperatures (-50°F, which I've experienced many times), at highway speeds, just the air flow through the grill, even with a properly-functioning thermostat, can prevent the engine from getting up to operating temperature. That is when you have to partially or fully block the grill off, and what those "flaps" or "curtains" you sometimes see on semi-trucks in the north country do.
 

NashvilleTJ

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Understanding a cooling system is key to effective diagnosis of system issues. The purpose of the thermostat is to allow the engine operating temperature to be more controlled. With today's emissions requirements, coupled with tight tolerances that must be maintained to avoid friction-induced damage to key engine components, the temperature range for an engine is pretty narrow.

The thermostat is a device that alters the flow rate of coolant through the radiator, depending upon the temperature of the coolant. It opens at a certain temperature (195°F for the stock TJ thermostat). This means that if your Jeep has cooled down to ambient temperature, the thermostat is initially closed. When it is closed, the coolant does not flow through the radiator so that the engine can get up to operating temperature as quickly as possible.

Once the engine gets up to the thermostat opening temperature, it opens and allows coolant to start flowing through the radiator. If ambient temperature is low (cold winter day), the thermostat may immediately start to close because the radiator is very efficient in the winter. This opening and closing may happen quite frequently in cold weather, but if it's summer, the thermostat will open initially, and stay open until you shut the Jeep off and it cools down.

Now, if the thermostat sticks open, and it's a slightly cold day, at low vehicle speeds, the radiator may not cool enough to drop the temperature. It depends upon the ambient temperature and the cooling efficiency of your system. However, at higher speeds, the efficiency increases, and now the radiator cools too well, dropping the temperature of the coolant. Since the thermostat is stuck open, the coolant temperature stays low until you slow down, reducing the radiator's ability to cool and bringing the coolant temperature back up.

In really cold weather, a thermostat stuck open will actually prevent the engine from ever getting up to operating temperature. And in extremely cold temperatures (-50°F, which I've experienced many times), at highway speeds, just the air flow through the grill, even with a properly-functioning thermostat, can prevent the engine from getting up to operating temperature. That is when you have to partially or fully block the grill off, and what those "flaps" or "curtains" you sometimes see on semi-trucks in the north country do.
Well, you got it mostly correct. One miss is that if the thermostat is opening and staying open in the summer, you have a capacity problem with the cooling system. If the thermostat is sticking open, the engine should run below normal operating temp - winter or summer. In normal operation, regardless of the ambient temp, or whether or not you are moving, the proper operation of the thermostat is to cycle open and closed to maintain a minimum operating temp.
 
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