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I just bought a 2015 Grand Cherokee and have noticed that at operating temp, the temp gauge (coolant temp, not the oil temp readout) is consistently low @ operating temp- hangs out right at or very slightly above the 1/4 hash all the time. I’m not getting any codes for bad thermostat or anything, heat seems to work (although it’s been so hot out since I’ve bought it, hasn’t been tested in cold weather.) Does this seem normal, or should it be closer to the 1/2 hash like every other car I’ve owned? This is my first Jeep, so maybe I’m just being neurotic and that’s just how the gauge is calibrated. I’m considering replacing the thermostat although I’d prefer to live by the “if it ain’t broke, don’t fix it” rule if this low gauge reading is normal for a GC.
Any insight is appreciated!!

PS oil temp with normal highway driving fluctuates between about 165 and 181, very rarely getting into the 190’s.
 

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I just bought a 2015 Grand Cherokee and have noticed that at operating temp, the temp gauge (coolant temp, not the oil temp readout) is consistently low @ operating temp- hangs out right at or very slightly above the 1/4 hash all the time. I’m not getting any codes for bad thermostat or anything, heat seems to work (although it’s been so hot out since I’ve bought it, hasn’t been tested in cold weather.) Does this seem normal, or should it be closer to the 1/2 hash like every other car I’ve owned? This is my first Jeep, so maybe I’m just being neurotic and that’s just how the gauge is calibrated. I’m considering replacing the thermostat although I’d prefer to live by the “if it ain’t broke, don’t fix it” rule if this low gauge reading is normal for a GC.
Any insight is appreciated!!

PS oil temp with normal highway driving fluctuates between about 165 and 181, very rarely getting into the 190’s.
They say the hotter it gets the more wear to the components. If everything is working fine including the heater core...I wouldn't worry about it. If you were our here in the Vegas heat I would be envious lol
 

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No, often running cold wears more. The specs, clearances and dimensions for the engine are all designed for operating temp, if you're running colder then the parts have not expanded to their proper dimensions to produce the proper clearances and if anything a little bit more wear will happen.

Don't confuse that with overheating, overheating will cause far more damage than running cold, because you are exceeding temperatures the parts can withstand as well as all of above increasing wear.

Its best to run at Operating Temp, for the best fuel economy, least emissions, most power and the least wear. It varies from engine to engine, but if anything the studies and experiments have shown running a little hot produces more power and economy, not cooler. And rarely does the running cold to trick the ignition to use more advanced maps actual produce any results.

There are no sensors in the Thermostat to set a code. Not sure if there is even a code description that includes the thermostat. The "takes to long to reach operating temp" would certainly imply the thermostat, but in my experience if the engine reaches operating temp, its not the thermostat, its usually a leak in the cooling system and being low on coolant.

The temp gauge should be just a tiny bit below half-way mark (less than half the next graduation mark) for normal operating temp. The Radiator Fan is designed to come on Full Power when the engine hits 222°F to increase air flow through the radiator and cool the engine back down to normal op temp. So the temp gauge going from normal to one mark above half-way, when engine load is high or the vehicle is stopped or low speed, and come back down again in a cycle is also normal.

Thermostats are cheaper than temp gauges, and tools. So likely either your temp sensor or thermostat is bad or a previous owner had installed a low temp thermostat for what ever reason. I'd change the thermostat first. then the temp sensor. you could measure the temp yourself with a tool, but that work and cost for a tool when you can just get a $7 thermostat and unbolt two bolts to replace it? I'd just replace the thermostat with the correct one and see what happens.

Yes, true, if you replace the thermostat, you're going to have to top off on coolant so you have to buy the coolant.
 

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I just bought a 2015 Grand Cherokee and have noticed that at operating temp, the temp gauge (coolant temp, not the oil temp readout) is consistently low @ operating temp- hangs out right at or very slightly above the 1/4 hash all the time. I’m not getting any codes for bad thermostat or anything, heat seems to work (although it’s been so hot out since I’ve bought it, hasn’t been tested in cold weather.) Does this seem normal, or should it be closer to the 1/2 hash like every other car I’ve owned? This is my first Jeep, so maybe I’m just being neurotic and that’s just how the gauge is calibrated. I’m considering replacing the thermostat although I’d prefer to live by the “if it ain’t broke, don’t fix it” rule if this low gauge reading is normal for a GC.
Any insight is appreciated!!

PS oil temp with normal highway driving fluctuates between about 165 and 181, very rarely getting into the 190’s.
 

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What Mongo said.
You can also buy or borrow an IR temp tool and actually check the system temps at the rad hose connections.
 

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I have a 2013 and when my thermostat died my Jeep did throw a P0128 error. Changing out the thermostat isn't hard, but I'm like you, if it ain't broke don't fix it. Just as an FYI, I did think it was running cold before it threw the error. Eventually it did throw an error code and when I cleared it, it came back. Swapping out the thermostat definitely fixed the error. I made a how to video of how to change out the thermostat and I'm pretty sure I posted it on here somewhere. Let me know if you need it and I'll dig it up.
 

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Discussion Starter · #8 ·
I ended up changing out the thermostat. Running warmer than it was before now (3/8 hash on gauge, oil temp running right around 203.) Old thermostat was the correct one (95 stamped on it, in C that’s 203F which is correct), must have been something wrong with it and was opening too early or letting some coolant leak by or something.

thanks for the insight!
 

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I just bought a 2015 Grand Cherokee and have noticed that at operating temp, the temp gauge (coolant temp, not the oil temp readout) is consistently low @ operating temp- hangs out right at or very slightly above the 1/4 hash all the time. I’m not getting any codes for bad thermostat or anything, heat seems to work (although it’s been so hot out since I’ve bought it, hasn’t been tested in cold weather.) Does this seem normal, or should it be closer to the 1/2 hash like every other car I’ve owned? This is my first Jeep, so maybe I’m just being neurotic and that’s just how the gauge is calibrated. I’m considering replacing the thermostat although I’d prefer to live by the “if it ain’t broke, don’t fix it” rule if this low gauge reading is normal for a GC.
Any insight is appreciated!!

PS oil temp with normal highway driving fluctuates between about 165 and 181, very rarely getting into the 190’s.
I just checked my dash thermostat today while idling. 103 degrees outside here, mine was just above the 1/4 mark too. Keep in mind my GC was just bought three days ago and the dealer just certified it, so I doubt there's a legitimate issue. All other temps were where they are supposed to be. Are yours?
 

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Discussion Starter · #10 ·
I just checked my dash thermostat today while idling. 103 degrees outside here, mine was just above the 1/4 mark too. Keep in mind my GC was just bought three days ago and the dealer just certified it, so I doubt there's a legitimate issue. All other temps were where they are supposed to be. Are yours?
Before I changed the thermostat my oil temps were a little low (about 170-185 fully warmed up on the highway/driving around town, 90+ degrees outside.) After replacing the thermostat the oil temp is consistently around 203.
 

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Before I changed the thermostat my oil temps were a little low (about 170-185 fully warmed up on the highway/driving around town, 90+ degrees outside.) After replacing the thermostat the oil temp is consistently around 203.
Oh ok. My oil temp was exactly at 203 degrees too lol...while idling.
 

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I am a newbie here and hope that it is ok to post my problem in this thread... I think it is relevant.
Vehicle is a 2013 Jeep GC Laredo 5.7 Hemi
We have had plenty of the typical cooling issues with it:
The leaky water pump, bad thermostats etc - all been fixed...

This one has me stumped. CEL came on and the engine temperature refuses to show anything above about 138 degrees. It threw the following...

P0128 code.
  • Checked coolant for level and purity (replaced a year ago with water pump)
  • Checked thermostat using IR gauge; at about 160 the top hose starts to carry very hot coolant (Thermostat was also replaced a year ago with a Mopar OEM)
  • Replaced ECT sensor
  • Bled system and double checked bleed valve.

So the temperature gauge reads cold and the digital reading in the instrument cluster reads <100. It is always reading "cold" which is triggering the error. Today it also threw P0118.

- I checked the wiring to the ECT sensor - one wire has 5v DC, the other I am not sure how to check back to the computer (don't have a wiring diagram)

What else could it be?

Please help and THANK YOU !!!

Phil
 

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P0118 JEEP Code - Engine Coolant Temperature Sensor Circuit High
P0128 JEEP Code - Engine Temperature takes to long to reach Operating Temperature


For taking to long to warm up, the first thing everyone suspects id the thermostat, but in my experience if the engine is reaching full operating temp it isn't the thermostat, something is causing the engine to take longer than normal to warm up. Or detect its actual temperature.

Every time I've gotten P0128, I was low on coolant, it was also cold out as well. I solved the problem that caused the low coolant level it solved the P0128. Remember, you have to check the coolant level in the radiator by removing the pressure cap (ONLY WHEN THE ENGINE IS COLD, YOU CAN BE SCALDED REMOVING IT HOT). If you have a leak in the cooling system it is very common for the overflow jug not to reflect the level of coolant in the system.

The Temp sensor is in the Temp sensor circuit, if its bad it could cause the circuit to be high, that or a short. With the additional code, it makes sense a bad temp sensor might be behind it all. You could do a search on how to test it if you have a multimeter and thermometer. Or just risk it and buy a new one, while they ain't cheap they aren't that expensive either.
 

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P0118 JEEP Code - Engine Coolant Temperature Sensor Circuit High
P0128 JEEP Code - Engine Temperature takes to long to reach Operating Temperature


For taking to long to warm up, the first thing everyone suspects id the thermostat, but in my experience if the engine is reaching full operating temp it isn't the thermostat, something is causing the engine to take longer than normal to warm up. Or detect its actual temperature.

Every time I've gotten P0128, I was low on coolant, it was also cold out as well. I solved the problem that caused the low coolant level it solved the P0128. Remember, you have to check the coolant level in the radiator by removing the pressure cap (ONLY WHEN THE ENGINE IS COLD, YOU CAN BE SCALDED REMOVING IT HOT). If you have a leak in the cooling system it is very common for the overflow jug not to reflect the level of coolant in the system.

The Temp sensor is in the Temp sensor circuit, if its bad it could cause the circuit to be high, that or a short. With the additional code, it makes sense a bad temp sensor might be behind it all. You could do a search on how to test it if you have a multimeter and thermometer. Or just risk it and buy a new one, while they ain't cheap they aren't that expensive either.
Thanks for the quick reply Mongo...

what you said above about "...Or detect its actual temperature...." I think is relevant. It seems that the ECU is going by the data that is shown in the console. In fact reading that says to me that the engine is not reaching operating temperature EVER!

I did test the ECT sensor for changes in resistance as the temperature is changed (I used a bowl of ice-water and a bowl of almost boiling water) but that is redundant since I replaced the unit anyway.

I don't know what else, unless the wiring to the ECT is bad?

I will check all fluids once again, but I am pretty sure they are good.

Phil
 

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what you said above about "...Or detect its actual temperature...." I think is relevant. It seems that the ECU is going by the data that is shown in the console. In fact reading that says to me that the engine is not reaching operating temperature EVER!
If you think the engine isn't reaching full operating temp, then the Thermostat is the most likely cause. They are not that expensive, just get a new one and replace it.

I can see it totally possible the engine consistently running too cold triggering the code for ECT sensor.

The way the PCM uses the temp sensor is that it sends 5vdc to down one wire to the sensor, which changes resistance according to temperature, the resistance causes a voltage drop and the return wire will carry what ever the resulting voltage is to the PCM to tell it the temperature.

So you can measure if you have the full 5vdc on contact on the connector, and then turn the vehicle off and test the resistance at the other connector. I "think" you should see Open Line on the other contact, maybe ground with zero or almost zero ohms. What you should not see is a resistance of more than a couple ohms all the way up to anything less than Open Line, that would be a big indication of a short.

Again, check the coolant level in the radiator (only when the engine is cold), if you have a leak and are low on coolant, I've seen it cause codes for too long to warm up. Since the ECT is at the top of the cooling system, if it was too low coolant might not be touching the ECT, and that could result in unexpected returns and trip codes for the sensor.
 

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If you think the engine isn't reaching full operating temp, then the Thermostat is the most likely cause. They are not that expensive, just get a new one and replace it.

I can see it totally possible the engine consistently running too cold triggering the code for ECT sensor.

The way the PCM uses the temp sensor is that it sends 5vdc to down one wire to the sensor, which changes resistance according to temperature, the resistance causes a voltage drop and the return wire will carry what ever the resulting voltage is to the PCM to tell it the temperature.

So you can measure if you have the full 5vdc on contact on the connector, and then turn the vehicle off and test the resistance at the other connector. I "think" you should see Open Line on the other contact, maybe ground with zero or almost zero ohms. What you should not see is a resistance of more than a couple ohms all the way up to anything less than Open Line, that would be a big indication of a short.

Again, check the coolant level in the radiator (only when the engine is cold), if you have a leak and are low on coolant, I've seen it cause codes for too long to warm up. Since the ECT is at the top of the cooling system, if it was too low coolant might not be touching the ECT, and that could result in unexpected returns and trip codes for the sensor.
Brilliant! Thank you so much. I will do all this in the a.m. once the beast has had a chance to cool down :)
Thank you for your insight and help.

Phil
 

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Update...

So after a little research... I realize that of the two wires going to the ECT, one "signal wire" carries the voltage and the other is supposed to go to ground. The PCM gets its signal voltage from the voltage wire, with reference to the reference voltage as part of a voltage divider circuit.

So here is what we have:
ECT sensor - good
Thermostat - good
Coolant level and bleed - good
Wiring - good
No shorts
PCM / ECU - good (otherwise we would not detect any voltage whatsoever)

So the final problem is that both the temperature gauge and the digital temperature readout in the cluster both indicate a temperature of less than 100 F when the engine is running, while registering an actual measured temperature of about 190 using an external thermometer!

THEREFORE...

I think the only thing left is that the vehicle's computer needs a reprogram / reset.

Does anyone concur? and if so does anyone know what is involved? I am thinking this may require to be a visit to the Jeep service, who here in Las Vegas are notoriously not a great experience.

I hope that this thread proves useful to anyone else and thank you for your time.

Cheers

Phil
 

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Which of the 2 dozen computers in the WK2 do think is bad?

You say the engine is not reaching operating temp, but then say you know the ECT and Thermostat is good?
What do you see on the actual gauge on the dash and how do you know the ECT and Thermostat is good?
 

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Which of the 2 dozen computers in the WK2 do think is bad?

You say the engine is not reaching operating temp, but then say you know the ECT and Thermostat is good?
What do you see on the actual gauge on the dash and how do you know the ECT and Thermostat is good?
The computer that sometimes needs to be re-programmed, and that reads the Vdiff between the signal voltage from the ECT and the Vref. I am not interested in the other 23.

Please read my previous posts about the steps taken to ensure that the thmst and ECT are good... but here you go just for convenience...

(Mopar brand) Thermostat - measured temp of hoses while engine warmed up and saw the top hose rise as housing reached operating temp. Also took it out and heated to 203F - watched it open.

ECT - measured resistance off engine and checked resistances with various temperature bowls of water. Compared data to Jeeps resistance table.

When the engine actual temperature, measured with an IR thermometer reads 210f, the guage says cold (all the way to the left) and the digital readout in the "Engine info" reads <100 (that sign means "less than" 100)

Furthermore... Voltage at signal wire reads 4.98V, ground wire 6.2 ohms to ground, putting a 470 ohm resistor across the connector terminal and measuring the PD measures about 4.46 volts - which should indicate a temperature of about 225F, Gauges say nothing.

Figure that one out!
 

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I haven't read everything in the thread but my 2015 has always read around the 1/4 mark or the next tick up. The only time I see it go higher is when towing in 90* ambient air.

Typically there is a dead zone built into the temp gauge to eliminate concerns of the vehicle owner. My LJ reads nearly dead center from 180 to 230.

I would snag a code reader than can show real time stats and see what the actual temp the sensor is reporting.
 
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