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Good morning all, happy Monday. So I recently acquired a 2012 4-door Rubicon for a silly low price and I knew I would be inheriting some issues because the price was low, but the cooling system issue I'm having has me completely stumped! I know the 3.6L Pentastar runs hot so you may tell me that what I'm experiencing is acceptable, but after reading dozens of posts on this forum I still believe that I have a real issue that needs solving.

Here's the problem: The Jeep heats up to normal operating temps (200-205) as it should and I'm able to drive it roughly 15 minutes before the problems start. The gauge stays straight up and down (have also confirmed steady temps with Torque app) idling, accelerating, climbing hills, whatever. Out of nowhere the gauge will start to climb and usually makes it almost to the 3/4 mark on the gauge (a few times it's hit the 3/4 mark). I've seen temps as high as 245-250 on the Torque app but I don't let it go higher. It takes the Jeep roughly 1 minute to go from normal to 3/4 on the gauge. Typically it will come back down on it's own after a few minutes which also only takes about 1 minute once it starts to fall. The driving I'm doing with it right now is just cruising close to home so I'm not pushing it, this is not climbing a mountain pass with a trailer. In fact, the driving has been pretty low RPM on flat roads. I live in Colorado at about 6000 feet and air temps outside have been in the 60s so I can't blame Phoenix-type air temps. I can also let the Jeep idle in the driveway for more than an hour with no excess heat issues. Aside from the random heating issue the Jeep runs great.

Specs on the Jeep: 2012 Rubicon, 2 inch lift, 37" tires, stock 4:10 gearing, factory bumper, no grill inserts, no winch, stock hood, front inner fenders removed.

Now for the fun part, here's the work I've done in pursuit of "fixing" this issue. All of the work listed below I've done in the last few weeks.
  • New radiator (standard two row)
  • New thermostat (two actually). I pulled out the new MOPAR stat that came with the Jeep, tried a new MOPAR stat and have also tried a Mahle stat.
  • New MOPAR water pump
  • New MOPAR coolant temp sensor
  • New radiator cap
  • I've bled the system countless times (easily 15+ times) so I'm pretty sure air is not the issue. I also use the funnel that attaches to the top of the radiator so I can leave it running as long as necessary to purge air (sometimes 45 minutes to an hour)
  • I've flushed the system twice with distilled water and only use OAT (orange) coolant.
  • I've bypassed the heater core to eliminate that from the equation (would rather not do that job)
  • I've run the vehicle with no thermostat to eliminate the highly unlikely situation where maybe I magically had three bad thermostats
  • I've pressure tested the cooling system and it holds pressure no problem for hours
  • I've done a compression test, all is good, less than 5% variance between cylinders
  • Tested oil and coolant, no signs of mixing
  • Tested the cooling system with the exhaust gas block tester (the test where the fluid changes color if exhaust is present in the system). Ran this test three times, no issues
  • Pulled inner fenders to allow for heat escape
  • I've confirmed that the fan is running as it should
  • I've confirmed that there's no muck in the A/C condenser or trans cooler that could restrict airflow
I am beyond frustrated with this thing and am ready to push it off a cliff, but before I do that I figured I'd reach out to the community to see if anyone else has experienced anything like this. I've read a ton of posts about Jeeps running hot but typically the person posting hasn't addressed the likely culprits like T-stat, mud, air in the system, etc so I haven't been able to search out any answers. I know I can take it to the dealer but I'm afraid I'll spend a ton of money to end up right back where I'm at now as I feel like I've done it all.

I welcome all feedback, even if it seems silly or unlikely because at this point I feel like I've tried or eliminated everything that's a possible cause of the problem.

Thanks for reading, fingers crossed on someone having some ideas.
 

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Have you tried replacing the temperature sensing unit?
 

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Discussion Starter #3
Have you tried replacing the temperature sensing unit?
Good morning, thanks for the reply. I have replaced the coolant temp sensor and actually plan to replace it again this week as I've reached the point where I'm replacing even the stuff I've already replaced in hopes that I got a bad replacement part.
 

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Good morning all, happy Monday. So I recently acquired a 2012 4-door Rubicon for a silly low price and I knew I would be inheriting some issues because the price was low, but the cooling system issue I'm having has me completely stumped! I know the 3.6L Pentastar runs hot so you may tell me that what I'm experiencing is acceptable, but after reading dozens of posts on this forum I still believe that I have a real issue that needs solving.

Here's the problem: The Jeep heats up to normal operating temps (200-205) as it should and I'm able to drive it roughly 15 minutes before the problems start. The gauge stays straight up and down (have also confirmed steady temps with Torque app) idling, accelerating, climbing hills, whatever. Out of nowhere the gauge will start to climb and usually makes it almost to the 3/4 mark on the gauge (a few times it's hit the 3/4 mark). I've seen temps as high as 245-250 on the Torque app but I don't let it go higher. It takes the Jeep roughly 1 minute to go from normal to 3/4 on the gauge. Typically it will come back down on it's own after a few minutes which also only takes about 1 minute once it starts to fall. The driving I'm doing with it right now is just cruising close to home so I'm not pushing it, this is not climbing a mountain pass with a trailer. In fact, the driving has been pretty low RPM on flat roads. I live in Colorado at about 6000 feet and air temps outside have been in the 60s so I can't blame Phoenix-type air temps. I can also let the Jeep idle in the driveway for more than an hour with no excess heat issues. Aside from the random heating issue the Jeep runs great.

Specs on the Jeep: 2012 Rubicon, 2 inch lift, 37" tires, stock 4:10 gearing, factory bumper, no grill inserts, no winch, stock hood, front inner fenders removed.

Now for the fun part, here's the work I've done in pursuit of "fixing" this issue. All of the work listed below I've done in the last few weeks.
  • New radiator (standard two row)
  • New thermostat (two actually). I pulled out the new MOPAR stat that came with the Jeep, tried a new MOPAR stat and have also tried a Mahle stat.
  • New MOPAR water pump
  • New MOPAR coolant temp sensor
  • New radiator cap
  • I've bled the system countless times (easily 15+ times) so I'm pretty sure air is not the issue. I also use the funnel that attaches to the top of the radiator so I can leave it running as long as necessary to purge air (sometimes 45 minutes to an hour)
  • I've flushed the system twice with distilled water and only use OAT (orange) coolant.
  • I've bypassed the heater core to eliminate that from the equation (would rather not do that job)
  • I've run the vehicle with no thermostat to eliminate the highly unlikely situation where maybe I magically had three bad thermostats
  • I've pressure tested the cooling system and it holds pressure no problem for hours
  • I've done a compression test, all is good, less than 5% variance between cylinders
  • Tested oil and coolant, no signs of mixing
  • Tested the cooling system with the exhaust gas block tester (the test where the fluid changes color if exhaust is present in the system). Ran this test three times, no issues
  • Pulled inner fenders to allow for heat escape
  • I've confirmed that the fan is running as it should
  • I've confirmed that there's no muck in the A/C condenser or trans cooler that could restrict airflow
I am beyond frustrated with this thing and am ready to push it off a cliff, but before I do that I figured I'd reach out to the community to see if anyone else has experienced anything like this. I've read a ton of posts about Jeeps running hot but typically the person posting hasn't addressed the likely culprits like T-stat, mud, air in the system, etc so I haven't been able to search out any answers. I know I can take it to the dealer but I'm afraid I'll spend a ton of money to end up right back where I'm at now as I feel like I've done it all.

I welcome all feedback, even if it seems silly or unlikely because at this point I feel like I've tried or eliminated everything that's a possible cause of the problem.

Thanks for reading, fingers crossed on someone having some ideas.
Question... Is it BOILING Over? If NOT .. The post on Temp Sensor is a valid component for replacement.
 

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Agreed with above to do the temp sensor check/replace. The factory service manual says to check it cold (engine below 120F) near the ECT sensor and it should be within 13F of what the sensor is stating; and hot, warm up the engine to operating temp, shut off, wait 10 min, measure again near ECT, should be within the same 13F. Edit: I would say based on how quickly it's changing, it's more than likely your sensor going bad.

If that wasn't it, I'd be curious if this is only happening at slower speeds when the engine is dependent on the cooling fan or if it's happening at highway speeds when the fan is less necessary for cooling. If it was only at slower speeds, I'd be looking at the electric fan. Its speed is controlled by the PCM via a pulse-width modulated (PWM) system rather than a simple on-off system and I've read instances where it has failed and is not going fast enough. I'd think at those higher temps you'd hear it pretty well.

After those two things, I'd be looking at the water pump. There seems to be some reports of failing water pumps on earlier 2012-2013 Pentastar engines, may google that some more. I also read at pentastars.com a reference to incorrect serpentine belt placement can cause the water pump to rotate backwards -- but I've never done a belt on my 3.6 so have no idea if that's plausible/possible.
 

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Discussion Starter #6
Question... Is it BOILING Over? If NOT .. The post on Temp Sensor is a valid component for replacement.
Hello, thanks for the thoughts. Fortunately it's not boiling over. I replaced the temp sensor two weeks ago but it wouldn't be the first time I've installed parts that were bad out of the box. I've ordered a second sensor and will be swapping those out later this week.
 

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Discussion Starter #7
Agreed with above to do the temp sensor check/replace. The factory service manual says to check it cold (engine below 120F) near the ECT sensor and it should be within 13F of what the sensor is stating; and hot, warm up the engine to operating temp, shut off, wait 10 min, measure again near ECT, should be within the same 13F. Edit: I would say based on how quickly it's changing, it's more than likely your sensor going bad.

If that wasn't it, I'd be curious if this is only happening at slower speeds when the engine is dependent on the cooling fan or if it's happening at highway speeds when the fan is less necessary for cooling. If it was only at slower speeds, I'd be looking at the electric fan. Its speed is controlled by the PCM via a pulse-width modulated (PWM) system rather than a simple on-off system and I've read instances where it has failed and is not going fast enough. I'd think at those higher temps you'd hear it pretty well.

After those two things, I'd be looking at the water pump. There seems to be some reports of failing water pumps on earlier 2012-2013 Pentastar engines, may google that some more. I also read at pentastars.com a reference to incorrect serpentine belt placement can cause the water pump to rotate backwards -- but I've never done a belt on my 3.6 so have no idea if that's plausible/possible.
Thank you for the spec on the ECT test cold vs. heated, I hadn't thought of that test, (guess my next purchase is a factory service manual). I agree with you guys that it just seems like something obvious like a temp sensor, that's what has been so frustrating with all of this, after replacing everything. I accept it when a complex computer-controlled system stumps me, but a cooling system is about as basic as it gets, sucks to be struggling with it. I'll test that ECT sensor and will likely swap it out again when the part arrives in a few days.

Thank you also for the thoughts on the fan and pump. The fan is cycling through the various speeds as it should and the water pump is a new MOPAR part (unfortunately the pump I took out was also in great shape but once apart I wasn't putting the old part back in). Funny on the serp belt, I'm curious how backward routing would even be possible, I may glance at that just out of curiosity. It is a funky set up too as the alternator is mounted with the pulley facing the back of the engine so the belt routing is interesting.
 

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Before you spend another dime on it, go purchase a Infra-red heat gun. They are not very expensive. Then shoot the thermostat housing. This way you can determine, if you are actually running that hot, or just getting a false reading. If it is running that hot, it still sounds like you have air in the system.
 

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Try plugging in a ScanGuage or similar and reading the temp during regular use. It’s possible the gauge and/or cluster is bad.
 

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Discussion Starter #11
Sounds like air in the system. I had the same problem and was only fixed by taking it to a shop that had one of these.
That's a good idea. I hadn't thought to have it vacuum pulled. I'm going to do some more testing with the ECT over the next few days but will likely to it in to someone with a vacuum setup if the ECT work doesn't help. I'll post in a few days on progress and if I ended up getting a vacuum done. Thanks
 

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Before you spend another dime on it, go purchase a Infra-red heat gun. They are not very expensive. Then shoot the thermostat housing. This way you can determine, if you are actually running that hot, or just getting a false reading. If it is running that hot, it still sounds like you have air in the system.
Funny you mentioned that, I just ordered an infrared gun yesterday. I'm at the point of assuming my sensors and gauges are crap. I'll get the temp gun in a few days and will post with findings. Thanks again for commenting.
 

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Try plugging in a ScanGuage or similar and reading the temp during regular use. It’s possible the gauge and/or cluster is bad.
I think you guys are all on the right track that this must be a sensor/gauge issue (or possibly still air in the system like TagBannedAgain suggested). I've been confirming temps with the Torque app but if the sensor is bad the data to the app is bad. I just ordered an infrared temp gun so I'll be using that as well to confirm temp readings. I'll update this post over the next few days as I follow the suggestions on here and do more testing. Thanks
 

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If you're using Torque with an bluetooth/wifi OBD adapter, you should be able to use Jscan or similar app with your OBD adapter to read ECT voltage. It's a 5V reference signal that goes lower as the engine temp increases. Although it's similar to seeing the actual temp, it could be useful to see if the signal is good and showing appropriate voltage from cold start through operating temp. It's also possible to record that data to catch when the temp goes up and see what the voltage is doing.
 

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If you're using Torque with an bluetooth/wifi OBD adapter, you should be able to use Jscan or similar app with your OBD adapter to read ECT voltage. It's a 5V reference signal that goes lower as the engine temp increases. Although it's similar to seeing the actual temp, it could be useful to see if the signal is good and showing appropriate voltage from cold start through operating temp. It's also possible to record that data to catch when the temp goes up and see what the voltage is doing.
That's a good point, if I can monitor ECT voltage while driving I should see a drop in voltage when the temps climb to the high levels (assuming things are actually getting hotter and I don't have a bad sensor). Or, like you said, if my voltages don't start at 5v and drop as expected with temp increase it's further confirm of a sensor issue. I'll make sure to update this post as I keep testing over the next day or two as I guarantee someone out there had (or is having) the same issue I'm having and will appreciate stumbling across all the good feedback you guys are providing.
 

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Yes that would be my next step, graphing the voltage to see how that reads and looking for odd spikes/drops/etc. and then comparing that to the actual temp that the temp gun reads next to the sensor. Also, in the FSM it's recommended to check the three values here to see how the sensors compare.

"Check and compare the values of the Ambient Temperature Sensor, Coolant Temperature Sensor and Fuel Temperature Sensor with the key on. The values should read within 5°C (41°F) of each other."
 

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Sounds like air in the system. I had the same problem and was only fixed by taking it to a shop that had one of these.
Interesting, I watched Bitchin Rides use that tool on a car last week. Very cool device. Now I know where to purchase. Thanks for the post.
 

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Before you spend another dime on it, go purchase a Infra-red heat gun. They are not very expensive. Then shoot the thermostat housing. This way you can determine, if you are actually running that hot, or just getting a false reading. If it is running that hot, it still sounds like you have air in the system.
Summit Racing and Harbor Freight BOTH show Infrared Temp guns for around 25 bucks.
 

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So I have an update....and it really sucks. I went ahead and replaced the coolant temp sensor, no change in the problem, then had a shop vacuum pull the coolant system to guarantee no air pockets, unfortunately no change there either. Feeling completely defeated I did the coolant block test one more time after the engine was nice and hot and the fluid changed from blue to yellow instantly, indicating exhaust in the cooling system...super bummed. Just for giggles I also bypassed the heater core again but this time I used clear tubing so i could watch the coolant flow. Above 2000 rpm the coolant is showing a bunch or small bubbles as well as several large air pockets in the tubing so i think it's safe to say that unfortunately I have a head gasket issue.

The worst part of all this is that I suspected a head gasket from the start because the price of the Jeep was so low, that's why I started with a compression test, cooling system pressure test and chemical block test, all came back fine. Apparently, my head gaskets are only leaking at high temps with the engine under load. I could have avoided weeks of work and hours spent replacing parts that were just fine if one of those tests had come back differently.

I also have a small leak at the rear main seal so I may just yank the engine and do head gaskets and other seals all at once.

Thank you all for the input on this issue, I really appreciate all the feedback.
 

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Hey, sorry to hear. Sounds like that's it. Just out of curiosity, how did you do the block test -- directly from the radiator I suppose? And did you ever see any other symptoms -- loss of coolant, lack of power, etc?
 
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