2018 RAM 3.6 Running Hot

[Jose Pineda]

Good afternoon, I just got a 2018 Ram 3.6 V6 and am having issues with it running hot. Coolant temps will fluctuate on the highway between 212F-242F whether I have the A/C on or off. I get the high temps with the A/C on the coldest setting but as soon as turn the temperature dial one setting over to get coolant flow to the heater core temps drop sharply to 190F-203F. Here's what I've done to it:
- Checked coolant levels, radiator and expansion tank were full
- Pressure tested the system and was able to hold pressure
- Replaced thermostat and water pump (overheated due to stuck thermostat)
- Bled all the air in the system via the valve on the thermostat
- Flushed heater core (currently having issues with water sloshing sound inside, heater works)

This may sound like a situation where I still have air in the system but as soon as I turn on the heater, coolant temps drop down to normal range and stay there. Previously I had air in the system and even with the heater on temps would go up beyond 235F. That has not happened since the last time I bled the system. High temps only happen with A/C on coldest setting or with it off. Previous owner did say that he put some Bars Leak head gasket repair and drained all the coolant before refilling the system. I have also block tested the system and it did not show any combustion gasses. Any ideas what could be causing these spikes?

Thanks in advance

 

[grizzstang]

The Pentastar does run hot and those temps don't seem too crazy to me.

They are difficult to get all the air out of the system but it helps to put the front up higher (like on ramps or something).

It is also pretty normal to get warmer temps when running the AC as the condenser sits in front of the rad so when it is on your radiator sees warmer air pushed through it. When you switch to heat your condenser cools quickly and you will get additional cooling through your heater core.

I am curious why the previous owner added Bars Leak. I believe that stuff can clog up the system. You may want to run a good engine coolant cleaner through the system and put more new coolant in.

Have to checked your active grill shutters? They stay closed a lot more than you think and block air flow across your rad in the name of aerodynamics.

If you have them you can remove all the shutters but the top and bottom ones and increase the air flow across your rad substantially.

I noticed my truck runs cooler and the temp jumps around less with most of my shutters removed.

 

[Jose Pineda]

The Pentastar does run hot and those temps don't seem too crazy to me.

They are difficult to get all the air out of the system but it helps to put the front up higher (like on ramps or something).

It is also pretty normal to get warmer temps when running the AC as the condenser sits in front of the rad so when it is on your radiator sees warmer air pushed through it. When you switch to heat your condenser cools quickly and you will get additional cooling through your heater core.

I am curious why the previous owner added Bars Leak. I believe that stuff can clog up the system. You may want to run a good engine coolant cleaner through the system and put more new coolant in.

Have to checked your active grill shutters? They stay closed a lot more than you think and block air flow across your rad in the name of aerodynamics.

If you have them you can remove all the shutters but the top and bottom ones and increase the air flow across your rad substantially.

I noticed my truck runs cooler and the temp jumps around less with most of my shutters removed.

I've done numerous cycles of purging the air out on my driveway which is on an incline but I will give it a few more tries.

I am aware that the previous owner suspected a bad head gasket hence the Bars Leak, he was fully transparent about the issue. Upper radiator hose was swollen which was the obvious sign that it has overheated previously. I got it at a good enough price that if it didn't fix the issue I have no problem with replacing the engine or maybe even making a Hemi swap. I am trying to understand why it only runs hot with the A/C on the coldest temp or off, turning the dial one setting over from the coldest immediately cools it down. Every thread points to either air in the system or a coolant flow problem.

I have removed all the shutters except the top and bottom ones and it helped lower the temps significantly but am still running hot with the A/C.

 

[grizzstang]

It is easier to have the front end up very high so unless you have one hell of an incline on your driveway it would probably help to put the front on ramps to purge the air out.

While you are purging air out of the system you have your heat set all the way to the hot side, right?

The setting doesn't matter as long as the AC is not cycling. You want heat only.

 

[Lee Peterson]

Did you ever solve this? The radiator probably needs to be replaced if it had leak stop junk ran threw it. Are you using a factory thermostat heard about lots of after markets faulty even new.

 

[Jose Pineda]

Did you ever solve this? The radiator probably needs to be replaced if it had leak stop junk ran threw it. Are you using a factory thermostat heard about lots of after markets faulty even new.

I ended up flushing the radiator and it fixed it. 200 miles later the head gasket sealer failed and the truck started to consume coolant and eventually overheat from the lack of coolant after driving for 20 minutes. I gave Blue Devil a try and it worked perfectly, been running it for 3,000 miles and have not seen any loss of coolant. I will update if Blue Devil fails.