FYI there is a TLDR at the bottom if you don't want to read all of this.

Just wanted to post this out there because I am not a mechanic, and while I like to pretend to be one on the weekends in my garage, I am often ignorant on how some things should work and maybe this will save some people some time and trouble.

After completing my Hemi swap a few months ago, I never had any issues with overheating, but what I did have is some weird random bubbling-water-flowy type sounds in my heater core and I noticed that the heat wasn't as warm as it was with the 3.8 and the passenger side air vent wasn't nearly as warm as the center vents.

I hoped that it was just air in the core and needed to be purged. So I finally decided to have a go at it. I disconnected the heater hoses, ran water with the water hose both directions through the core to make sure it wasn't plugged up. It wasn't clogged and the water ran clean almost immediately.

According to Google, the method for purging the air out of your coolant loop is to take the radiator cap off, make sure that the radiator is full, start the engine, turn on the heater and and let it run until it gets to operating temperature (hot enough to open the thermostat ) and add water/coolant as needed and the air will eventually burp out. When I tried to do this, once the engine would get to the point that the thermostat would open up.. some air would burp out and then I would get a sudden surge of coolant and it would overflow and then suck down coolant. I would try and quickly fill the coolant quickly and repeat the process.. but it kept surging over and over and I feel like I wasn't really getting the air out of the system, because I wasn't fast enough pouring water back in the radiator.

Behold! The Lisle No Spill Funnel (aka the $40 funnel).



The funnel has a radiator cap attachment so you can connect it to the radiator. I tried the process again.. this time when the system burped air, it surged... almost to the top of the funnel and started sucking coolant back down again (very quickly). I thought this was a bit odd but went with it and kept enough water in the funnel that it wouldn't run out. I repeated the process a few times until I was satisfied that no more air was burping out. The surging of fluid kept happening over and over, but no more air bubbles. Cool.

One day later, at a stop light, had the A/C on, and I checked the temperature (Using OBD Adapter and phone app Torque) and it was running around 227 degrees. Now, I was told that the Hemi 5.7 (205 - 225 degrees) is intended to run warmer than the 3.8, but I kept noticing that it would spike real hot (up to 230 degree) and then cool down again and repeat this process over and over, especially at redlights. Guess what? I heard bubbling sound again behind the dash. Pretty frustrated at this point.

I went to Harbor Freight, purchased the Radiator pressure testing kit, connected it to my radiator and pumped it up to 15 psi. The radiator side held at 15 psi. No leaks. I then set up a connector to use the radiator pressure kit directly on the heater hose and plugged the hose outlet on the engine... pumped it up to 15 psi -- Still held! No leaks in the heater core!

At this point, I was pretty sure the Hemi gods were playing some sort of cruel trick on me. Then I started to wonder if my hoses were set up correct. I remember distinctly during the Hemi swap, having a conversation about whether or not it mattered the direction of flow to the heater core. I assumed that since there wasn't a valve in the heater hose circuit, like in some other cars and that the coolant is always flowing through heater core, that it didn't really matter. I was wrong.

I had the flow from the thermostat going to the top of the heater core... flowing through the heater core... out the bottom and then back to the water pump. If you are wondering about which connection is the top and bottom at the firewall (since they are at the same level), the connection on the left is to the bottom of the heater core and the connection on the right is to the top of the heater core.

I swapped the lines... refilled the coolant system with the lisle funnel.. started the Jeep.. thermostat opened up... kept the funnel filled with water, and burped all the air out. This time however, unlike before I swapped the hoses, there was no surging of coolant. Once the coolant burped all the air out, the level in the funnel always stayed the same even with the temp below & above thermostat-open temp (203 degrees- Stock Hemi Thermostat).

Since then.. no more random bubbling in heater core, no more temperature spikes, and now I have hot heater air flow again.

Theory: I assume that because of heat soak (heat rises) in the heater core and because of the wrong flow direction, that the much cooler water coming back from the core into the pump was possible keeping the temperature in the pump housing/thermostat area lower than the temperature of the coolant in the rest of the loop and would keep the thermostat closed until the operating temperature got REALLY hot (spike), it would open the thermostat up, and the coolant would run through the radiator like it should, dropping the temperature quickly drop back down again.


TLDR: Had bubbling sounds behind the dash at the heater core/sporadic temp spikes/coolant surges/poor heating at the air vents without any leaks in the system -- Had heater hoses swapped. After swapping hoses (flow in bottom of core and out the top) it fixed all of the issues.

 

Thanks for posting this. Just curious - did you have to modify the Ram heater hoses to reach the JK heater fittings?

 

I extended mine with a brass barb from AUtozone.

 

Thanks! Seems like all these little things will take the longest to sort out.