idahofreq

So, after fighting this issue for six months and finally figuring it out, I felt the need to submit this write-up. The problem? Coolant was not being drawn from the reservoir after shutting off the motor and letting it cool down.

Background
I browsed through countless threads looking for the same issue. While there were many, none of them described this exact problem - they were always somehow slightly different. So, for clarity, these were my symptoms:

1. No leaks.
2. Coolant overflowed out of the reservoir - it would go into the reservoir, but it wasn't being drawn out.
3. Air kept coming into the system from somewhere, and needed to be bled out weekly.
4. None of the online suggestions (radiator cap, possible head gasket leak, reservoir tank and hose, etc.) actually worked or applied. The problem persisted.

Trial and error: here's what I replaced, with no results:
- Oil cooler (upgraded to aluminum)
- Oil cooler coolant hose
- Thermostat (three of them, from O'Reilly's)
- Water pump
- Radiator (it came with a new cap as well)
- Upper and Lower Radiator hoses
- Hose clamps for heater core hoses

The discovery? Air was slowly being drawn into the system through the bleeder valve under the throttle body. Coolant residue was present, not only because it's a bleeder valve and coolant is bled out when burping the system, but also because there was a very slow and slight leak! The leak was so small, that it never ran down the thermostat housing or otherwise made itself obvious. Maybe I was tightening it too tight - I don't know. How did I figure this out? After coating it with RTV gasket, coolant began drawing back into the radiator from the reservoir. I added it when the motor was hot, so that any vacuum would suck the freshly applied gasket in, as the motor cooled.

Please, for the love of everything holy, if you have this issue as well, try the RTV gasket first! In the meantime, I'll be shopping for an upgraded thermostat housing design.

resharp001

Gonna ask for a little clarification here. The bleeder valve on the 3.6L is right on the thermostat housing. The thermostat and the housing are 1 unit. You tried 3 different thermostats, from o'reilly. Were those not coming with the thermostat and a new housing as 1 piece? I have to imagine this was not an issue with 3 different thermostat housings. I'd always suggest sticking with factory when it comes to sensors and thermostats, but that is just me.

idahofreq

Yep! This was indeed for the 3.6L and it was all one piece. I read multiple threads of JK owners getting defective one-piece thermostat/thermostat housings, so that had to be ruled out. Those were all because the plastic would crack. It didn't occur to me that it might have something to do with the bleeder valve, though.

resharp001

So you are saying that each of the new thermostats/housings you tried had the same problem of letting air in the system through the bleeder valve?

idahofreq

That is correct. My thinking is, I was simply tightening it too tight. There doesn't seem to be any torque specs on it, anywhere that I could find. All in all, it's a bad design for that O-ring, I think. Take a look at this one. See how that O-ring really sits inside? On the OEM/O'Reilly's version, it is just sandwiched. So, over-tightening it could have very well caused this to happen. Coating it with RTV and fixing the problem supports the theory.