S4 is overheating only when driving for an extended period of about 15 mins. The stock gauge creeps up to 1/5, then 1/4, and would go higher if I continued to drive it. I saw it go to 1/3 and remember it going to near 1/2.

Won't overheat at idle, and some steam is barely escaping from the exhaust.

Heater works fine until it begins to overheat, at that moment it only blows out outside air.

Bubbles can be seen coming from the filler neck and no amount of bleeding (unless I'm doing it wrong) can solve this.

I replaced the thermostat with a Fail-safe Stant (I know that others recommend OEM but I've read alternative threads where others have installed an aftermarket one with no problems. Also, I work in a parts store and have yet to see an issue with Stant.)

I previously had a leak at the o-ring/gasket at the LIM which I replaced. (I'm not so sure I did it properly at the moment; however this is no leak gurgling up when I pressure test it). I initially attributed my overheating to this problem; however this isn't the case anymore.

Coolant Pressure test comes back with almost a 1 psi lost every 5 min.

Now here is the funny part; I did a combustion leak block test [Napa PN # B.K. 700-1006] which came back negative for any combustion gases present in the coolant system. (Although, once again, I am not sure I did it properly).

Any ideas rx7club?

 

Sounds like you have a coolant seal failure and are losing coolant into the combustion chamber. Bubbles in the coolant are a pretty sure sigh of that.

 

you need to pressure test the cooling system,as the Coolant "seems to be" entering your Combustion cycle...Not the Other way around(gases entering your Cooling system).
If the pressure drops off when you do the test,it will tell you one of Two things,
If your cooling system is leaking,or if Coolant is entering the internals.

 

Coolant leaking into the combustion chamber would make sense except there are a steady stream of bubbles that are visible at the filler neck. If there was a bad seal, the bubbles would be caused by combustion chamber gases right? And if that was the case, then my combustion leak block tester would have come back positive.

I have pressure tested the system that only has a 1 psi loss per 5 min with no visible external leaks. If it was leaking into the combustion chamber, from where would leaking into? I have heard of incidents where it traveled down the LIM from small leaks there and caused false signs of a bad seal.