Coolant leak

3.6L 2015 Jeep Wrangler Sport. Aftermarket Cold Case radiatior (because 2 factory radiators leaked at the plastic seams).
145k miles
Won't leak usually, until I drive for a bit, get engine temperatures up, and build pressure. I will stop for gas, or groceries for 5 minutes. Start it again, then it starts leaking somewhere, but drips out of the passenger side low.
It is not the supply hose, that is bone dry for leaks. And I dont think heater core, because nothing has come through the cab on passenger side ground. Has happened twice now, in 2.5 weeks

Where could it be leaking?

Anybodyhome said:

Where on the passenger side? Toward the front end, up near the radiator? Or, further back, closer to the firewall?

Click to expand...

Near radiator, I believe. Somewhere in between the engine fan and the back of the radiator. I don't see any cracks, from the angles i can get at. And it has only happened 2 times in 2.5 weeks. First time, was just a dribble. This recent second time, it spewed about half a cup of coolant. Then it stopped. And it didn't leak on the drive this morning. I can't figure it out.
Only happens when I have driven it. Turned it off. Then turn it on for a short drive after. Pressure and temperature?

Isn't radiator cap.

The cap and the hoses and clamps, are bone dry.

I am working with a weird theory that the reservoir bottle, on the opposite side of the radiator mind you, is leaking and trickling down to here.

Because the radiator was still "full" even after it leaked that much

Anybodyhome said:

There's a chance it was over-filled? There are 2 separate hot and cold reservoir levels, have you checked both of them before driving (cold) and after driving (hot)?

Click to expand...

Filled the reservoir to max, and not a shade over. Wouldn't have mattered, because when it was leaking it was in between mid and max. Which is crushing my theory that it is this reservoir that is the culprit. But it still could be.

I just don't know why it would leak from reservoir, to the passenger side.

Better wording:
I filled the reservoir to the MAX LINE. Very aware that there is an excess drain hole at the very top. I was stating this to remove that as a possible location where the leak could be coming from.
Auxiliary to the reservoir:
I twisted off the cap, which is good and has no signs of it being the part that is leaking, in order to check if the radiator needed filling after it pissed out the significant amount, as see in the picture.
The kicker there is, it was full. With the cap off still, I squeezed the intake and outlet tubes, hoping to move a bubble that could have formed from the loss. Nothing. No air came out.
Replaced the cap.
No leaks this morning. But leaked a little yesterday. Last weekend, I added 300 miles at 85mph, no leaks.

It only happens when I have driven it 5-7 minutes on highway (75mph), stop at the grocery store for 5 minutes. start it back up again.

As if the already pressurized system, can't take more pressure from this next startup.

I do not see cracks in the solid aluminum radiator. And even if there were, it would be leaking constantly, not sporadically like it has been doing.

Unless, the 230 deg F operating temperature is somehow "welding" the crack or hole together. And it is only rare circumstances that expansion or contraction open it back up???