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Another Coolant Question

redsyphon

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So, I've been tracking down a coolant issue for quite some time now...

Long story short, I believe the reservoir cap was put on (probably by me) at one point not 100% level (i.e. cross threaded).

I've never gotten a temperature alarm, and I've never caught the Temps running hot (nothing above 224).

But, I did find the reservoir bottle low one morning (cold) and filled it to the min line with Mopar 50/50. I then drove it for a few days and noticed smoke from the hood at a stop light.

I pulled over immediately and popped the hood. The coolant was boiling over and dripping onto the exhaust. I took a look at the Temps and it was at 220. I let it cool down and drained some coolant from the tip of the bottle, then drove it home.
Jeep Wrangler JL Another Coolant Question VideoCapture_20230823-181240


It was sitting about 1inch above the max line cold, so I pumped it to the min line and buttoned everything up. All went well until a few days after with same issue (stop and go traffic again, same Temps 220).

Since then I've kept an eagle eye on it, and have only noticed it boiling once, but it wasn't high enough in the reservoir to boil over and start smoking (i.e. boiling inside the bottle).
Jeep Wrangler JL Another Coolant Question IMG_20230823_181310


Want some input as the engine isn't running hot (210-220 without a/c, 195-210 with a/c on), but I don't notice the cap being pressurized when taking it off the reservoir (engine off) and the coolant level keeps trying to boil over if I fill it to the min level. It'll eventually drain down slowly after initially rising even more post engine shutdown to about the mid-line.

When I'm running the engine and monitoring it (i.e. sitting in driveway), the coolant level stays just above the min/max line and never moves (cap on or off). So, I'm assuming it needs to be under load to act up? Even though Temps are about 215 when testing...
Jeep Wrangler JL Another Coolant Question IMG_20230823_181256


Any input welcome.

Note: I did have a local mechanic perform a pressure test, and it passed fine. I have not had the coolant system bled or anything though.
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redsyphon

redsyphon

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I should note, I do have a new cap inbound to make sure it's not as simple as a bad cap not creating a good seal...
 
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redsyphon

redsyphon

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James
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For anyone that comes across this, it was indeed a failed cap. More specifically, the bottom O-ring had a small chunk missing. I was not able to locate the missing chunk in the reservoir tank, so I've no idea where it went.

It's holding pressure now and hasn't gotten past 215 while driving.
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