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In short: Any ideas why a coolant leak would be sporadic and suddenly not be found during a pressure test?

2020 Jeep Cherokee / 3.2L V6.

Had what was almost certainly a coolant leak on Saturday. Usually just in front of the front passenger tire or at least on the right side of the front bumper. Not huge, reservoir level seemed unaffected, but first noticed it as a very obvious little puddle after parking the car. Inspected the drip, it's a bright pink color, a bit slick but not oily.

Opened hood, and got that sickly sweet smell of burning coolant. Hard to see without taking things apart, but could maybe see a wet spot under the corner of the radiator that I could imagine was dripping down inside the bumper towards the wheel. Assume it's from radiator or maybe connection to hose near it but not sure.

Shops were busy and/or closed for weekend, and nearest appointment I could find was for Tuesday. Since then, was driving it real gingerly. Short trips only. Nothing where I would be long distance or in traffic. After parking, the drips are super variable. Sometimes nothing. Sometimes a few drops that doesn't accumulate before subsiding.

Take it to the shop this morning. They did a pressure test but apparently couldn't find any leak.

I think it's related to engine heating up, so they're gonna let it idle for a half-hour then test it again. But I'm worried they might find nothing still.

I have a road-trip this weekend so really need the peace of mind of having this fixed.

Curious if there are any experiences or other ideas that might help the shop figure this out.

Thanks.

Edit (+1 day): They couldn't find anything. I drove it around a bit that evening doing errands and nothing dripped. Then this morning a very small drip after one short drive. Looking through gaps in the engine bay, I can see wetness on inside of bumper and how it would've trailed towards the wheel well. Seems to point towards bottom/starboard corner of radiator, but can't trace it much further due to stuff in the way. And front of radiator doesn't show anything. Drove it around a bit more, hoping to coax it out then take it back to mechanic, but nothing.

Underside wetness around rim of front bumper (can see skid plate and CV joint for front-right wheel to orientate yourself)

Color of liquid

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Sometimes when mechanics do a pressure test they forget to test the radiator cap. These oftentimes fail and cause the exact symptoms you are experiencing. If the shop cannot find the problem just replace the cap with a new one and make sure there is no air in the system and you should be good to go, assuming that the pressure test was done correctly.

I have also seen coolant hose leaks that would pass a pressure test. When I pressurized the system it would hold pressure until I wiggled the hoses and I'd find the leak in the hose, usually in the area around the clamps. You could try going to an auto parts store and purchase a water based florescent dye and a black light. Clean your engine around where the leak might be. When engine is cold add the dye to the coolant. Drive as you normally would. After a couple days use the black light and look for any glowing fluids. Do this frequently until you find the leak.

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  • Wouldn't hurt to replace anyway. But location is on left side (compared to drip on car's right) and lack of any wetness around the cap or nearly parts when it occurs. Suppose it could've traveled weirdly and evaporated quickly, but can't currently find evidence for it. Jul 26, 2023 at 17:39
  • If it helps, the design is unusual in this car. There is no cap on the radiator itself, only the cap on the reservoir. But it does appear to be under pressure and has a PSI rating, so it seems to function as the radiator cap as well. Jul 26, 2023 at 18:21
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    I've a edited my answer to add more possibilities.
    – Jupiter
    Jul 27, 2023 at 9:19
  • Thanks, the dye idea might be helpful for sure. At the least then the mechanic could trace it better too when my car decides to "boy who cried wolf" at the most inconvenient time. I have an inkling it's coming from the back corner of the radiator, which could be the connection to the hose there, so I think this is a likely culprit. Jul 27, 2023 at 17:26

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