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I have a very slow coolant leak that has been driving me crazy for some time now, starting in 2019. After several visits to the mechanic, no one seems to be able to solve this mystery, so I'm hoping someone here has had a similar experience and can help out.

My vehicle
2008 Malibu LTZ with LY7 3.6L V6

Cooling System Repairs to Date:
Thermostat - Replaced 2018
-Several months after this repair, I noticed the reservoir was filled considerably above the max limit
-I started noticing the leak in 2019
Water pump - Replaced 2020
Reservoir cap - Replaced 2020

With the system pressurized and running I see no leaks around the water pump, thermostat, or any of the visible hoses from the engine bay. There are, however, obvious wet spots on the control arm of the passenger side of the vehicle.

There is typically a small wet spot on the garage floor, not a puddle, at the same location several hours after the engine has turned off, but not before several hours have passed.

If I fill the reservoir to the maximum cold level, I can typically use an entire tank of gas before the reservoir is just about empty. If I leave the car sitting for several days, there is no change in the coolant level in the reservoir tank.

I'm to the point of considering using some sort of stop leak product as a hail mary attempt at fixing the issue. Please help if you can!
 

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2011 Malibu LTZ 3.6L V6 Red Jewel Tintcoat
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Is that wet spot slick or have a particular smell to it?

Do you ever smell a sweet kind of odor from the engine bay or the exhaust, or maybe while driving down the road?

Use some leak-detection dye if you can find some. Make sure to read the package. Not all dyes are made for the cooling system and could cause issues if it's not made for the type of coolant you have.

You'll need a black light flashlight, which you can find all over the internet. It'll cause the dye to fluoresce.

If you don't use dye, look around the water pump, particularly behind the pulley, which is where the weep hole is. If there's wet coolant or a slick feel of build-up then you should probably replace the pump.

When I had to replace the t-stat in my 2011 3.6L in 2019, I ended up choosing to replace the water pump and do a complete cooling system flush. It just so happened to be only days before making a 1500-mile round trip in hot weather, so I filled it with water only. In case there were any new leaks, I wouldn't lose any expensive coolant. I learned that lesson the hard way 2 times in my earlier years, so now I practice it and recommend it to others. About a month later I drained the water and added the proper amount of coolant.

If you find no leaks externally, then you should check the coolant for exhaust components and the oil for coolant.
 
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