2006 Mustang V6 Coolant Leak, no overheat

I drive a 2006 Ford Mustang V6, about 96,700 miles and from time to time it will smell of burning coolant. When I power off the car, it ll begin to leak coolant. However I notice that the temperature gauge does not go anywhere near the H or Hot symbol. It's not a regular thing, seems like it only happens once a month or so. Is this a faulty thermostat? Broken gauge? Asked my local mechanic but he told me it might be the radiator cap that was missing a gasket. Changed that out but it happened again. Need help and advice before going to the big guys!

Your system is pressurized so that it will run at a higher temperature without the coolant turning to a vapor. So a leak does not make your car run hotter until it starts to run low on coolant. The leaky cap was a wrong call. You would also have seen fluid/steam coming out of the cap when the engine was hot or at least some staining around it.
Your coolant also cools your transmission fluid and it is more difficult to see leaks in that system because it is under the car instead of regular system leaks that are on the top portion of the engine.
Anyways, you can find low end pressure testers for the coolant system on-line for about $40.

Change mechanics.

"When I power off the car, it ll begin to leak coolant". Where is it leaking? Fix the leak. If you do not know where it is leaking, get the system pressure tested.

Pressure test the system and the cap. Visually inspect. Then repair.

Sometimes what you describe is a bad cap and it sounds like the mechanic may have suggested that before offering to empty your wallet. You weren't lucky with that so ask the mechanic to diagnose it now.

Get the system pressure tested. It'll take a few minutes (free usually)
Go to a radiator specialist or competent tech.

Get the cooling system pressure tested. It's leaking somewhere if you can smell the leak. Try to determine if it smells inside or outside the vehicle. If you smell it inside and not around the motor area you might have a heater core starting to leak.

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