[z28pat]

We’ve had a bit of a cold spell in Florida and the one morning I woke up and the pressure was down to 17 on left front tire , filled it up no problem that day. A day later it was fine all day, Drove to work, sat at work parked all day and no issues , this morning the pressure is down to 13! Dropped off at Costco and they haven’t found anything yet. before taking there I took wheel off and looked, filled it up and waited for a leak but nothing .
Any ideas what could be causing this ?

[Trochoidal]

Ah a mystery leak. Had this happen on a different vehicle many years ago.

Pulled the wheel off and grabbed a spray bottle full of soapy water. Nothing showed a leak.

But wait for it, just for the heck of it I decided to fill up the valve stem without cap in place. Well guess what, the schrader valve was loose. Blew it out with compressed air, tightened it and life was good.

The soapy water could also show if you had a cracked wheel too.

[c4racer]

It must be those air stealing mice at your house!!

There could be a small nail or similar that only leaks when it's on the bottom when contacting the ground, but it seals up when pressure is relieved. I noticed that about a temporary plug in a tire before I had it patched properly. I would take the wheel off and go over it very carefully visually and see if you can find anything that doesn't look right. Keep a screwdriver handy to pop out rocks that are lodged in there to rule those out as a nail. Sometimes its very difficult to tell the difference.

[DIYguy]

^^^^^^^^^^^^^

I had that happen on my car.
Mystery leak, couldn't find it using soapy water.
Then I looked for and pried out anything non-rubber I could find in the tread no matter how small or insignificant it seemed.
After that it became a constant leak that I could deal with.

On my wife's truck we had a valve stem core go bad about a year ago.
She went to drive it one morning and one of the rear tires was completely flat.
I had checked the tire pressures just a couple of days earlier and apparently it didn't seal completely (but it was tight in the stem).
I keep spare cores on hand and after replacement it hasn't been a problem again.

[Vtor_ZL1]

refinished wheel ? I had issue once where finish peeled off (mostly during tire installation ) and depending where tire was sitting, it would "fizz" out. Easy to spot w some bubble water after some head scratching and not finding any punctures.

[z28pat]

And woop there it is !!! The soapy water did the trick but only with the tire on the car , the smallest little slit that’s blowing bubbles and of course it’s right by the sidewall not on it but close enough
, f#*k! Now what do I do?, car has under 5000 miles , so do I get 2 stock tires for the front? Because eventually as a daily driver I was gonna change all the Goodyears and get Michelin ps4s.

[c4racer]

bummer! And yes - 2 stock tires for the front. Maybe you can find some take offs with lower miles.

[DIYguy]

the smallest little slit that’s blowing bubbles and of course it’s right by the sidewall not on it but close enough

Bummer indeed.
As c4racer said, haunt some tire shops or Craigslist and see if you can find some low mileage takeoffs.
Just to get you by until you feel that you have gotten your money's worth.
Then go for a whole set of new ones like the Michelins.

.....or maybe get your Michelins now and the other three become someone else's great find on Craigslist....?