Yesterday went to practice slow flight and stalls.
At some point during flight I saw the MP dropping on its own, I suspected was the throttle was returning by itself, so I tightened it added power and flew for a bit.
That seemed to fix it.

Then loosened throttle a bit and was normal.
Was just scratching my head (I dont like when things fixes themselves).

Practiced a power off stall, pull all the power, I could feel all power was out but MP gauge showed 15MP, it will not go below 15 no matter what I did.

So went back and landed.

At idle the MP is higher than 15, about 17
Idles is smooth though.

Faulty gauge? Line?
Induction leak?

If it's running the same as always I would have to say gauge problem

My manifold pressure gauge copper line taps in just above the carburetor on the intake "Y". Mine wasn't reading right and I discovered the nut was loose causing a leak.

Carb ice?

I've seen a number of issues causd by a loose nut, but it's usually the one in the pilot seat.

Tested another gauge, (had the same reading) blew air thru the line, checked for induction leaks, nothing.

Flew the plane, all smooth and nice, power off (idle) then MP increased.
Idle when on the ground gauge reads 17MP at idle.

So the high reading is only happening at idle, could it be a carburation problem?

Sounds like an induction or gauge line leak. Are you still getting an RPM rise at shutdown?

I've had two leaks that presented like the idle behavior you are seeing...but they didn't have the same behavior at cruise you describe, only rough running at times.

One was a cracked primer tube. The other was an induction boot leak that I readily found by moving a propane wand along the induction with the engine running without the cowl at idle. The RPM raced as soon as the wand got near the offending boot.

Propane sounds like an interesting trick---will remember that