I have a 1993 Fiat Panda 1.0 i.e. (gasoline) with single injector. Last year it started to present a very specific issue: from time to time (but basically now it happens every time I drive the car) it "gives gas" by itself for some seconds, usually noticed when idling.
The engine can be warm or cold, it does not matter. Also, the minimum rpm never drops below the factory correct value: it only goes up. The fuel consumption does not appear to be affected at all (I keep track all).
For example, at the red light when idling and without touching the gas pedal, the engine in some cases starts "hunting", oscillating between the usual minimum rpm and an higher value like I was giving rpetitive pushes to the pedal, or, more frequently, the engine start keeping a quite high rpm (I estimate 2000-3000 rpm), VERY constant, before dropping back to normal.
When this happens, pressing repetitively the gas does not help: when I start touching the pedal the rpm goes down a little (I mean for half second), then picks up again following my foot (it increases further). Sometimes it's even worse and the gas I give just moves the "minimum" to an even higher rpm, even after releasing the pedal.
Sometimes I'm not idling but I'm driving (highway, city, it doesn't matter) and I notice that when I release the pedal the engine brake is very weak. I engage the clutch and I can confirm it's idling at high rpm. This can be dangerous, the brakes have to do more work.
The issue disappears by itself within 10-20 seconds, usually, but I have the feeling that when I give a lot of gas (that means I'm driving) it gets shorter.
Another example: once the issue was showing up and I had to give full gas. After the usual delay the car lost power, going back to the "normal" power.
It seems like somewhere additional air is let in. Am I correct?
I tried to spray brake cleaner in the engine bay (very far from the air intake) and I noticed a reproducible increase in rpm, but very minimum. I don't think there is a leak. I also tried moving the fuel lines and air ducts (quite old) in case there was a crack, but no effect.
The butterfly looks very clean.
The stepper motor for the bypass air is fixed (very tightly) with two screws, one of which has a destroyed head, so I couldn't take it out last time.
What should I focus on, when trying to fix the issue?