hello, I'm a new member ...
Sorry for the long speech... lol
I spent so many times on these topics, I have a Fiat Punto 1.2 8v active, it has been a super reliable car, revisions made every 10,000 in 10,000 km, since 2003 until today it currently has 173,000 km.
Moving on to what happened a few days ago, I changed the radiator and the fan, because they already had many kilometers, and I usually change one part or another for prevention, even if it is still perfectly functional, in a third dismantled the old parts and assembled the new ones, filled the radiator with liquid indicated, was waiting for the fan to fire what happened, everything ok.
The next day I go to the next city in the morning, the car arrives at the ideal operating temperature, everything is ok i think, but i notice that the idle engine goes from + -700 / 600 or less and goes back to 700 rode like this in this cycle.
That same day, connected an obd II scan via bluetooth, looked for errors, no errors, the hardware was all right, you only noticed the rotations going below and above in the obdII scan.
About three years ago I had changed the throtle body, but I didn’t put a new o-ring on it, I reused it, with this in mind I thought it must be a leak, I went to the store and bought the kit of intake joints, I dismantled the throtle body , changed the gasket, reassembled the throtle body, started the engine, and waited, until the same thing happened again ...
I went back to looking, in the intake manifold that is plastic and prone to cracking, I didn't find anything, I looked at the tubes and the ventilation valve of the fuel tank, I even changed the two o´ring of this valve, the engine did the same rpm up and down, I examined the brake assistance vacuum tube, nothing ...
With no options in sight, i examine the original throtle body, thinking about reassembling it to see if the problem would be with the hardware.
I examine the old throtle body and compare it with the new one that is assembled and I immediately notice a difference, the old one has the screw that advances or delays the degrees in the throtle almost all tangled up.
In the new one, the screw is completely unscrewed, the repair was simple, I turned on the obdII scan, I looked for a tps sensor, it was 0 degrees, I started to tighten the screw, I made a new scan, in order to raise the idle until it remained and did not sob.
When it still indicated 0 degrees and the engine at the ideal temperature was already stable, I switched on the lights and the vent in order to overload the engine and check if it could keep up with the rotation, + - 800 rpm, I gave the screw and the tps a little bit more it was indicating 0.29 degrees (it is less than 1 degree the ideal is to keep 0 degrees), but as it is I think there will be no problem.
It stayed that way and so far it has never been a problem.
What I think happened was when I assembled the new throtle body, I never adjusted anything I installed and the ECU took care of the rest, with time the screw may have unscrewed a little while it took time for this problem to happen.
Put some loctite in the screw.
I hope this help.