Technical Idle Problem

[Mike1alike]

Fiat Punto mk2 1.2 8 V, I'm experiencing some idling issues. I've identified the cause being faulty throttle positioning sensor and I'm making this post about it because I've not seen many posts providing a solution in this case, but I have seen this problem described multiple times. This is one case.
Current issue is engine is revving by itself, especially when it should idle, but also while driving and in gear. The TPS gives erratic data, value moves by itself without touching the throttle pedal.
So, with MES connected, throttle positioning is bogus and not stable value. Pedal untouched, reading is 5⁰, 8⁰, 9⁰ - just dancing around those numbers. When pressing down the pedal the value changes but again, it does not reflect the real position. I think it went up to max 19⁰ although the pedal was to the floor.
What bugged me in this case was that even with the TPS unplugged there was still a bogus reading. Lower than with it connected but still bogus, was dancing around 2.x⁰.
I've also put some wires in the plug to measure the voltage and had the feed 5 V and the signal voltage 1.20 V on closed throttle to 1.40 V when opening. Normal values should be ~0.5 V closed to ~4.5 V opened.
I ordered a new TPS.
Found the code is 71738921, although Fiat gives no code for this one, nor for the Idle Air Control Valve.
(The IACV code is MEAT & DORIA 84055, FISPA 87046, HOFFER 7514055, MAGNETI MARELLI IB0100, SIDAT 87046)

I ordered the Magneti Marelli TPS. Unfortunately, I didn't find to order here the Ricambi Originali one, would have preferred that as I see it's a set including new screws and new TB gasket.

Never replaced the TPS. From pictures I see it has a holding mechanism resembling to the IACV's, that one I've replaced. It's some sort of starry spring.

 

[Mike1alike]

Yeap, fixed it.
Got my sensor and it was very easy to replace it, knowing how it is held into place. I put a screwdriver under that little plastic bit and pried it away, and it started to come out. Then under the other plastic bit (it's underneath in the picture) and the same with the little corner of the plug. That is very small but it's enough, ones it starded coming out. Cleand the inside and put the new sensor in. It has an oring so I put some oil onto it and it went in just by pressing it by hand. I used a cloth because there is that bit o plastic that hurts if you press it directly.

Measured the resistance on new sensor and it was:
*out of the box: 1.01 kOhms position closed
2.03 kOhms position opened
* mounted on Throttle body: 1.57 kOhms position closed
2.39 kOhms position opened.
Also, using MES I could see it reads the correct position of the throttle. It was 0⁰ pedal up, going accordingly up as I pressed the pedal to almost 70⁰ I think (was 60 something), pedal down.

One little problem was you cannot buy the TB gasket individually, found it just in set. And I had the TB down last week to clean it and afterwards the gasket was loosing vacuum. I put Renzosil sealant on it and it held vacuum perfect, the engine would cut off if I covered the top of the TB with the palm.
This time I was going to do the same, use sealant, but I remembered I had one gasket left in the Intake manifold gaskets set I bought about 4 years ago, Elring 728670. It's the number 2 marked in the photo and it matches perfectly although it does not have the specific shape of the number 1. It's the same lengh and it was much higher now than the used one. It holds vacuum perfect, engine cuts off if I cover the TB opening.

One little detail about cleaning the TB, because I've cleaned it in the past using petrol or different sprays but never got the result I got this time. The spring got really clean and I am totally impressed with how tensioned the pedal feels after this, like on a new car. I goes down smooth when I press it, but feels better than ever before. This time I washed it in hot water mixed with citric acid and laundry detergent, without putting the part with IAC valve and TPS under water.

Also, this bad Throttle positioning sensor started manifesting itself some time ago. At first the RPMs would drop while pulling away. It happened because ECU wouldn't "see" I'm accelerating so it wouldn't inject more fuel.
The only stored errors that would pop up were about Mixture strength P0170 and P1170. After cleaning the Throttle body the engine started revving itself up while driving and in neutral (at stop signs/lights) because the (false) TPS signal was the throttle is opened and so ECU would inject more fuel. Therefore the Mixture strength error.