I didn't fully understand your answer, but I will read and investigate better tomorrow. You're saying that my calculations and schematics are right, but in order to improve the value readed by the sender I should do what you suggest, right ?
Which part don't you understand.
I suggested to supply the voltage divider from the 3.3volt pin.
That is more stable than the 5volt rail, and a lower voltage on a low-resistive sensor is less power wasted.
Enabling the internal bandgap reference allows you to use the full A/D range.
Diagram #1 is ok for testing. #2 has added protection.
In addition, R5 and C1 as shown in the schematic are used as a low pass filter (and current limit) to provide signal conditioning to the input of the A/D. Especially in automotive etc. type applications (which tend to have all sorts of strange transients), you need to provide input protection like that to help prevent blowing your board up. You will also need to make sure the ground of the board and the ground of the sender are very close together to avoid all sorts of goofy readings due to current in the ground wires.
Everything is almost good, except one thing. I have a ebay pressure sender that for now I use a 100 ohm resistor to "emulate". The voltage divider I use also a 330 Ohm in a 3.3v with a LD33V.
This is the scheme, https://tinyurl.com/zqcuegn, and the analogRead should read 157, but I got 164. Some times, depending where i put the negative of my voltmeter i get the 157, but most of the time is 164 or higher. Could be some interference ? I already change the Arduino Nano clone to a Arduino Uno original that improved, but still not perfect.
I've commit the code to my github repo, although I have some problem with printing values on bluetooth serial and computer serial, everything is working fine.
I will try to add a diagram of all the components wire together but I have to find a software that have a MPX4250AP sensor and a bluetooth component to map.
Yesterday, before I go to sleep I tried one more thing. Because the sensor causing problems was the oil pressure, I have wire the 3.3v from arduino directly to that sensor (using LD33V for the other 2) and a dedicated ground also for that sensor (you can view on this diagram): https://circuits.io/circuits/2527463-car-sensor-controller/
Note: I test this circuit using 100 ohms resistor for oil pressure sensor, 220 ohms for the oil temperature and 10k ohms for water temperature to simulate their values.
Remember the 1.1V internal reference needs calibration - there is a lot of variation between
devices, but for any given device it is stable and pretty accurate - this means measuring
your particular Arduino's response and either putting it in the code (or better in the EEPROM
so the code doesn't have to be changed for each Arduino).