Hi Magos,
1) In your 1st post you used an LM2907. But your schematic shows an LM2917. What do you have?
2) Most circuits in the datasheet use a 9V-16V car battery and the 470 ohm resistor you are using drops it for the zener diode regulator in the LM2917. You say that your Vo supply is only 7.5V but the IC's current through your 470 ohm resistor drops the 7.5V to about 6V for Vcc (pin 9). This 6V isn't regulated nor decoupled and is probably swinging all over the place. If you really do have only a 7.5V supply voltage (where from?) then your 470 ohm resistor should be replaced with wire only if you are not using an LM2917 with its zener diode.
3) Most circuits in the datasheet use a 20nF cap at pin 2 but yours has 100nF. I think that 100nF can't charge and discharge properly with input frequencies above about 200Hz. Your frequency is much higher, requiring a cap with a lower value.
4) Most circuits in the datasheet use a 100K resistor at pin 3 but yours has 10K. If you use a 100K variable resistor you should get a 3,5V output with it near half, when the cap at pin 2 is 22nF, and the 470 ohm resistor is shorted out.
5) What is the voltage of the circuit's input pulses from your car's ECU?