by using zeners in this manner, you're setting yourself up to have reliability problems.to start with, a 3V3 zener is way too close to the operating voltage. In this case, it is probably pulling the voltage down too close to the threshold voltage. That is way too much leakage. You would be better off using a Optical coupler. You can get them for both digital and analog signals. The input of a optical coupler has a wider voltage range, that it can work with. As long as you keep the current down below the maximum. And if one should go out, there's no way you can harm your processor. even a MOSFET should give better reliability and Still give good protection. all you need to do is properly bias the input. I recommend you rethink your design.I cannot recommend using zeners in this manner.
Remember to design for the maximum peak voltage and not just the battery voltage. I had a problem a long time ago that resulted in blowing every single lamp in the car, including the headlights, from overvoltage.I think most "automotive grade" chips are rated for 20V, most even take surges up to 40V without damage. Don't forget to allow a big safety factor on your capacitors too. A 16V capacitor is not suitable for automotive use.Yes a "3.3V" zener actually starts to conduct below 3.3V. That voltage is measured with a specified current like 200mA. Pick a higher voltage or other surge suppressors like polyfuses may be more appropriate.