Hi,
Yes, the schematic would be needed to know what it is.
If you dont have a schematic and cant get one, you can try a low value first like 3v, then around 4v, then around 5v, then 6v, etc., until the car works normal again.
It's important to start with the lowest value though, not the highest, and work your way up, testing the car completely after each try.
You can sometimes get away with using a signal diode like 1N4148 in series with the zener to boost it's apparent voltage up by about 0.7v or so. That will provide you with other zener voltages for the tests so you dont have to buy as many. Two diodes will boost the zener up by 1.4v roughly.