Use 2 ecu's for 1 car. 1 for fuel 1 for waterinjection?
 

like the titel says:

is it posible to use a 2nd ecu (tap the wires) to control 4 injectores to inject water of methanol.

or will the 2nd ecu interrupt the signals to the 1st ecu, causing damage to the injectors. (in mind: amps voltages an ohms) and wrong readings from sensors?

its for a honda 4th gen d16z2 (same as a6)
but the principle is the same for every other car.

I don't know of an injector that will stand up to water/methanol mixture without rusting shut.

If you know of one let me know!

Negative temp. coefficient resistor based sensors will read wrong on both ecu ifyou tee.
You would have to install extra coolant and intake air temp sensors for the 2nd ecu.
There may be other NTC sensors on some ecu also(oil temp., trans temp, EGR temp ect...)

Potentiometer sensor can be shared, they have a power, ground, and signal(throttle, map, evap press. ect...)

Hall effect sensors could should be sharable. I have not tried but don't see any problem with on paper. (vehicle speed, crank & cam postition with 3 or 4 wires, square wave DC output)

VR sensors are similar to Hall sensors but with 2 wires, probably tee-able not sure.

Second ecu will probably throw emission codes. The outputs should not be hooked together. If for example ecu1 turned a solenoid on and ecu2 had the output off it could damage the circuit on ecu2. The 2nd ecu will most likely go into limp home mode and stay there unless the whole emission system is duplicated or removed from programing.

May be simpler to use a Megasquirt for W/I control instead of 2nd stock. MegaSquirt-II - Advanced Electronic Fuel Injection Computer by Bowling & Grippo ©2005

On a 4th gen there are no emission's related to the ecu. So there should be no problem with anything emission's related. I would think that this idea would actually work. My only concern would be with the timing maps, if timing on both ecu's were set the same, would it be read just like a regular single timing map or would it do something crazy with it since there are 2 sets of maps. It would be very easy to set the Ms on the injector's to 0 until the point at which you wanted them to start operating since they would be entirely on there own harness. The only thing I can see as a concern would be dealing with the timing, and I don't think it would be that big of a problem.

that would b inovative

lol a electrical mess i bet too