I have a 1990 745 with a B234F (2.3l,16 valve,non-turbo)and Bosch LH 2.4. The engine will not start when it is about -25c or colder outside. It turns over fine but will not fire. I checked spark - OK. I checked both fuel pumps - both OK. Checked the radio interference suppression relay - OK. The battery is new this winter. It may have to do with the fact that this engine does not have a cold start injector (fifth injector).

The injectors are fed with constant 12 volts (as long as the ignition switch is on). The ECU grounds the negative side of the injector to energize it. The injectors are connected to pin 18 of the ECU, through the injector driver, out pin 17 to the ground connection on the intake manifold.

That led me to an idea:

If I ran a wire from the negative side of the injectors (= pin 18) through a momentary switch to ground I could energize the fuel injectors and give the engine a good shot of fuel when starting on those really cold days. Like a manual fifth injector.

The trouble is I don't know if the injectors are across a full 12 volts or is there an impedance in the ECU to drop the voltage across the injectors. The injector pulses are so short you would need an oscilloscope to read the voltage, which I do not have. And would this damage the ECU or injectors? I could put a resistor in series with the switch to drop the voltage on the injectors.

If anyone has tried this or can share some knowledge on this it would be much appreciated. It gets real cold here in Winnipeg.

Pat