Fuel injector voltage compensation

I've noticed that the engine sound changes when the cooling fan kicks in. Not a big deal, I thought, but now I understand it is something worth a bit of attention.

Injector "dead time" is the time between pulse and fuel is delivered. What makes it a little bit complicated is that the dead time is not only voltage dependent but also fuel pressure dependent.

When the fan kicks in, or the headlights are lit, the battery voltage is dropped. The ECU have a voltage compensation map that add some time to the fuel map. If that map isn't correct the engine rpm may drop, causing the alternator to generate less current, even less fuel, and the engine could eventually stall. If we use closed loop lambda control the ECU would stable the engine within a second, but what would be written to the long term fuel map?

In my case the compensation map set up was not correct for my injectors.

The easiest way to measure injector dead time is to have the engine running and measure the lambda. Then remove the alternator control cable to drop the voltage about 2v, and see if the lambda changes. Adjust the map until the lambda reading is as before the alternator was disabled.

A more accurate way is to start the engine, remove the battery and connect a variable voltage source, and then disable the alternator and measure lambda at different voltage levels and adjust the map accordingly.

Or, you could do as I did and Google the injector's specs.

Bosch "Green giant" injectors at 4 bar, MBE 9A4:

It worked.