What it does
As soon as the ECU sees the engine cranking and has synced to the trigger, it fires a single priming injection. The Fuel Prime Pulse table sets how big that shot is. Its job is to get enough fuel into the engine to light it off. After that, normal cranking fuel takes over, and once RPM rises above the Max Cranking RPM, the ECU moves on to the main fuel tables.
It's different from the pre crank prime pulse, which squirts fuel before the engine turns at all.
The units depend on your tuning method
| Tuning method | What the table value means |
|---|---|
| Volumetric Efficiency or Mass Air Flow | A percentage of the fuel mass the engine would need at 100% VE, at rest |
| Injection Time or Fuel Flow Rate | The actual injection time or fuel quantity delivered as the prime pulse |
How we tune it
A cold engine needs a much bigger prime than a hot one, because less fuel vaporizes on cold surfaces. Start modest, then check it on real starts:
- Long crank before it fires, then it catches: prime is probably too small.
- Coughs, smells rich or needs throttle to clear: prime is too big.
- Hot restarts are hard but cold starts are fine: pull prime out at high temperatures.
Test cold, warm and hot starts, and change one table at a time so you know what fixed it. See fuel tuning for how the rest of the fuel model fits together.
