What open loop idle means
With idle control in open loop, the ECU doesn't look at engine speed to decide what the idle valve should do. There's no rpm feedback and no correction. The Base - Open Loop table is the only thing setting the idle motor duty, and the valve simply goes to whatever position the table asks for.
The table supports up to 2D mapping. We usually map it against coolant temperature, so the valve opens further on a cold engine and closes down as it warms up.
When we use it
- First start-up and diagnosis. Open loop lets you confirm the valve moves and airflow changes, without a controller hiding problems.
- Finding your base positions. The duty that gives a good idle in open loop is a great starting point for the Base - Closed Loop table.
- Simple setups where a steady valve position is good enough and you don't need rpm control.
Tuning it
Warm the engine up in steps, and at each coolant temperature adjust the duty until idle speed is where you want it. Expect to go back and adjust as the engine gets hotter, because a warm engine needs less air. Remember that anything which changes load, like the A/C, fans or power steering, will pull idle down in open loop, since nothing is correcting for it. The idle up settings can help with that.
For the full picture, see idle control.
