Idle Tuning

Tuning the open-loop idle solenoid duty cycle by coolant temperature

The Idle page helps you set the open-loop idle control duty cycle for each coolant temperature breakpoint. It extracts stable idle periods from your log data and shows you what duty cycle was being used and what RPM was achieved, so you can work backwards to the duty that hits your target idle speed.

Idle Tuning page showing scatter of logged idle duty cycles across coolant temperatures

What counts as a valid idle point

Not all low-throttle data is useful for idle tuning. DigiTune GTS applies several filters to extract only genuinely stable idle periods:

Throttle position must be near zero (no driver input).
The idle control system must have been active for at least the Idle Settling Time (default 5 s) to allow closed-loop control to stabilise.
RPM and MAP must both be stable within the expected variation thresholds for at least the Idle Req. Time for Valid (default 5 s).

Points that pass all filters are shown as scatter on the graph. The X axis is coolant temperature; the Y axis is duty cycle. Colour indicates data quality as on other pages.

How to tune

Collect logs that include cold-start idle (for low CLT cells) and fully warm idle (for high CLT cells). Let the engine idle for at least 10–15 seconds at each condition.
On the Idle page, look at where the scatter sits relative to the current table line at each CLT breakpoint.
If idle RPM was too low at a given temperature, the duty needs to increase at that CLT. If too high, decrease it.
Use the Idle RPM Response Rate setting (RPM per % duty, default 20) to estimate how much to change the duty value to achieve a given RPM correction.
Edit the table values, click Update, save, and collect a new log to verify.

Relevant settings

Setting	Default	Description
Idle RPM Response Rate	20 RPM/%	Expected RPM change per unit of duty cycle. Used to scale correction suggestions.
Idle Averaging Filter Time	2.0 s	Time window used to smooth duty readings before averaging.
Idle Settling Time	5.0 s	Time the idle controller must have been active before data is considered valid.
Idle Req. Time for Valid	5.0 s	Minimum consecutive stable idle time required for a point to be included.
Idle expected RPM variation	3.0 RPM	Acceptable RPM jitter for a period to be considered stable.
Idle expected MAP variation	3.0 kPa	Acceptable MAP jitter for a period to be considered stable.
Idle Allowed RPM error	15.0 RPM	Maximum RPM deviation from target before data quality is penalised.

See Settings Reference to adjust these.

Tip If you find the page is showing very few valid data points, the settling time or stability thresholds may be too strict for your engine. Try reducing Idle Settling Time or increasing the expected variation thresholds slightly in Settings.