Temperature Trims (IAT / CLT)

Fuel correction tables for intake air temperature and coolant temperature

The IAT Trim and CLT Trim pages let you tune the temperature-based fuel correction tables. These tables apply a multiplier to the base fuelling to compensate for how air density and engine fuelling requirements change with temperature. Getting them right is particularly important for cold-start enrichment and for engines that operate across a wide ambient temperature range.

IAT Trim page

The X axis of the IAT Trim graph is intake air temperature (−40 °C to +80 °C). The Y axis shows the trim correction factor or percentage. Each scatter point represents the measured fuel correction required at that IAT.

2D vs 3D IAT Trim

2D IAT Trim (ME, MegaSquirt, Link, ECUMaster) — A single trim curve across IAT. No load selection is shown. You can optionally colour the scatter points by data quality or load using the Z-axis dropdown.
3D IAT Trim (Haltech) — The trim table has both an IAT axis and a load axis. A Load column dropdown appears. Select a load breakpoint to view and edit the trim slice for that load band.

CLT Trim page

Identical in layout to the IAT Trim page, but the X axis is coolant temperature (−40 °C to +130 °C). Cold-start enrichment is visible as higher trim values at the left side of the graph.

2D vs 3D CLT Trim

Same as IAT Trim — Haltech uses 3D tables with a load axis; other ECUs use 2D tables.

How to tune trim tables

Collect log data that covers the temperature range you want to tune. For CLT this means logs from cold start to fully warm; for IAT it means different ambient conditions or different seasons.
On the trim page, inspect where scatter points sit relative to the current trim line. Points above the line mean the engine is lean at that temperature; points below mean it is rich.
Adjust the table values to move the trim line towards the centre of the scatter. Use cell increment/decrement or paste from clipboard as appropriate.
Click Update to commit changes, then save and verify with a new log.

Note — Trim interaction with AutoTune If IAT and CLT trim channels are present in your logs, the AutoTune algorithm accounts for their effect when calculating VE corrections. This prevents trim adjustments from being inadvertently absorbed into the base VE table. For best results, set your trim tables to a reasonable baseline before running AutoTune.

Tip — Cold start data CLT trim data at very low temperatures (below 20 °C) requires logs taken from a cold start. Let the engine idle until fully warm and ensure the log covers the entire warm-up period. Several cold-start logs at different ambient temperatures give the most complete picture.