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Coolant Temp (CLT / ECT)

Engine coolant temperature — the master signal for everything that depends on how warm the engine is.

What it is

The coolant temperature sensor (CLT, also ECT — engine coolant temperature) reports how warm the engine is. It's the single most influential temperature signal in the ECU, because so much of cold- versus-warm behavior keys off it.

Why it matters

Coolant temperature drives a long list of corrections:

  • Enrichment — cranking, after-start, and warmup enrichment all scale with it. Cold = richer.
  • Cold idle — the fast-idle target is a function of coolant temperature. Cold = higher idle.
  • Knock limit and timing — temperature shifts the knock threshold and the derived timing.

A wrong coolant reading cascades everywhere: a sensor stuck cold makes a warm engine run rich and idle high; stuck warm makes a cold engine hard to start.

How to tune it

CLT is an input. What you tune are the curves that read it — the enrichment and cold-idle curves that must all reach neutral at operating temperature (around 80–90 °C).

Watch out

A warm engine that idles high and rich often traces back to enrichment/idle curves that don't fully taper out at operating temperature — check them against the coolant reading.

In the game

The coolant temperature gauge climbs from ambient at startup to operating temperature. Use it to know which part of every temperature-based curve is active.