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26 Here’s the coolant temperature sensor, which takes the system from open loop (cold engine) to closed loop (engine at operating temperature). In open loop, the system is on a pre-programmed rich mixture (longer injection pulse width) for engine warm-up. When coolant reaches operating temperature, it goes into closed loop and normal operation with the Holley-programmed fuel curve. Fuel curve (map) can be calibrated via the hand-held programmer provided with the kit.