The use of an electric pump instead of the standard crankshaft-driven one in Internal Combustion Engines, gives the possibility of controlling the coolant flow rate independently of engine speed, allowing therefore, the use of much lower coolant flow rates than usually adopted and the development of nucleate boiling flow regimes within the engine cooling system. In order to take advantage of nucleate boiling and of the associated high heat transfer coefficients, the onset of this heat transfer regime must be correctly identified. This work presents the results of an experimental campaign, which was carried out on a small displacement spark ignition engine (1.2 dm(3), 60 kW) with the aim of detecting the occurrence of nucleate boiling within the engine cooling system. The test rig was properly instrumented in order to measure coolant temperatures at engine inlet and outlet, coolant pressure at several locations in the circuit, coolant flow rate and engine metal temperatures. Operating conditions involving different coolant flow rates were selected in order to enforce both the usual single-phase heat transfer regime and nucleate boiling conditions. Several experimental quantities were analyzed with the aim of establishing the coolant flow rates ranges where the nucleate boiling occurs. The agreement in the trend of coolant temperature and pressure and engine wall temperature provides hints to identify experimentally the onset of nucleate boiling. (C) 2015 Elsevier Ltd. All rights reserved.