Hydrogen is the most promising alternative fuel for spark-ignited engines. This paper experimentally investigated the performance of a pure hydrogen-fueled SI engine at idle and lean conditions. The experiment was carried out on a four-cylinder gasoline-fueled SI engine equipped with an electronically controlled hydrogen port-injection system and a hybrid electronic control unit which was used to govern the hydrogen injection duration. The engine original electronic control unit was used to adjust the opening of idle bypass valve and spark timing to enable the engine to be run around its target idle speed. The test results showed that the fuel energy flow rate was reduced with the increase of excess air ratio for the pure hydrogen-fueled engine at idle and lean conditions. When excess air ratio increased from 2.08 to 3.2, the hydrogen energy flow rate was decreased from 11.79 to 9.97 MJ/h. Both the flame development and propagation periods were increased with the increase of excess air ratio. Because of the increased opening of idle bypass valve and dropped cylinder temperature, both pumping and cooling losses were reduced when the engine was leaned out. NOx and CO emissions were negligible, but HC and CO2 were still existed for the pure hydrogen-fueled SI engine due to the possible burning of the blow-by lubricant oil gas. Copyright © 2013 John Wiley & Sons, Ltd.