A diesel engine intake system including a timing valve which is positioned in the intake system in engine starting and warming-up periods under a cold engine condition. The timing valve is opened in the final period of the intake stroke so that a strong suction pressure is produced in the combustion chamber before the timing valve is opened. When the timing valve is opened, the intake air as rushes into the combustion chamber at a high speed, whereby the intake air is compressed under the inertia of the high speed flow to thereby increase the temperature of the intake air. The engine is also provided with a blow-by passage and an exhaust gas recirculation passage, both opening to the intake passage upstream of the timing valve, so that it is possible to prevent the blow-by gas and the recirculated exhaust gas from being drawn under the suction pressure when the timing valve is closed.