A diesel engine intake system including a timing valve which is positioned in the intake system in engine starting and warming-up period 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 is rushed 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 timing valve is responsive to the suction pressure in the combustion chamber and opened when the suction pressure becomes stronger than a predetermined level.