My RPM go to high

Hi folks. I have a range rover P38 4.0 Se 1996 Automatic. When i start the car my RPM odometer go to high and stay there 1900RPM or 1700RPM. My Range is LPG conversation. This problem do in LPG and Petrol to. And is a littel difficult to start. No misfire or error. Where is the problem ???

Idle control valve or air leak in pipe work to it.

Make sure there is plenty of free play on the throttle cable too.If all these checks don't fix it then you need a diagnostic test to see if the voltage output from the throttle position sensor is equal to or lower than the value stored in the engine ecu(With the throttle closed).If its higher it needs resetting and checking that the sensor is giving a plausible output.