my 240 keeps its idle normal for a while then it goes up to 1500 for a long time when i have it in neutral it does it violently sometimes like from 700 to 1500 rpm really quickly...any ideas on what the cause of it might be?

 

Manual or auto? Any check engine light coming on?

 

manual... it came on and it turned it self off... at one point the read out was ping sensor... but that stopped..

 

ECU code check will help here.

 

I have noticed before that if a code stored on the computer is not current, then it may not show the code to you manually. You may want to take the vehicle to a pro to diagnose it for you or at least invest in a code reader. It sounds to me that a sensor my not be functioning properly, or it could be simpler like a stopped up catalytic converter. BTW by manually I mean using the method of reading the blinks of the check engine light.

 

i havent done much research but do nissans have a IACV ( idle air control valve) ? thats what sounds like to me

 

yes they do have a iacv... also look into broken or cracked vaccum lines or air leaks in the intake...