A friends '89 300zx likes to rev up and down when idleing, but it doesn't do it all the time, what could possibly cause this
Thanks

try unplugging your FCID or AAC unit and see if that levels out the idle... also look for vacuum leaks... usually they just cause high idle but if it's an intermittant leak then it could cause fluctuating... this (http://www.thumper300zx.com/z31/vacuum/diagram.jpg) is a map of all of the vacuum lines in the engine bay minus the stuff for climate control...

it's also a good idea to do a tune up, it probably won't help the idle issue, but it's always a good idea... if that doesn't solve it, i'd go after the cylinder head temp sensor (CHTS) as they go bad it seems every time you look at them.

Im not quite sure what the FCID or AAC units are, im sure there could be a vacume leak, the motor has 106k from an 87 and the body has 166k on it, but my friend doesnt really care about it
Thanks

the idle control thing on the drivers side of the intake plenum, right in the middle of it

So just umplug them when its bouncing? it doesnt do it all the time, usually when cold, but i will try, thanks