Hot IdleProblems

[d6]

Here's my recentley purchased '86 ae86 4age levin



My problem is hot idle . when the engine is cold it revs fine at idle ( occasionly 2300 rpm , but pick the throttle and that settles out. ) when the engine reaches 45% on the scale and the engine is under no load ( neutral , no rear heated window ) the engine revs up and down between 700 and 1800 rpm this makes crawling in traffic difficult and sounds terrible too. ( grief from the missus ) if a load is put on the engine , half clutch or sometimes lights , heater & fans on, the idleing settles for a few mins.

i've checked coolant level , this was low but filled and no change. btw a sports air filter is fitted. i was told this could affect it. the valve cap in the throttle body was repalced with a tyre cap that didn't make any diff either. a friend mentioned it could be a leaky vaccuum pipe somewhere.

where and what should i check next ?

i'm still kinda green ya see.