1990 legacy LS 5 speed

 

well driving on the freeway at 60 mph@2500 RPM then shift into neutral to coast down hill. the idle goes down to 400rpm then 700rpms, back down and then dies, does this almost every time I go into neutral from any rpm but mostly off of free way speeds. and when starting the car on a cold day the idle starts off high around 1700rpm then drops down to 700rpms and then back up back and fourth.does that until the car is warm. or until I step on the gas and the idle drop and stays at 700rpms. besides that the car does fine.

 

What is the deal?:-\

Maybe a vacuum hose leaking. Had something similar with other brand car turned out to be simple rubber stopper disappeared.

Might be the coolant temp sensor. They are relatively cheap to replace. Around $20 $25

Neutral switch in tranny ...yes the 5 speed has one, should be near the reverse switch. Thats how the ECU knows to take over the idle instead of just having it decellerate while in gear.

On the 5MT's it really doesn't do much, however on the auto's is does.