[deadair]

whats causing my prelude vtec to idle up and down

 

[spray004]

whats causing my prelude vtec to idle up and down

Check engine light on?

 

[deadair]

i wish but nope its not on

 

[spray004]

i wish but nope its not on

What rpm range is it fluctuating between?

 

[deadair]

if the engine is cold it'll rev up to around 3200 and back down to 900 over and over and when its warm it'll only rev up to about 2500

 

[spray004]

What year prelude? Could be a number of different things, idle air control valve possibly. MAP or MAS depending on what year prelude?

 

[deadair]

its a 93 prelude vtec

 

[Snoopy]

sounds like a possible vacuum leak or bad iacv.

 

[explosivpotato]

check timing

check the timing belt/position.

I had an uneven idle and pinging at low rpms when my timing was bad.