Poor Idle

1. An air inlet leak causing unmetered air to enter the inlet manifolds. These can be split or disconnected vacuum pipes, a loose EGR pipe, faulty EVR, split/loose air bypass pipe.

2. Faulty EVAP system, loose or faulty petrol filler cap. Make sure the filler cap is ratcheted tight.

3. A 'Hanging throttle' - engine delays at higher revs when accelerator is released, then revs die suddenly, revs stick high and engine races irregularly without touching the accelerator - is probably caused by a sticky throttle butterfly valve, particularly on the DOHC models. Grime and burnt oil (from the PCV system) can gather round the throttle plate and cause it to stick slightly open. Clean the inside of the throttle body with good quality carb cleaner and an old toothbrush.

4. Faulty connections on or flatlining upstream HO2S (Lambda) Sensor. These are the ones located on the engine side of the Catalytic Converter - those AFTER the Cats do not affect engine performance.

 

Do you have access to or know of any shops with a smoke machine. If so hook the machine up to a port on the intake and look to see where smoke escapes. You could do this to make sure you dont any vacuum leak at all. I used this machine at school a couple times and it works great. Anyone know if those small fog/smoke machines used during like halloween would work??

Have you checked your fuel pressure yet?

 

changed the o2 sensors, no change, changed the plugs, no change, there must be a vacuum leak somewhere

 

probably the 90* vacuum fitting on the back side of the intake manifold... fun to replace.