Alfa Romeo Repair: alfa 156 2litre, air flow meter, alfa 156

Question
When i clutch to change down gears the the idler goes right down causing the car to stall. i have changed the air flow meter but no improvement does any one know what might be the problem ? Thanks ger

Answer
First is to clean the carbon out of the idle motor channels and the throttle body. Then check for excess back pressure in the exhaust system. Take out the O2 sensor and install a fitting to attach a pressure gauge. At 2000 rpm there should be less than 2 psi showing.