Leaky areas in the intake system allow uncontrolled secondary air to pass through. This disrupts the control unit computer and leads to distorted results, for example when preparing the fuel-air mixture: the injectors supply too little fuel to the engine - the mixture becomes uncontrollably lean. The malfunction is most clearly recognized by the "sawtooth" number of idle speeds. At full load, engines that receive an excessively lean working mixture are prone to an audible ringing sound. To protect the Zetec engines in the Focus from costly indirect damage (e.g. burnt piston bases, piston scuffing), the knock sensor on the engine block constantly monitors their noise effects and, if necessary, corrects the ignition timing in the direction of retarded ignition timing. Attentive drivers notice the interference: their Focus responds somewhat more slowly to the gas pedal.
Stages of work
Check the vacuum hoses for cracks and tightness. Check all hoses that are connected to the injection module or to the intake manifold (absolute pressure sensor, brake booster, fuel evaporative control system, etc.).
Warm up the engine, then let it idle; open the hood; disconnect the lambda probe connector.
Spray all injection hose connections and flange gaskets one by one with a commercially available cold engine starting spray: where the engine speed changes when sprayed, there is a leak.
(The original source of this article can be found at: «FORDBOOK»)