Leaks in the suction system allow uncontrolled stray air to pass through. This disrupts the operation of the control unit computer and leads to distorted results, for example, when preparing the air-fuel mixture: the injectors supply too little fuel to the engine - the mixture becomes leaner uncontrollably. Most clearly, a malfunction is recognized by «sawtooth» idle speed. At full load, engines running too lean are prone to audible ringing. To protect Zetec engines in Focus from costly collateral 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 retarded direction. Attentive drivers notice interference: their Focus is somewhat slower to respond 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 vapor recovery system, etc.).
Warm up the engine, then start it at 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 start spray: there is a leak where the engine speed changes when sprayed.
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