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If one injector shows a smaller pressure drop, it is not flowing fuel properly, which causes hesitation in that cylinder under load. Ignition faults would not affect injector flow. PCM driver faults would typically cause no flow, not restricted flow. MAP sensors affect overall fueling, not one injector. ASE stresses injector balance testing to identify cylinder-specific fueling issues. Technicians must use this test to separate injector faults from fuel system or ignition problems.

Detonation under boost often points to inadequate fuel delivery. If fuel pressure is normal at idle but drops under load, the pump cannot maintain the required flow. This results in a lean mixture under boost, which promotes detonation. A restricted return line would increase pressure, not decrease it. A faulty regulator typically shows issues across all conditions, not just under load. A clogged air filter might reduce airflow but would not cause fuel pressure to drop. ASE stresses the importance of load testing the fuel pump to reveal failures that static idle tests miss. Fuel delivery systems must be evaluated under dynamic conditions because high demand situations, such as boost, expose pump weaknesses. Recognizing this scenario helps avoid misdiagnosing air induction or ignition as the cause.

Hesitation at tip-in with a lagging TPS signal confirms faulty TPS response. The PCM depends on TPS for transient fuel enrichment; a slow signal prevents proper enrichment. Ignition and catalytic faults would not create tip-in-only hesitation. Dirty injectors may cause lean under load, but not voltage lag. ASE emphasizes dynamic sensor testing, not just static values, for drivability complaints.

High exhaust backpressure confirms a restriction, most often a clogged catalyst. Ignition, TPS, and injector issues wouldn’t raise backpressure. ASE emphasizes using mechanical pressure tests to verify restrictions when MAP is low.

During loaded operation, the catalytic converter should operate at high temperatures as it oxidizes HC and CO. A significantly lower-than-expected temperature indicates that the substrate is no longer functioning effectively. O2 sensors are working normally, confirming proper mixture monitoring. EGR affects combustion temperature, not catalyst temperature. A weak fuel pump would cause lean conditions and drivability issues, not low catalyst activity with normal O2 readings. ASE emphasizes using thermal analysis as a supplemental diagnostic tool for catalytic efficiency, ensuring that physical evidence supports scan tool data. Detecting catalyst failure through low operating temperature is a key exam scenario.