* Insufficient Fuel Delivery: A weak or failing fuel pump might not be able to deliver enough fuel at the required pressure to the engine. This leads to a lean fuel mixture, causing misfires, hesitation during acceleration, and a rough idle.
* Intermittent Problems: A fuel pump that's failing might work intermittently. This could explain why the hesitation and rough idle are not consistent. Sometimes it works fine, other times it struggles.
* Low Fuel Pressure: A pressure regulator issue *could* also be the cause but the fuel pump is the most likely culprit. A diagnostic check of the fuel pressure would help differentiate.
Other potential causes (to rule out): Before jumping to conclusions about the fuel pump, it's important to consider other possibilities for hesitation and a rough idle, such as:
* Spark plugs and wires: Worn or faulty spark plugs and ignition wires can cause misfires leading to rough idling and hesitation.
* Air filter: A clogged air filter restricts airflow, leading to a lean fuel mixture and similar symptoms.
* Mass airflow sensor (MAF sensor): A faulty MAF sensor provides inaccurate readings to the engine control unit (ECU), resulting in incorrect fuel delivery.
* Throttle body: A dirty or faulty throttle body can also contribute to these problems.
* Oxygen sensor: A bad oxygen sensor will provide the ECU with bad information leading to an incorrect air/fuel mixture.
Diagnosis: To determine if the fuel pump is the culprit, a mechanic would typically perform a fuel pressure test. This directly measures the fuel pressure delivered by the pump. If the pressure is low or inconsistent, it strongly points to a failing fuel pump. They will need to test other components as listed above.
In short, while a bad fuel pump *can* cause hesitation and a rough idle, it's crucial to have a proper diagnosis to pinpoint the exact problem before replacing parts unnecessarily.
