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What Is Engine Hesitation and What Causes It?


Engine hesitation is a stumble, misfire, or loss of power when the throttle is pressed. In most cases, engine hesitation is an indication of the air and fuel mixture being too rich or too lean.

Under normal operation, when the throttle is pressed, it adds more air and fuel to the engine. Several things can cause either of these to become restricted or unresponsive to throttle input. They are an indication that the fuel trim attempts by the engine's computer, based on oxygen sensor input, are unable to compensate.

Purely mechanical engines will have the throttle adjusting both air and fuel intake. This is relatively reliable, but very inefficient. Modern engines with fuel injection, however, are more sophisticated. Most are one of three types of air control for fuel injection: speed density, vane airflow, or mass airflow.

Speed density systems do not use a dedicated air sensor to read air intake to adjust fuel intake, but instead rely on the throttle's position to do so. Inputs from the manifold absolute pressure sensor and air temperature sensor are combined with engine RPM to estimate airflow and adjust accordingly. These systems are mainly found on early fuel injected engines.

More common are the airflow sensor-driven injection systems. These use either a vane airflow or a mass airflow sensor to determine air intake and adjust fuel in response. Input from the airflow sensor is taken and compared to the throttle position and manifold absolute pressure.

In all of the mentioned injection systems, any fault in a sensor or loss of pressure through vacuum leaks can result in engine hesitation issues. This is especially true of vane and mass airflow systems.

So causes for engine hesitation can include bad MAP or throttle position sensors, a bad or dirty mass airflow sensor, vacuum leaks, low fuel pressure, dirty fuel injectors, or bad gasoline.

In coming articles, we will talk specifics for diagnosis and repair of these issues related to engine hesitation. Get started on that here.