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Stalls like this are often ignition-related and happen when the engine loses spark. The underlying cause is often a bad crankshaft position sensor, or sometimes a failing ignition coil (if the engine has only one coil). A faulty ignition switch that loses contact intermittently may also cause the engine to suddenly die for no reason.
When this happens, open the hood and check for spark. This can be done by pulling off a plug wire (if the engine has plug wires), and placing the end near the block while a helper cranks the engine. DO NOT hold the wire as it may shock you if the ignition system is working. If you do not see a spark or hear the plug wire snapping when the engine is cranking, the fault is in the ignition system.
Another possibility is a bad PCM (engine computer) relay. The power supply to the PCM is often routed through one or two main power relays. If one of these relays loses contact momentarily, it's like pulling the plug on the PCM. The PCM shuts down and turns off the ignition and fuel injectors, causing the engine to stall. One way to see if this is a possibility is to switch or replace the PCM power relay(s). If the problem goes away, the cause was a bad relay. If it continues, the fault is something else (possible a wiring fault in the PCM relay or PCM power circuit).
Yet another possibility is low system voltage, loss of voltage, or overcharging. The PCM and other control modules require a steady 12 volts to operate correctly. If the supply voltage suddenly drops below 9 volts, or surges about 16 volts, or cuts out, the PCM may temporarily kill the injectors or ignition circuit. The underlying cause may be an intermittent short somewhere in the electrical system or charging system that causes a momentary drop or surge in voltage. These can be very difficult to find, and often require hooking up a scan tool that can capture snapshot data when the stall occurs. By looking at the data, a technician can see the chain of events that caused the stall, and hopefully identify, isolate and repair the fault.
Hesitation is when your engine misfires, stumbles or lacks power when you accelerate or step on the throttle. The problem often means the air/fuel mixture is not being properly enriched or is going lean, or the ignition system is weak and is misfiring when the engine comes under load or the air/fuel mixture goes lean.
When you step down on the accelerator and the throttle opens, the engine sucks in more air. The computer should respond by adding more fuel.
If the engine has a speed-density type of fuel injection system (no airflow sensor), the computer uses inputs from the throttle position sensor, manifold absolute pressure sensor, air temperature sensor and engine rpm to estimate airflow and how much fuel the engine needs. NOTE: Speed-density systems are much less sensitive to vacuum leaks than EFI systems that use an airflow sensor.
If the engine has an airflow sensor (vane airflow or mass airflow), it looks primarily at the airflow signal from the airflow sensor, but also takes into account what the throttle position sensor and MAP sensor (if equipped) are telling it. NOTE: Airflow EFI systems are very sensitive to vacuum leaks, and air leaks downstream of the airflow sensor.
Consequently, if the inputs from any of these sensors is inaccurate or missing, the engine computer may not add enough fuel, allowing the fuel mixture to go lean causing a misfire that produces a hesitation or stumble when accelerating or opening the throttle.
The amount of fuel added by the computer when the throttle opens may also be insufficient if the fuel injectors are dirty or fuel pressure is low. The oxygen sensors in the exhaust monitor the air/fuel mixture so the computer can adjust fuel trim as needed to maintain the proper air/fuel ratio. Fuel trim adjustments can compensate for dirty injectors and/or low fuel pressure to a certain extent, but occur too slowly to offset a throttle hesitation problem.
Vacuum leaks will typically cause the fuel trim to run rich as the computer tries to compensate for the extra air being sucked into the engine through the leak.
Possible Causes of Engine Hesitation or Stumble:
Dirty fuel injectors (cleaning the injectors often fixes this).
Bad MAP (manifold absolute pressure) sensor
Bad TPS (throttle position) sensor
Bad or dirty MAF (mass airflow) sensor
Low fuel pressure (leaky fuel pressure regulator or weak fuel pump)
Vacuum leaks (intake manifold, vacuum hoses, throttle body, EGR valve)
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