Hesitation, Sag, Stumble. Chevrolet Metro 1998

For 1990-2009 cars

Checks | Action
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DEFINITION: The vehicle has a momentary lack of response when pushing down on the accelerator. The condition can occur at any vehicle speed. The condition is usually most severe when trying to make the vehicle move, as from a stop sign. The condition may cause the engine to stall if severe enough.

Preliminary Check

• | Refer to Powertrain On Board Diagnostic (OBD) System Check .

• | Refer to Symptoms .

Fuel System Checks

• | Check for a restriction in the fuel supply system. Check for the correct fuel pressure. Refer to Fuel System Diagnosis .

• | Perform the Fuel Injector Coil Test Procedure and the Fuel Injector Balance Test Procedure. Refer to Fuel Injector Solenoid Coil Test - Engine Coolant Temperature Between 10-35 Degrees C (50-95 Degrees F) and Sequential Multiport Fuel Injector Balance Test .

• | Check for water contamination of the fuel.

Ignition System Checks

• | Check for adequate secondary ignition voltage with a J 26792 spark tester or an equivalent.

• | Check the spark plugs for any of the following conditions:

- | Wet (fuel fouled) plugs.

- | A terminal screw that is bent or loose (rotate and pull).

- | A ceramic insulator that has cracks, carbon tracking or deposits (red or black dust)

- | A tip insulator that has cracks, carbon tracking, deposits (oil, carbon, glazing, fuel additive), or is loose (rattles).

- | A center electrode that is loose (wiggles side to side)

- | An incorrect spark plug gap that is too large or too small.

• | Check for bare or shorted ignition wires.

• | Check for faulty spark plug wires.

• | Check the CKP sensor engine reference signal with a scan tool. Observe the Engine Speed parameter while operating the engine at the speed of the concern. The scan tool should indicate steady and linear RPM readings while running. If the erratic values, such as sudden spikes or drops in engine speed are displayed, the engine reference signal is not stable enough for the engine to run properly. Check for a short to ground in the CKP sensor Ignition Reference Low circuit . A short to ground in the CKP sensor Ignition Reference Low circuit will cause a faulty engine reference signal to be sent to the PCM, but will not set a CKP sensor DTC (P0335).

Additional Checks

• | Check the TP sensor for binding or sticking. Voltage should increase at a steady rate as the throttle moves toward Wide Open Throttle (WOT).

• | Check the MAP sensor for a quick and accurate response to changes in intake manifold pressure.

• | Check the EVAP control system. Refer to Evaporative Emission Control System Diagnosis .

• | Check the generator output voltage.

• | Check for excessive exhaust system backpressure. Refer to Restricted Exhaust System Check

• | Check the Service Bulletins for any updates.