Hesitation, Sag, Stumble

WARNING: This page is about a different car, the 2004 GMC Sierra, 2004 GMC Cab & Chassis Sierra, 2004 Chevrolet Silverado, and 2004 Chevrolet Cab & Chassis Silverado. However, it is still accessible from the selected car via links, so may be relevant.

Inspection	Action
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 from a stop. If severe enough, the condition may cause the engine to stall.
Preliminary Inspection	Refer to Symptoms - Engine Controls .
Fuel System Inspection	Inspect the fuel supply system vacuum. Refer to Fuel System Diagnosis (Pickup) or Fuel System Diagnosis (Cab Chassis) . Inspect for water contamination in the fuel. Refer to Contaminants-in-Fuel Diagnosis . Perform the Fuel Injector Balance Test. Refer to Fuel Injector Balance Test with Tech 2 (with Automatic Transmission) or Fuel Injector Balance Test with Tech 2 (with Manual Transmission) . Inspect for high fuel supply system vacuum after a cold start or during moderate or full throttle acceleration. If the vacuum jumps above specification, there is a restriction in the fuel system.
Sensor Inspection	Turn ON the ignition with the engine OFF. Observe the actual fuel rail pressure with a scan tool. The actual fuel rail pressure should be between 1.0-1.8 MPa. If it is not, inspect for high resistance in the fuel rail pressure (FRP) sensor circuits or the FRP sensor. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Test for an intermittent FRP sensor signal by wiggling the harness between the sensor and the engine control module (ECM) with the ignition ON and the engine OFF, while monitoring the parameter with a scan tool. Observe the FRP sensor and the fuel pressure regulator commanded percent at idle. FRP should be near 40 MPa, and commanded percent should be near 40 percent. If the command is high, a fuel pressure or sensor exists. Inspect for an intermittent crankshaft position (CKP) sensor signal. Refer to Intermittent Conditions . Inspect the CKP sensor reluctor wheel. Remove the CKP sensor and attempt to move the reluctor wheel front to back or side to side with a probe. If there is any movement the reluctor wheel retaining bolts are loose.
Additional Inspection	Review the service bulletins for control module software updates. Compare the scan tool data at idle with the scan tool data list. Refer to Scan Tool Data List . Inspect the generator output voltage. Refer to Charging System Test (CS/AD/SI Generators) in Engine Electrical.