Hesitation, Sag, Stumble. Chevrolet Impala 2000

For 1990-2009 cars only

Makes 🢒 Chevrolet 🢒 2000 🢒 Impala 🢒 Engine 🢒 Engine Controls - 3.4L 🢒 Hesitation, Sag, Stumble

Inspection/Test

Action

DEFINITION: Momentary lack of response as the accelerator is pushed down. Can occur at any vehicle speed. Usually more pronounced when first trying to make the vehicle move, as from a stop sign. May cause the engine to stall if severe enough.

Preliminary

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

•	Refer to Intermittent Conditions .

•	Search for bulletins.

Sensor/System

Notice: Contamination of the oxygen sensor can result from the use of an inappropriate RTV sealant (not oxygen sensor safe) or excessive engine coolant or oil consumption. Remove the HO2S and visually inspect the portion of the sensor exposed to the exhaust stream in order to check for contamination. If contaminated, the portion of the sensor exposed to the exhaust stream will have a white powdery coating. Silicon contamination causes a high but false HO2S signal voltage (rich exhaust indication). The control module will then reduce the amount of fuel delivered to the engine, causing a severe driveability problem. Eliminate the source of contamination before replacing the oxygen sensor.

•

Monitor the heated oxygen sensors (HO2S). The heated oxygen sensors should respond quickly to different throttle positions. If not, inspect the HO2S for silicon or other contaminants from fuel or the use of an improper RTV sealant. The sensors may have a white powdery coating. Silicon contamination causes a high, but false, HO2S signal voltage and a rich exhaust indication. The PCM will reduce the amount of fuel delivered to the engine, causing a severe driveability problem.

•

Monitor the 24X crankshaft position (CKP) sensor and the camshaft position (CMP) sensor on the scan tool. If neither sensors is responding, test the sensor feed circuit. Both sensors use a separate feed circuit but are internally connected to power. Test all of the 24X and the CMP sensor circuits for intermittents. Refer to Testing for Intermittent Conditions and Poor Connections in Wiring Systems.

•

Monitor the 3X parameter on the scan tool. If the 3X is not responding, inspect the 7X CKP sensor and the circuits for intermittents. Check the ignition control (IC) circuit, the bypass circuit, the 3X reference high circuit, and the 3X reference low circuit for intermittents. If these circuits become open or shorted, the circuits may not set a DTC immediately, but are capable of causing driveability complaints. Refer to Testing for Intermittent Conditions and Poor Connections in Wiring Systems.

•	Inspect the throttle position (TP) sensor and the related wiring.

•	Inspect the mass air flow (MAF) sensor and the related wiring. Inspect the air intake ducts for collapsing, damaged areas, looseness, improper installation, or leaking, especially between the MAF sensor and the throttle body.

•	Test the exhaust gas recirculation (EGR) system for proper operation. Refer to Exhaust Gas Recirculation (EGR) System Description .

Fuel System

•	Test the fuel pressure. Refer to Fuel System Pressure Test .

•	Test for contaminated fuel. Refer to Alcohol/Contaminants-in-Fuel Diagnosis .

•	Test the fuel injectors. Refer to the following procedures:

•	Fuel Injector Balance Test

•	Fuel Injector Solenoid Coil Test - Engine Coolant Temperature Between 10-35 Degrees C (50-95 Degrees F)

•	Fuel Injector Solenoid Coil Test - Engine Coolant Temperature Outside 10-35 Degrees C (50-95 Degrees F)

•	Inspect any items which may cause an engine to run rich. Refer to the Diagnostic Aids for DTC P0172.

•	Inspect any items which may cause an engine to run lean. Refer to the Diagnostic Aids for DTC P0171.

Ignition System

•	Inspect for the proper ignition voltage output using the following steps:

1.	Attach the J 26792 spark tester to engine ground.

2.	Connect the spark plug end of the spark plug wire to the J 26792 spark tester. Leave the other end of the spark plug wire connected to the coil being tested.

3.	Connect the spark plug end of the companion spark plug wire to ground.

The companion spark plug wire is the wire attached to the corresponding coil tower.

4.	Crank the engine while observing the J 26792 spark tester. A crisp blue spark should be observed.

5.	Repeat the above steps for each coil.

•	If spark is not present at the coils, inspect for the following conditions:

-	Check the coils for cracks, carbon tracking or arcing, or a resistance value outside the specified range.

Coil Resistance
5000-8000 ohms

-	Check the spark plug wires for signs of arcing, cross firing, cracks, carbon tracking, plug boot damage, pinching, improper routing, or a resistance value outside the specified range.

Spark Plug Wire Resistance
1 968 ohms per meter (600 ohms per foot).

Important: : Spraying the secondary ignition wires with a light mist of water may help locate an intermittent problem. The ignition voltage will arc to ground when a secondary component is faulty.

-	Check for a faulty ignition module.

-	Check the ignition system wiring for a loose ignition module feed or ground connection, or damaged system wiring.

•	Remove the spark plugs and inspect for the following conditions:

Fouling

Cracks

Wear

-	Improper gap

-	Burned or damaged electrodes

-	Improper heat range or reach

•	If the spark plugs are gas or oil fouled, the cause of the fouling must be determined before replacing the spark plugs. Refer to Spark Plug Visual Diagnosis in Engine Electrical.