Hesitation, Sag, Stumble

Definition: Momentary lack of response as the accelerator is pushed down. Can occur at all vehicle speeds. Usually most severe when first trying to make the vehicle move, as from a stop sign. May cause the engine to stall if severe enough.

Perform careful visual and physical check as described at the beginning of Symptoms.

Perform "Diagnostic Circuit Check."

^ CHECK
- Ignition wires for shorts or faulty insulation
- Ignition system for moisture, dust, cracks, burns, etc. Spray plug wires with fine water mist to check for shorts.
- For possibility of misfiring, crossfiring, or cutting out under load or at idle. If present, refer to the "Ignition System Check" Chart. Chart C-4C Ignition System Check (Remote Coil)
- Secondary ignition voltage output with J 26792 (ST-125) spark tester or equivalent.
- Ignition coil connections.
- ECM and ignition system for faulty grounds.
- Proper operation of EST.
- Spark plugs. Look for wear, wetness, cracks, improper gap, burned electrodes, and heavy deposits.
- Spark plugs for correct heat range.
- Fuel for poor quality, "stale" fuel, and water contamination
- Fuel system for restricted filter or improper pressure. Use CHART A-7. Chart A-7 Fuel System Pressure Test
- EGR operation. Use Code 32 chart. Powertrain Controls - ECM/PCM
- For vacuum leaks at intake manifold gasket.
- Air cleaner element (filter) for dirt or plugging.
- Idle Air Control system. Use Code 35 chart.
- Check electrical system voltage. IAC valve will not move if voltage is below 9 volts or greater than 17.8 volts. Also check battery cables and ground straps for poor contact. Erratic voltage will cause the IAC valve to change its position, resulting in poor idle quality.
- PCV valves for proper operation by placing finger over inlet hole in each valve end several times. Valve should snap back. If not, replace valve. Ensure that valve is correct part. Also check PCV hoses.
- Canister purge system for proper operation.
- PROM for correct application (Consult Service Bulletins.).
- Throttle shaft or TPS for sticking or binding TPS voltage should read less than 1.25 volts on a "Scan" tool with the throttle closed.
- MAP sensor output. Use CHART C1-D and/or check sensor by comparing it to the output on a similar vehicle, if possible. Chart C-1D Manifold Absolute Pressure (MAP) Output Check
- Oxygen sensor for silicone contamination from contaminated fuel or use of improper RTV sealant. The sensor will have a white, powdery coating and will cause a high but false signal voltage (rich exhaust indication).
- The ECM will reduce the amount of fuel delivered to the engine, causing a severe driveability problem.
- Coolant sensor for a shift in calibration. Use Code 14 or Code 15 chart.
- A/C relay operation. A/C should cut out at wide open throttle. Use CHART C-10.Chart C-10A A/C Clutch Control Diagnosis
- A/C refrigerant pressure for high pressure. Check for overcharging or faulty pressure switch.
- P/N switch circuit on vehicle with automatic transmission. Use CHART C-1A. Chart C-1A Park/Neutral Switch Diagnosis
- Generator output voltage. Repair if less than 9 volts or more than 16 volts.
- Transmission torque converter operation. See TRANSMISSION. Transmission and Drivetrain
- Transmission for proper shift points. See TRANSMISSION. Transmission and Drivetrain
- Transmission [1][2]torque converter clutch operation. Use CHART C-5. Chart C-5 Electronic Spark Control (ESC) Check
- Vehicle brakes for dragging.
- Engine valve timing and compression. Refer to ENGINE. Engine
- For the possibility of an exhaust restriction or improper valve timing by performing the following test.
1. With engine at normal operating temperature, connect a vacuum gauge to any convenient vacuum port on intake manifold.
2. Run engine at 1000 rpm and record vacuum reading.
3. Increase engine speed slowly to 2500 rpm. Note vacuum reading at steady 200 rpm.
4. If vacuum at 2500 rpm decreases more than 3" Hg from reading at 1000 rpm, the exhaust system should be inspected for a restrictions.
5. Disconnect exhaust pipe from engine and repeat Steps 3 & 4. If vacuum still drops more than 3" Hg with exhaust disconnected, check valve timing.
- For worn or incorrect basic engine parts such as cam, heads, pistons, etc. Also check for bent pushrods, worn rocker arms, and broken or weak valve springs. Refer to ENGINE. Engine
- For overheating and possible causes. Look for the following.
- Low or incorrect coolant solution. It should be a 50/50 mix of GM #1052753 anti-freeze coolant (or equivalent) and water.
- Loose accessory drive belt.
- Restricted air flow to radiator, or restricted water flow through radiator.
- Faulty or incorrect thermostat.
- Faulty fan clutch. See CHART C-12. Chart C-12 Charge Air Cooler (CAC) Pump
^ If the system is running RICH (block learn less than 118), refer to "Diagnostic Aids" of Code 45. Powertrain Controls - ECM/PCM
^ If the system is running LEAN (block learn greater than 138), refer to "Diagnostic Aids" of Code 44. Powertrain Controls - ECM/PCM