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 Section "B".
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 "Ingition System Check" Chart in Section "C4.''
  • Secondary ignition voltage output with J 26792 (ST-i 25) spark tester or equivalent.
  • Ignition coil connections
  • ECM and ignition system for faulty grounds
  • Proper operation of EST See Section "C4".
  • Spark plugs. Look for wear, wetness, cracks, improper $ap, 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.
  • EGR operation. Use Code 32 chart.
  • 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. Use CHART C-3.
  • 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 Cl- D and/or check sensor by comparing it to the output on a similar vehicle, if possible.
  • 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-i 0.
  • 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-i A.
  • Generator output voltage. Repair if less than 9 volts or more than 16 volts.
  • Transmission torque converter operation. See AUTOMATIC TRANSMISSION (Section 7A1) in the 1991 Light Truck S/T Models Service Manual.
  • Transmission for proper shift points. See AUTOMATIC TRANSMISSION (Section 7A1) in the 1991 Light Truck S/T Models Service Manual.
  • Transmission torque converter clutch operation. Use CHART C-8.
  • Vehicle brakes for dragging.
  • Engine valve timing and compression. Refer to ENGINE (SEC. 6A) in the 1991 Light Truck S/T Models Service Manual.
  • 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 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 4.3L V6 (SEC. 6A3) in the 1991 Light Truck S/T Models Service Manual.
  • 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.

• If the system is running RICH (block learn less than 118), refer to "Diagnostic Aids" on facing page of Code 45.
• If the system is running LEAN (block learn greater than 138), refer to "Diagnostic Aids" on facing page of Code 44.