DEFINITION:
A mild to severe ping, usually worse under acceleration. The engine makes sharp metallic knocks that change with throttle opening.

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
  • For possibility of misfiring, crossfiring, or cutting out under load or at idle. If present, refer to the "Ignition System Check" Chart in Section "C4."
  • Spark plugs for correct heat range.
  • Ignition timing. Refer to underhood emissions label for correct procedure.
  • Fuel for poor quality, "stale" fuel, and water contamination
  • Fuel system for restricted filter or improper pressure. Use CHART A-7.
  • For excessive oil entering combustion chamber. Oil will reduce the effective octane of fuel.
  • EGR operation. Use Code 32 chart.
  • For vacuum leaks at intake manifold gasket
  • 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 valves are correct part. Also check PCV hoses.
  • MAP sensor output. Use CHART C1-D and/or check sensor by comparing it to the output on a similar vehicle, if possible.
  • Coolant sensor for a shift in calibration
  • 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
  • Vehicle Speed Sensor (VSS) input with a "Scan" tool to make sure reading of VSS matches that of vehicle speedometer. See "Diagnostic Information" in Section "6E."
  • Transmission torque converter operation. Refer to AUTOMATIC TRANSMISSION (SEC. 7A1) in the 1991 Light Truck SIT Models Service Manual.
  • Transmission for proper shift points. Refer to AUTOMATIC TRANSMISSION (SEC. 7A1) in the 1991 Light Truck SIT Models Service Manual.
  • Transmission torque converter clutch operation. Use CHART C-8.
  • Vehicle brakes for dragging.
  • PROM for correct application. (Consult Service Bulletins.)
  • 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.
    
    
  • Engine valve timing and compression. Refer to ENGINE (SEC. 6A) in the 1991 Light Truck SIT Models Service Manual.
  • 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 4.3 LITER V6 (SEC. 6A3) in the 1991 Light Truck SIT 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 2500 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.
  
  
• Remove internal engine carbon with top engine cleaner.
• If the system is running LEAN, (block learn greater than 138), refer to "Diagnostic Aids" on facing page of Code 44.