DEFINITION:
Fuel ignites in intake manifold or in exhaust system, making a loud popping sound.

Perform careful visual and physical check as described at the beginning of Section "B".
Perform "Diagnostic Circuit Check."

• Check
  • EGR operation for valve being open all the time. Use CHART C-7.
  • Intake manifold gasket for leaks
  • 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. Look for wear, wetness, cracks, improper gap, burned electrodes, and heavy deposits.
  • Ignition coil connections
  • Ignition system for moisture, dust, cracks, burns, etc. Spray plug wires with fine water mist to check for shorts.
  • ECM and ignition system for faulty grounds
  • Secondary ignition voltage output with J 26792 (ST-i 25) spark tester or equivalent.
  • For vacuum leaks at intake manifold gasket.
  • 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 ENGINE (SEC. 6A) in the 1991 Light Truck SIT Models Service Manual.