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HOMESERVICE MANUALSGMC1999FORWARD CONTROL 5.7 R, AUTOMATICREPAIR AND DIAGNOSISEXTERNAL PAGESDIFFERENT CARSECTION 601 (ENGINE CONTROLS - 3.6L (LY7) (TROUBLESHOOTING & DIAGNOSIS))TROUBLESHOOTINGBACKFIRE
1999 GMC Forward Control 5.7 R, Automatic

Section 601 (Engine Controls - 3.6L (Ly7) (Troubleshooting & Diagnosis)): Troubleshooting: Backfire

1999 GMC Forward Control 5.7 R, AutomaticSECTION Backfire
WARNING: This page is about a different car, the 2004 Buick Rendezvous. However, it is still accessible from the selected car via links, so may be relevant.
Backfire

Inspections Actions
DEFINITION: Fuel ignites in the intake manifold or in the exhaust system, making a loud popping noise.
Preliminary
  • Refer to Important Preliminary Inspections Before Starting in Symptoms - Engine Controls .
  • Search for bulletins.
  • Inspect the engine control system grounds for being clean, tight, and in the correct locations. Refer to Power and Grounding Component Views in Wiring Systems and Engine Controls Schematics . Ensure the resistance between the engine control module (ECM) housing and the battery negative cable is less than 0.5 ohms.
  • Verify that the ECM and ECM bracket fasteners are at the correct torque.
Fuel System
Sensor/System
Ignition System
  • Test for the correct ignition voltage output with a J 26792  Spark Tester. If you suspect an ignition system condition, refer to Electronic Ignition (EI) System Diagnosis .
  • Remove the spark plugs and inspect for the following conditions:
  • If the spark plugs are gas, coolant, or oil fouled, determine the cause before replacing the spark plugs. Refer to the following procedures:
    • For diagnosis of a rich condition, refer to DTC P2178 or DTC P2188 .
    • For diagnosis of coolant contaminated spark plugs, refer to Loss of Coolant in Engine Cooling.
    • For diagnosis of oil fouled spark plugs, refer to Oil Consumption Diagnosis in Engine Mechanical - 3.6L (LY7).
    • Inspect the crankshaft position (CKP) sensor for the correct resistance. The sensor resistance may be out of range after a hot soak. Measure that the resistance remains within 700-1,200 ohms at all temperatures.
Engine Cooling System
Engine Mechanical
Additional
  • Inspect the vacuum hoses for splits and kinks. Verify that the connections and routing are as shown on Vehicle Emission Control Information. Refer to Emission Hose Routing Diagram .
  • Inspect the intake manifold and the exhaust manifold passages for casting flash.
  • Test the transmission torque converter clutch (TCC). The scan tool should indicate a drop in engine speed when the TCC is commanded ON. Refer to Torque Converter Diagnosis Procedure in Automatic Transaxle - 4T65-E.
  • Inspect the following exhaust system components:
    • The exhaust system for damage
    • The exhaust manifold for a collapsed inner wall.
    • The mufflers for heat distress or possible internal failure
    • The three-way catalytic converters for restrictions

      For more information, refer to Symptoms - Engine Exhaust in Engine Exhaust.

  • Electromagnetic interference (EMI) on the CKP sensor signal circuits can cause an engine misfire condition. You can usually detect EMI with a scan tool by monitoring the engine speed parameter. A sudden increase in the engine speed parameter, with little change in actual engine speed, indicates that EMI is present. Inspect for high voltage components near the ignition control circuits if a condition exists. Also, inspect the engine ground shield of the CKP sensor circuits.
  • Test the park neutral position (PNP) switch circuit. Refer to DTC P0850 (3.6L) in Automatic Transaxle - 4T65-E.
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When to See a Mechanic

Stop DIY work and contact a certified mechanic immediately if any of the following apply:

  • • You smell fuel, burning insulation, or see smoke.
  • • Brakes feel soft, pull hard to one side, or make grinding noises.
  • • The engine overheats, stalls repeatedly, or misfires under load.
  • • You are missing required tools, torque specs, or safe lifting equipment.
  • • You are not confident in the next step or safety outcome.