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HOMESERVICE MANUALSGMC1999FORWARD CONTROL 5.7 R, AUTOMATICREPAIR AND DIAGNOSISEXTERNAL PAGESDIFFERENT CARSECTION 83 (ENGINE CONTROLS - SELF-DIAGNOSTICS)DIAGNOSTIC TESTSDTC P0300: ENGINE MISFIRE DETECTEDDIAGNOSTIC PROCEDURES
1999 GMC Forward Control 5.7 R, Automatic

Diagnostic Procedures

1999 GMC Forward Control 5.7 R, AutomaticSECTION Diagnostic Procedures
WARNING: This page is about a different car, the 2002 Saturn LW200, 2002 Saturn L200, and 2002 Saturn L100. However, it is still accessible from the selected car via links, so may be relevant.
  1. Ensure fuel level is more than 10 percent. Connect scan tool. Operate engine for one minute while monitoring scan tool MISFIRE CURRENT CYLS #1-4. If cylinders No. 1 through 4 are randomly filling up, go to next step. If paired cylinders No. 1 and 4, or cylinders No. 2 and 3 misfire counters dominant, go to step  3 .
  2. If a specific cylinder misfire is dominant, see DTC P0301-P0304: CYLINDERS NO. 1-4 MISFIRE DETECTED  . If a specific cylinder misfire is not dominant, problem is intermittent. See DIAGNOSTIC AIDS  .
  3. If paired cylinders No. 1 and 4 or cylinders No. 2 and 3 are dominate, go to next step. If misfire is random, check the following for possible causes:
    • Large vacuum leak.
    • Fuel pressure (cycle ignition 3 times) 50-60 psi (345-414 kPa).
    • Leaking injectors (injector leakdown test) maximum 8 psi leakdown within 5 minutes.
    • Clogged fuel injectors (injector balance test) plus or minus 1.5 psi (10 kPa).
    • Fouled plugs or incorrect gap. Gap should be 0.45" (1.1 mm).
    • Sticking valves.
  4. Turn ignition off. Remove EI module from ignition coil housing and install interconnect to EI module and ensure connection is tight. Connect Noid Light (J-34730-2A) between primary ignition coil voltage pin (Pink wire) and White wire (cylinders No. 1 and 4) or Orange wire (cylinders No. 2 and 3) at EI module interconnect. Crank engine. If noid light pulses, check connector at ignition coil housing. If connector is okay, replace ignition coil housing. If noid light does not pulse, go to next step.
  5. Turn ignition off. Disconnect PCM connector J1. Turn ignition on. Touch test light between battery voltage and PCM connector J1 terminal No. 26 (White wire) for cylinders No. 1 and 4, or terminal No. 25 (Orange wire) for cylinders No. 2 and 3, for one second. If noid light pulses when circuit is connected to battery voltage, check PCM connector J1. If connector is okay, replace PCM. If noid light does not pulse when circuit is connected to battery voltage, go to next step.
  6. Disconnect EI module connector. Check continuity on White wire (cylinders No. 1 and 4) or Orange wire (cylinders No. 2 and 3) between EI module and PCM. If continuity does not exist, repair open in appropriate circuit. If continuity exists, go to next step.
  7. Check continuity between ground and PCM connector J1 terminal No. 26 (White wire) for cylinders No. 1 and 4, or terminal No. 25 (Orange Wire) for cylinders No. 2 and 3. If continuity exists, repair short to ground in White wire or Orange wire. If continuity does not exist, go to next step.
  8. Reconnect PCM connector J1. Turn ignition on. Measure voltage on EI module connector terminal "B" (White wire) for cylinders No. 1 and 4, or terminal "G" (Orange wire) for cylinders No. 2 and 3. If voltage is present, repair short to voltage on White wire or Orange wire. If voltage is not present, check EI module connector. If connector is okay, replace EI module.
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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.