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Home › Saturn › 1998 › SL2 Automatic, MP6 › Repair and Diagnosis › Engine Performance › System › Engine Controls - Tests W/Codes › Diagnostic Trouble Codes › DTC P0300 - Random Cylinder Misfire Detected › Diagnostic Procedures

Diagnostic Procedures

  1. Connect scan tool. Start engine and operate for one minute. Check if any active misfire DTC(s) are present. If DTC(s) are present, go to next step. If DTC(s) are not present, problem is intermittent. See DIAGNOSTIC AIDS.
  2. If DTC P0300 is set with no other active injector fault or EI system DTC, go to next step. If other DTC(s) are present, diagnose affected DTC(s) first. Check for cause of misfire.
  3. Check fuel grade and condition (i.e. water). Start engine and check for vacuum leaks or arching spark plug wires. Repair as necessary. If no vacuum leak or faulty spark plug wires are found, go to next step.
  4. Using scan tool, note current misfire counts. Run engine for one minute. If misfire counts for cylinders No. 2 and 3 or cylinders No. 1 and 4 are dominant, go to next step. If counts on either cylinders are not dominant, go to step 7).
  5. Remove spark plus on misfiring cylinders. Check for spark using a spark tester. If spark is consistent, go to next step. If spark is not consistent, check coil secondary resistance. Resistance should be 700-10,000 ohms at 70°F (21°C). If resistance is not as specified, replace coil and retest engine. If resistance is as specified, check for problems in wires, coil pack or module. See DIAGNOSTIC AIDS. Also check for open or short in circuits to EI module terminals "C" and "E".
  6. Perform compression test. Normal compression should be 185-205 psi (13-14 kg/cm2 ). If compression is okay, replace spark plugs and retest engine. If compression is not okay, check for internal mechanical engine problem. Repair as necessary.
  7. Perform fuel pressure test. Fuel pressure should be 38-44 psi (2.7-3.0 kg/cm2 ). If fuel pressure is as specified, go to next step. If fuel pressure is not as specified, diagnose fuel system. See SYSTEM/COMPONENT TESTS article.
  8. Perform fuel pressure leak down test. Reprime fuel rail and wait 5 minutes. Maximum leak down should be 3 psi (.2 kg/cm2 ). If pressure is holding, go to next step. If pressure is not holding, check for leaking injector and/or fuel pressure regulator. Replace as necessary.
  9. Perform fuel injector balance test. See SYSTEM/COMPONENT TESTS article. Prime fuel rail after each injector is tested. Note fuel pressure gauge drop. If pressure drop is consistent within 1.5 psi (.1 kg/cm2 ), go to next step. If pressure drop is not consistent, check injector resistance. Resistance should be 12 ohms. If resistance is as specified, check for stuck injector pintle valve or clogged injector. Clean or replace injector. If injector resistance is not as specified, replace injector and retest engine.
  10. Remove wires from spark plugs and test for spark on each cylinder. If spark is consistent on all cylinders, go to next step. If spark is not consistent, check for faulty wire, coil pack and/or module. Repair or replace as necessary. See DIAGNOSTIC AIDS.
  11. Perform compression test on all cylinders. Normal compression should be 185-205 psi (13-14 kg/cm2 ). If compression is okay, replace spark plugs and retest engine. If compression is not okay, check for internal mechanical engine problem. Repair as necessary.
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