1999 Jeep Wrangler Backfiring through intake when cold

  • 6 CYL
  • 4WD
  • 120,000 MILES
My 99 Jeep Wrangler backfires through the intake only when cold and only when accelerating. It has 120k miles on it. I had it tuned recently (first time) mainly because it had never been tuned and it was runnign rough hot or cold. The tune up made a huge difference, but now I get this back fire when cold. I assume it is a fuel mixture problem and probably not caused by the engine control chip, but possibly by the O2 sensor. Am I on the right track? What else could it be?
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have the same problem?
Tuesday, December 30th, 2008 AT 6:53 AM

1 Reply

Verify the Manifold Absolute Pressure sensor and Crankshaft Position Sensor are operating correctly.

Manifold Absolute Pressure (MAP) Sensor Test

Inspect the MAP sensor vacuum hose connection at the throttle body and sensor, then repair as necessary.
Test the MAP sensor output voltage at the sensor connector terminal B (as marked on the sensor body) with the ignition switch ON and the engine OFF. The output voltage should be 4-5 volts. The voltage should drop to 0.5-1.5 volts with a hot, neutral idle speed condition.
Test engine controller terminal 5 for the same voltage as in Step 2 to verify the wire harness condition. Repair any problems, as necessary.
Test the MAP sensor supply voltage at the sensor connector terminal C with the ignition ON. The voltage should be 4.5-5.5 volts. The same voltage should be present at terminal 6 of the engine controller wire harness connector. Repair or replace the wire harness, as necessary. If the engine controller is suspect, use Diagnostic Tester M.S.1700, or equivalent, to test engine controller function.
Test the MAP sensor ground circuit at the sensor connector terminal A and engine controller connector terminal 4. Repair the wire harness as necessary.
Test the MAP sensor ground circuit at the engine controller connector between terminal 4 and terminal 11 with an ohmmeter. If the ohmmeter indicates an open circuit, check for a defective sensor ground connection located on the right side of the cylinder block. If the ground connection is good, replace the engine controller.

Crankshaft Position Sensor Test

Disengage the speed sensor connector from the ignition control module, then install an ohmmeter between terminals A and B as marked on the connector. The ohmmeter should read 125-275 ohms on a hot engine for 1991 vehicles and no resistance for 1992-95 vehicles. Replace the sensor if the readings are not as stated.
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Monday, January 5th, 2009 AT 10:47 PM

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