High Idle Issue

[the_mechanic]

I have one that is stumping me a bit on my 2000 XJ 4.0 that I just got back together. It starts up no problem but as soon as it starts, it idles high (usually around 1800-2000 rpm) and if your rev it up, it will stay revved up and sometimes slowly work its way back down to the 1800-2000 mark after a delay. Thinking it was just the computer relearning, I just let it run for a while and then it dropped RPMs and stalled out, throwing out a CEL in the process. I run the codes and I get P1391, P0351, P0352, and P0353. After it stalls, it will always start back up no problem and continue running for a decent amount of time before stalling again in a similar manner; I haven't been able to see any sort of correlation to time, temp, action, etc. that causes it to stall. It doesn't ever seem to run rough or anything, just idles fast and then revs down and dies.

I tried replacing the coil to no avail and am working on getting my hands on a new cam and crank sensor along with a new IAC (thinking that may be the high idle issue or part of it). I am not seeing any vacuum leaks. Any other ideas or anything that would be worth trying to test and/or replace?

 

[Tim_MN]

I would suggest some of the trouble codes, symptoms, and faults are unrelated.

It probably needs new Idle Air Controller, and you should test the Throttle Position Sensor.


P1391 - Intermittent Loss of CMP or CKP Signal. Loss of the Cam Position Sensor or Crank Position Sensor signal has occurred.

P1391 JEEP Possible Causes
- Faulty Crankshaft Position Sensor (CKP)
- Crankshaft Position Sensor (CKP) wire harness is open or shorted
- Crankshaft Position Sensor (CKP) wire circuit poor electrical connection
- Faulty Camshaft Position (CMP) Sensor
- Camshaft Position (CMP) Sensor wire harness is open or shorted
- Camshaft Position (CMP) Sensor wire circuit poor electrical connection.

P1391 – Possible symptoms:
(1) Random misfire/stumble/bucking/stalling, often over bumps or right hand turns or when going into reverse.
(2) Shorted out the Fuel pump/ECU power wire.
(3) A improperly indexed distributor/cam sensor may exhibit surging, light bucking or intermittent engine misfiring. This will most likely occur when the vehicle is at operating temperature and under a light load at approximately 2,000 rpm. The proper indexing procedure must be used as outlined in the service manual.

Often replacing the faulty sensor fixes the P1391 code, but not necessarily. Therefore it's important to check the following:
• Check that the Cam and Crank sensor wiring isn't routed too close to sharp metal edges, or to hot exhaust parts.
• Visually inspect the wiring at the sensor for any burn marks, bare wires, or discoloration indicative of melting / chafing of insulation, and other damages.
• Visually inspect the cam sensor for damage. Make sure the magnet is firmly attached. The sensor can be tested.
• Visually inspect the reluctor wheel inside the distributor or Cam Sensor.
• Inspect the distributor assembly for excessive wear.
• If everything checks out okay, then test the Crankshaft Position Sensor.

If you buy any OBD-II engine sensors, make sure to buy genuine Jeep sensors.
Potential Fixes:
Fix #1:
Inspect your O2 wire harnesses for bent or pushed back wire plugs pins, damaged wires, chafed insulation, or failed wire splices. Use a volts/ohms multi-meter to check the resistance of the ground wire circuits from each sensor. Replace corroded ground wires/connectors and clean ground wire connection points.

Fix #2:
The timing may off. Re-index the distributor/cam sensor per the FSM instructions, or replace faulty distributor/cam sensor pickup module.
Read this article: Scan Tools, O-Scopes and Toothpicks here >> https://web.archive.org/web/20200820085756/http://jeep.blackonyx.net/pdfs/jcss.pdf
Read this thread >> https://www.jeepforum.com/forum/f19...352-p0353-crank-cam-sensor-issue-fix-3487474/

Testing the Throttle Position Sensor and Coolant Temperature Sensor is also a good idea.