• NAXJA is having its 18th annual March Membership Drive!!!
    Everyone who joins or renews during March will be entered into a drawing!
    More Information - Join/Renew
  • Welcome to the new NAXJA Forum! If your password does not work, please use "Forgot your password?" link on the log-in page. Please feel free to reach out to [email protected] if we can provide any assistance.

98 4.0L Weird idle?

Chris2552

NAXJA Forum User
Location
West Virginia
I bought a 1998 Cherokee Classic recently and it's giving me one heck of a headache. The Jeep will idle for a minute, then fall flat on its face. I've replaced the O2 Sensors, IAC, tried a different ECM, plugs and wires, TPS, and the MAP Sensor. I've cleaned a ton of grounds but still no luck. When I check for codes, I get P1391, I've replaced the CPS with 2 off of other jeeps and bought a new one, still no luck. It seems like it has a really bad misfire, but goes away in the higher RPMS. I've checked the fuel pressure and did a compression test, they checked out good. Also checked for vacuum leaks, didn't find any. I've ran out of things to check:helpme:
 
Timing chain? Distributor gear/shaft play?

Did you test grounds, shorts, signal, supply?
Test cmps, visual inspection as well, RCP on here recently got lucky finding his P1391, just by visually inspecting.


I'm in the same boat kinda, but I haven't replaced sensors yet lol, might find out this week.

I do advise taking it to get a diagnostic for lab scope reading, it's very informative and can point out a lot of issues meters can't.

Also a recent recommendation I've read it take fuel pressure reading under load, might change. Meh


Keep us posted!
 
Only hint I see in that is that it idles okay "for a minute". Could it be an issue once the motor warms up and the ECM goes into closed loop?



Do you notice a change at a particular engine temperature or does it die fairly quickly after starting?
 
Solve the P1391 code. I would suspect the Cam sensor synchronization is out of tolerance. A mis-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.


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

Possible causes
- Faulty Camshaft Position (CMP) Sensor
- Camshaft Position (CMP) Sensor harness is open or shorted
- Camshaft Position (CMP) Sensor circuit poor electrical connection.
-CMP SENSOR OUT OF SYNC
-DAMAGED TONE WHEEL/FLEX PLATE (CRANKSHAFT)
-DAMAGED TONE WHEEL/PULSE RING (CAMSHAFT)
-IRREGULAR LAB SCOPE PATTERN OF CMP SIGNAL
-INTERMITTENT CKP SIGNAL LOSS WHEN WIRING IS WIGGLED
-IRREGULAR LAB SCOPE PATTERN OF CKP SIGNAL
-WIRING HARNESS INTERMITTENT
-INTERMITTENT CMP SIGNAL LOSS WHEN WIRING IS WIGGLED
-CRANKSHAFT POSITION SENSOR CONNECTOR/WIRING
-FAULTY CRANKSHAFT POSITION SENSOR


Potential Fixes:
Fix #1:
Inspect your wire harnesses for 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:*
Timing may off. Re-index the distributor/cam sensor per the FSM instructions, or replace faulty distributor/cam sensor pickup module.
*
Google Scan Tools, O-Scopes and Toothpicks, and read the article.

Read this also : http://www.jeepforum.com/forum/f19/j...e-fix-3487474/

A mis-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.
 
Last edited:
Thank you everyone for the advice! I forgot to mention that the engine has been swapped in the past with a 1996 engine. During inspection, I found a bolt missing and a very loose bolt in the top of the bellhousing. Do you think it would cause the CPS to loose sync when the engine vibrates or moves?
 
CkPS

keep an extra in the glove box
 
Thank you everyone for the advice! I forgot to mention that the engine has been swapped in the past with a 1996 engine. During inspection, I found a bolt missing and a very loose bolt in the top of the bellhousing. Do you think it would cause the CPS to loose sync when the engine vibrates or moves?

Abso-damn-lutely
 
I found a bolt missing and a very loose bolt in the top of the bellhousing. Do you think it would cause the CPS to loose sync when the engine vibrates or moves?

The AW-4 has 2 locating pins and about 8 bolts. A few loose or missing bolts would be very unlikely have an effect upon the CranKshaft Position Sensor.
 
Probably not your problem, but if your battery is on the way out your car won't be able to hold idle without stalling out. Happened to my '98 and a new battery solved it (it was still starting fine).
 
Back
Top