99' Wrangler Sport Bucking and Shaking - JeepForum.com
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post #1 of 26 Old 05-13-2019, 11:24 AM Thread Starter
diegoa1997
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99' Wrangler Sport Bucking and Shaking

I have a 1999 Jeep Wrangler Sport 4.0L 6Cyl. Whenever I am accelerating from a stop it will start bucking and shaking like if it was going to stall out. This only happens when accelerating from a full stop. When I first turn the car on in the morning, it accelerates fine for the first few stop signs and red lights but then it starts doing it at every single stop. Once it picks up speed the ride is smooth and has no issues. The check engine light came on the other day but turned off before I got home to check it. When I checked it, it gave me a stored code of P1391. When I read online I see that this can mean the crankshaft and camshaft sensors which I already changed along with the distributor cap, rotor, cables and spark plugs. Any suggestions as to what the problem could be?

Thank you,

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post #2 of 26 Old 05-13-2019, 12:05 PM
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Unplug your upstream O2 sensor and take the Jeep for a drive. If the bucking and shaking problem goes away, replace the upstream sensor. You will almost certainly get a code and CEL illuminated with the sensor disconnected, ignore it for this test. Your upstream sensor is the one located closest to the exhaust manifold.

The first couple minutes of operation has your fuel injection working in open loop, ie, it ignores the O2 sensors. Once the engine and O2 sensors warm up sufficiently, it swaps over to closed loop where it then uses the O2 sensor signal(s) to adjust the mixture. It's common on earlier model TJs for the oxygen sensors to not trip a fault code when they go bad.

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post #3 of 26 Old 05-13-2019, 12:20 PM Thread Starter
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Thank you, I will give this a shot.
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post #4 of 26 Old 05-13-2019, 12:28 PM
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A quick note to add: if you do end up replacing your O2 sensor, be sure to only use an NGK/NTK brand sensor (they were the OEM part). Our Jeeps have shown a tendency to not do well with Bosch or most other brands.

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post #5 of 26 Old 05-13-2019, 12:43 PM Thread Starter
diegoa1997
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I see 4 sensors here and two of them are upstream. Which one should I unplug?
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post #6 of 26 Old 05-13-2019, 12:50 PM
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Since you're blessed with the Cailfornia-only '99 setup, you'll want to unplug both upstream sensors. Unfortunately unplugging one at a time is unlikely to narrow it down any further to a specific sensor as I'm pretty sure the PCM will ignore both signals if one is unplugged. You can either replace both if unplugging them clears the issue or you can make an educated guess as to which is the culprit by reading your spark plugs to see which bank has evidence of running an improper mixture. Make sure you take a close look at your connections for the two mini cats at the manifold, as leaks here can sometimes give false readings from the O2 sensors.

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post #7 of 26 Old 05-13-2019, 03:12 PM
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The 1391 code is of more concern to me. It indicates an intermittent loss of signal either from the cam or crank sensor. Not necessarily a bad sensor but can be a wiring problem too. Another known problem can be in the distributor where the pulse ring (half circle ring) can become loose on the shaft or the shaft may wobble from worn distributor bushings. I would remove the cam sensor & inspect those 2 things. Also note if there is any sign of the ring rubbing on the sensor (including the old one if you still have it).
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post #8 of 26 Old 05-14-2019, 07:44 AM Thread Starter
diegoa1997
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Unplugged the o2 sensor yesterday. Went for a drive and at the beginning it was fine as usual. After the engine warmed up, the check engine light came on which makes sense based on what mukluk said but then the bucking started just like always :/.

Should I replace the sensor anyways?
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post #9 of 26 Old 05-14-2019, 07:53 AM Thread Starter
diegoa1997
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I unplugged the sensor yesterday and took it for a drive. Check engine light came on after a little while because of the sensor but the car still start bucking like always. Should I still replace the sensor.

I will inspect the distributor again.
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post #10 of 26 Old 05-14-2019, 08:12 AM
jtec
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The p1391, you mentioned it was "stored" is the code current or in history?
Lets start with a fresh SCAN and post all codes...
tell us do you have a SCANNER, able to see live data?

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post #11 of 26 Old 05-14-2019, 08:26 AM
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You are using genuine Jeep OBD sensors, and not the cheap crappy Chinese clones from the auto parts store ?


The most common root cause is the magnet has separated from the tone wheel on the Cam Sensor. Wiring issues/damage would be the next suspect.



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/MAGNET (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 : https://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.

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post #12 of 26 Old 05-14-2019, 11:34 AM
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Quote:
Originally Posted by diegoa1997 View Post
Unplugged the o2 sensor yesterday... the bucking started just like always :/.

Should I replace the sensor anyways?
Nope, don't replace the O2 sensor(s) because the engine still exhibited bucking even with them disconnected (they were taken out of the equation when unplugged).

The P1391 code is currently the only thing we have to go with, but the behaviour you describe doesn't really match up with what one normally gets with that code, being a bucking only when accelerating from a full stop. I would recommend starting with a clean slate: either clear all codes with a scanner or disconnect your battery for a few minutes, then afterwards drive it normally to see if the P1391 or any other codes pop back up.

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post #13 of 26 Old 05-14-2019, 12:39 PM
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Has the CEL & P1391 been returning?

The CEL came on then went out & found the stored code. I assumed (can be a bad thing) that the CEL is coming on whenever it starts bucking. If not then that could have been a one time thing & unrelated to the problem.
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post #14 of 26 Old 05-15-2019, 08:08 AM Thread Starter
diegoa1997
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Cleared the codes yesterday. No code has come back. I checked the distributor and the cam sensor. I see a small scratch on the metallic part of the cam sensor. I can't remember if that was there when I installed it. I threw the old one away so I can't compare them. The rotor and the pulse ring have a little bit of rotation movement when I tug on them but it's very minimal like an 1/8" give.

Should I replace the whole distributor and get a new camshaft sensor?
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post #15 of 26 Old 05-15-2019, 08:33 AM
jtec
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some thoughts -
could the bucking be a trans/clutch issue and not an engine issue?
Not using AC to eliminate that slugging possibility?
Fuel pressure - the higher rpm in lower gears causing an issue?

Be sure to use a scanner to ck codes...

When I see the price of OEM I think aftermarket.
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