4.0 Won't Start; Throwing P0340

ShredAZ

ShredAZ

TJ Enthusiast
Joined
Aug 20, 2018
Messages
201
Location
Phoenix, AZ
Seems obvious eh? CMP most likely, but mine was replaced with an OEM unit a few years ago. I searched and had trouble finding folks with my exact issue.

Scenario:
This Jeep is a weekend toy. Stretches its legs once a week roughly.
In prep for a camping trip this weekend, I drove it a few extra days to smoke out any gremlins before taking it out,

Everything was working fine yesterday. Drove to work and back. Parked in my driveway. After dinner, fired it up and backed it into my garage where it sat all night.
When I attempted to start it this morning, it just cranked and cranked and cranked but the engine wouldn't start.

It had a CEL stored on the computer as of this morning. Just a P0340 and nothing else. Usually I see these accompanied by other CEL's or symptoms while driving (IE spurting, dying, bad idle, etc.). My jeep demonstrated none of that. It worked perfectly up until it refused to start the next morning.

It has a new battery (less than a few months) and the voltmeter seems to be reading fine.
I can feel the starter engage the flywheel, and I can feel it cranking/spinning just fine. Nothing feels sluggish. The engine just won't fire up.

Dropping it off at my mechanic on Monday, but wondered if anyone had some ideas I could poke around with over the weekend first.
 
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P0340 code definition​

Camshaft Position Sensor Circuit Malfunction

What the P0340 code means​

The camshaft position sensor calculates the rotational speed of the camshaft and where the camshaft is positioned in that rotation. The camshaft sensor sends a signal to the Power Control Module (PCM) to communicate this information. The PCM then uses the information collected by the camshaft position sensor to set the fuel injector timing and control the ignition spark.

When the signal to and from the camshaft position sensor and the PCM is broken, the ignition spark and fuel injector timing will fail and the PCM will store the P0340. This will trigger the illumination of the Check Engine Light.

What causes the P0340 code?​

  • A camshaft position sensor circuit wiring that is broken, shorted, or corroded
  • A camshaft position sensor circuit connector that is broken, shorted, or corroded
  • A faulty camshaft position sensor
  • A failed crankshaft position sensor
  • A faulty PCM

What are the symptoms of the P0340 code?​

  • The Check Engine Light will come on
  • The vehicle may not start, or may be difficult to start
  • The vehicle may idle rough or stall
  • The vehicle may misfire
  • There may be a loss of power while driving

How does a mechanic diagnose the P0340 code?​

  • Uses an OBD-II scanner to retrieve all of the trouble codes that have been stored by the PCM
  • Inspects the camshaft position sensor wiring for wires that are broken, corroded, or shorted
  • Inspects the camshaft position sensor connector for connectors that have broken or corroded
  • Inspects the camshaft position sensor circuit wiring for wires that are broken, corroded, or shorted
  • Checks the continuity of the camshaft position sensor circuitry
  • Completes any necessary repairs for other related trouble codes that have been stored by the PCM
  • Uses a scan tool or oscilloscope to check the camshaft position sensor voltage readings
  • If the voltage readings are abnormal, the camshaft position sensor will need to be replaced.
  • If the voltage readings for the camshaft position sensor voltage readings are normal, it is possible that there is a problem with the PCM which may require replacing or reprogramming.

Common mistakes when diagnosing the P0340 code​

Before replacing the camshaft position sensor, it is important to inspect the wiring and connectors to rule them out as the cause of the problem. This is the most common mistake when diagnosing the P0340 trouble code.

Another mistake that is made during the diagnostic procedure is neglecting to consider a misfiring problem or a crankshaft sensor problem as possible sources of the P0340 trouble code.

How serious is the P0340 code?​

This code is considered serious. The car may not start or be difficult to start. While operating the vehicle, the driver may also experience a lack of power. These symptoms make the operation of the vehicle dangerous for the driver and anyone else on the road. It is also possible for damage to be caused to other components in the engine if the P0340 trouble code goes unaddressed for an extended period of time. The P0340 trouble code should be diagnosed and repaired immediately.

What repairs can fix the P0340 code?​

 
Nimbley said:
You have no spark. Looks like you lost the CMP. Should be an inexpensive fix. Less than $100 if you replace it yourself.
Click to expand...
I always get the CMP and CPS confused. I had this assembly replaced a few years back:
1623454656648.png


The plug on top is effectively the CMP, correct?
 
Mopar CMP's are unobtanium these days. Guess they were discounted a few years back.

I've read folks having good luck with the Napa Echlin series CMP's. The sensors alone aren't cheap @ $60. I assume this is my best bet these days unless there are other recommendations?
 
Well I know I said I had the CMP and synchronizer replaced a few years ago. This was by my mechanic. I could have sworn I specified Mopar. Popped under the hood today, and to my dismay, there's a gosh dang Duralast under there. Very cool. I bet that's my problem based on what I've been reading.
 
(My now new) Mechanic called. Confirmed the synchronizer assembly failed. Big "L" for Duralast hah!
He also confirmed the Mopar part has been discontinued (like that needed confirming).

They recommended the Echlin Synchronizer and CMP (NAPA), and from what I've read, that seems to be your next solid bet to Mopar. They're also warranting the work for 5 years.

Aside from that, I did find a few of the Mopar CMP's floating around still, so I might buy 1 or 2 to keep on hand. From what I read, It appears the aftermarket synchronizers are typically good to go, but the sensors are what can be picky.

As for a Mopar synchronizer, I haven't had any luck finding one of those and I've been at it for almost an hour.
 
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