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Vehicle is a 2007 Dodge Nitro R/T with a 4.0L engine. I've had a lot of trouble diagnosing a cylinder 2 misfire and looking for help on next steps.

Symptoms

After engine is warm, stop and go traffic will result in a rough idle and repeated engine light warnings for cylinder 2 misfire, P0302.

Steps Taken

So far, the plugs and coil packs have been changed. Fuel injector replaced. Cylinder valves inspected/fixed. Some done by mechanic(s), some done by me.

More Data Gathered

In order to diagnose further, I've recently purchased a bluetooth OBD2 tool so that I can collect data at the time of failure.

The chart below shows an example of what happens to four data points (02 Bank 2 Sensor 2 [blue], 02 Bank 1 Sensor 2 [orange], EGR error [gray], and commanded EGR [yellow]. I identified the time of the problem in red, around the 11 minute mark in the trip. It occurred after sitting through several stop lights. What I see is that the commanded EGR went to 0% for about a minute, then three things happened simultaneously as the check engine light turned on: The voltage for 02 Bank 2 Sensor 2 went to 0 AND the rough idle started. The timing of the rough idle exactly coincided with Voltage on 02 Bank 2 Sensor 2 dropping to 0.

Problem Chart

Suggestions?

Do you have ideas of the culprit? I have been considering an EGR valve being stuck open causing the problems, but I'm not getting an EGR error code. I've also thought that maybe the 02 sensor is bad, but can the downstream sensor cause a rough idle?

Suggestions for next steps would be greatly appreciated.

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  • Is the misfire on that cylinder constant? Have you done a compression test?
    – Solar Mike
    Mar 27, 2018 at 21:05
  • The misfire is constant in the sense that once it starts (at a stoplight for instance) it will keep misfiring until I get up to speed. I haven't done a compression test, the spark plugs to get into the cylinders are difficult to access. You have to remove the air intake manifold and attached hoses. Would it be a problem doing a compression test with out this apparatus?
    – Underminer
    Mar 28, 2018 at 14:41
  • Can you do a power balance test when this happens to confirm it really is cylinder 2? The downstream sensor can be used for final fuel trim correction, but I’ve never seen one cause a rough idle or misfire. As others mentioned a running compression test would be a good idea.
    – Ben
    Mar 28, 2018 at 16:47

1 Answer 1

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My suggestions for next steps to take:

  1. Vacuum gauge testing at a warm idle, is it steady? A running compression test as well. Mechanical integrity should be verified before chasing any bad data theories.

  2. Freeze frame data, you mentioned the light comes on at a warm idle. Does your freeze frame agree? Correct me if Im wrong, but your vehicle will shut down the injector for the offending cylinder that misfires, for the remainder of that key/drive cycle.

  3. Graph the rear O2 sensors with the front. Although the one in the image you posted does go flat, the other side takes a bit of a dip as well. You want to know if the fronts are telling the same story as the rear.

One more thing that could change some of the possibilities is a more detailed description of what was actually done when the cylinder valves were inspected/fixed.

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