Chasing down an intermittent misfire

[Gareth Randall]

I have a 2005 Jeep Liberty 3.7L. About every 1000 miles or so I’ll get a misfire code, although I never actually notice any misfire. It’s usually P0300 at first, and then gets pinned down to cylinder 1 or 2 (it varies). I reset the code, and everything’s normal for another thousand miles.

Since it’s very intermittent, am I right in thinking that I’m probably only going to isolate the cause if I connect up an OBD2 data logger until it happens again, then read the data and see if I can work out what’s going on? I installed all new plugs and coils about six months ago, and the coils for cyls 1 and 2 pass the multimeter tests detailed in the FSM, so I don’t think it’s them.

Something that may or may not be relevant – the misfire code never triggers the Check Engine light while I’m driving, only ever at startup. I think I’m right in saying that the PCM will turn on the light any time it detects a relevant fault condition, not just at startup? In which case, if I only ever get the misfire code showing up at startup, it’s possible that the misfire condition is only actually occurring at startup, which might help narrow it down?

Topic

[Kevin Criswell]

When was the last time the plugs and wires were inspected/replaced?

[Bryan]

You can use a data logger or do a power balance test to isolate the cylinder then check igniter, spark and fuel presure. This will lead to the next step.

[college man]

Try doing a power balance test.

http://www.ericthecarguy.com/faq/solving-automotive-performance-issues

[Gareth Randall]

Raistian77 – the plugs and coil packs were replaced about six months ago. I haven’t tested the wires running from the PCM to the coils.

Thanks for the heads-up about the power balance test, I’ve never heard of that before. But will it necessarily be helpful, given that there’s about 1000 miles between each instance of a misfire code and the engine seems to run smoothly all the time? As I say, I’ve never actually noticed a misfire – if I felt a stumble and the light came on then that’s one thing, but that never happens. The light only ever comes on and stays on at startup, and if I don’t reset it myself it goes away after 40 key cycles, which means that the fault condition hasn’t recurred.

[crypkilla]

Have you recently pulled the plugs to check for any fouling?

[Kevin Criswell]

There is a TSB for your Jeep for a PCM reflash relating to random misfire codes and ignition coil codes being set when there is no problem.

Also check your belt tensioner I have seen loose ones cause the belt to jerk on the drive pulley setting false misfire codes also.

[Gareth Randall]

Thanks Raistian, I’ll check both of those out B)

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