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Unread 01-20-2013, 11:11 AM   #1
bluejunior
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Inconsistent Misfires Cyl 4 and 6

Howdy guys!

I'm having trouble with random misfires in cylinders 4 and 6. Typically they occur either right after startup or on the freeway (which would be 2200-2700 rpm). No error codes besides the misfires. It had a problem with misfires right after startup for awhile, finally it wouldn't start at all one day, which turned out to be the CPS. After replacing the CPS I had no problems at all for about 6 weeks. Now it randomly (can't seem to connect it to temperature outside, time since last run or anything else) idles rough after start and will misfire in 4 and/or 6 on the freeway. Not experiencing any noticeable power loss on the freeway so I have to think they're not missing many ignitions in a row.

So my question is: Knowing that my plugs, ignition rail and CPS are all new in the last 6 months, what are my target parts to test and how do I test them?

I'm on a shoestring budget but I have time, so even pain in the *** testing is better than "replace parts and pray" so I really appreciate whatever input y'all have. Thanks in advance!

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Unread 01-20-2013, 11:42 AM   #2
bluejunior
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Don't know how I failed to add this: it's a 2001 XJ Sport 4x4 (NP231) automatic with the 4.0L I6.
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Unread 03-04-2013, 01:02 PM   #3
bluejunior
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Okay I'm bumping this thread in desperation. Initially I swapped around injectors and plugs to see if I could get the misfire to follow, to no avail. Air filter, ignition rail and plugs are all good. Not sure how to properly check injectors, but the misfire didn't follow them. The misfires are still highly inconsistent but are getting more frequent. They still primarily happen at idle right after startup or while at freeway speed (2200-2700rpm).

Now I'm getting misfire codes for all 6 cylinders (p0300+p0301-06). How can I dependably test the fuel pump or the ignition coil? Those are the only things I can think of that could result in all cylinders misfiring intermittently....

Thanks for the help guys.
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Unread 03-04-2013, 01:15 PM   #4
buildin1XJ
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Rent a pressure gauge connect it to the Schrader valve should be 49 psi + or - 5 psi. because you have coil packs I doubt your problem is with them. could be a bad cam sensor
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Unread 03-04-2013, 04:37 PM   #5
bluejunior
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Thanks I'll do that....safe to assume they'll have one sitting around autozone, oreilly's or the like? Is there a way to dependably test the cam sensor? I had an issue with the crankshaft position sensor a few months ago even though it showed the appropriate resistance on the multimeter
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Unread 03-04-2013, 05:39 PM   #6
tjwalker
It's the crank sensor!
 
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4+6 are not "companion" cylinders for the coil rail, so I don't think the problem is that and you have replaced that item.

Just brainstorming; I agree with buildin1xj about the cam sensor (test and try synchronizing it), fuel pressure (less likely but I'd test it) and after that, I'd test compression on all cylinders as you don't want to get tunnel vision; can't forget about internals.
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Unread 03-04-2013, 05:54 PM   #7
buildin1XJ
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Where did you purchase the crank sensor from, really from your symptoms does not sound like the problem but the big box auto part sensors are known to fail right out of the box or have a very short life.
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Unread 03-08-2013, 09:56 AM   #8
bluejunior
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The crank sensor came from a local mom-and-pop. I had originally gone to O'Reilly's but the part they pushed across the counter didn't match stock at all (thank god I checked before I went home). Fuel pressure tests had the needle hovering around 50psi. The problem is getting worse, the engine started making a bad knocking noise for a minute after startup yesterday.

How do I test/sync a cam sensor? How do I test compression? I've done it on other cars by placing a sensor over the port for the spark plug, but given the rail setup I have no idea how to properly test it for a cherokee....

Thanks again for the help guys!
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Unread 04-16-2013, 10:51 PM   #9
bluejunior
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So, I lived with the problem for about 5 weeks after that last post, largely due to financial restrictions. Yesterday I installed a new Cam Position Sensor. So far it seems to be solving the problem, I'll edit this reply in a week or so with the longer-term results.

Thanks for your help guys!
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Unread 04-17-2013, 12:06 AM   #10
buildin1XJ
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Thanks for responding back and giving others here feed back. I myself do not like it very much when we all give advice and never hear back from the OP.
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Unread 04-19-2013, 10:52 AM   #11
bluejunior
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No problem. That's half of why I have to post questions on well-covered topics like this, because people forget to reply back with what actually worked.

As an update to any later readers, I now have several hours of road time and over a dozen start/stop cycles without issue. Unless I come back and post otherwise, the Camshaft Position Sensor worked. It was also ridiculously easy to replace, took me ~65 bucks at oreilly's and 5 minutes in the driveway to do, most of which was figuring out the right socket size for the two bolts involved. It's a little round sensor right next to the oil filter.
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