Cylinder 3 and 6 Misfire. -
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post #1 of 3 Old 07-09-2013, 01:19 AM Thread Starter
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1997 ZJ 
Join Date: Jun 2012
Location: Las Vegas
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Cylinder 3 and 6 Misfire.

Been having this issue for 6 months since i bought it. I have a 97 ZJ with the 5.2 V8. Ever month i get the same issue, the check engine light comes on, i take it to Auto zone, and it reads misfire on 6 and 3. I've replaced the spark plugs with platinum champions, then it goes off. then it came on again, so i replaced the spark plug wires. then it comes on the next month AGAIN so i replaced the cap and rotor. So whats next? this is really starting to get on my nerves.

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post #2 of 3 Old 07-09-2013, 04:52 AM
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1997 ZJ 
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it could be a fuel delivery problem. injectors, wiring, pressure.

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post #3 of 3 Old 07-09-2013, 08:41 AM
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2005 WK 
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A cylinder needs 3 things to properly fire - sufficient spark, fuel, and pressure. It appears that you've been throwing parts the spark area without any success, and this can become frustrating. Here's some thoughts on each of these areas, and how you might go about testing and diagnosing your problem.

Spark - I'm not as anal about the types of plugs as others, but be careful about putting some of the more advanced designs of plugs in your car because, as one wise person said, "these old fashioned Jeeps work best on old fashioned parts". Putting new designs in your car might be similar to putting a CD into a cassette player. I know that one Jeeper put in one of those fancy high-end plugs and it created a feed-back in through the PCM's coil driver that it just couldn't buffer ... and it had problems. In other words, your PCM and other electronics are not designed to accommodate anything other than the types of plugs that existed in 1997 ... which means ye olde copper is best.

Now, assuming that you've replaced all of your secondary ignition parts (and it seems you have), the only thing you have yet to replace is your coil. I don't know your mileage, but I believe replacement as a general maintenance item should be done after 100,000 to 150,000 miles - generally, this should give you a stronger spark which could be your problem IF you were getting random cylinder misfires or IF those were happening on the longest cables. You can also test the strength of your coil by using something like a Thexton spark tester, or by doing a resistance test across the two primary terminals (.95 to 1.2 ohms) and from the 12V to the secondary or high-tension terminal (11,300 to 13,300 ohms).

My initial guess is that the coil could your problem ... it would be the first thing I would test.

Fuel - Insufficient fuel delivery could cause problems here as well, and if this is your problem area then it is likely your fuel injectors (dirty, misfiring). So, after the coil, this would be the second area I would explore. Here is some testing you can do. (1) Put a fuel pressure gauge on your rail and ensure you have sufficient fuel pressure at idle and while revving the engine - 48 PSI, I believe for your Jeep. Bad PSI or PSI dropping when revving? Then replace the pump/filter/regulator. Good and solid PSI? Then (2) run a tank with fuel system cleaner (e.g. Seafoam or Techron) through your car to clean up those injectors. Also, clean the electrical connectors on the #3 and #6 fuel injectors, put on some dielectric grease and reconnect. . If you still have the problem after getting through this tank of gas, then (3) swap one or both of the fuel injectors to see if your misfire moves. For example, swap the #3 and #4 fuel injectors. If the misfire moves from #3 to #4, then you know your problem is a fuel injector.

Compression - Simply, do a pressure test on your cylinders and see if your compression is significantly lower on #3 and #6. If it is, then you will probably need to do a leak down test to diagnose further. By the way, the Seafoam or Techron treatment noted above could also break off some carbon deposits that could be causing some compression loss ...

Another issue could be a vacuum leak. I would suspect something on the intake manifold if the two cylinders were next to one another, but they aren't. I don't think a vacuum problem is present with your situation, but including it only because a vacuum leak can cause a misfire.

Let us know what you find.
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