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Old 09-25-2011, 01:05 PM   #1
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Question Need help with the misfire/CEL/loss of power thing

I've seen a lot of threads on this issue and there doesn't seem to be a single hard-and-fast solution to this problem. There are so many things that can cause these symptoms apparently.

Here's my situation. I just bought an '02 Wrangler Sport off this guy. He didn't seem too bright to be blunt. He was asking a very good (or so I thought) price for a very clean looking Wrangler in Phoenix where it seems everyone is getting a premium for these things. Anyway, I knew it wasn't perfect mechanically when I looked at it. Keep in mind though I know nothing about Jeeps and not much more about auto mechanics (although I am getting a crash course now!). It had a major oil leak but I figured even if I had to replace the rear main seal and more for the price it was worth it. Other than that it drove fine. No issues whatsoever. Ran smooth, accelerated fine, plenty of power, no unusual noises or anything. I did notice too that there are a lot of electrical issues. Fog lights didn't work. Windshield washer didn't work (had fluid but no pumping). Rear wiper didn't work and switch felt flaky. I was hoping maybe it just needed fuses. So far I've only had time to check the fog lights and whatever's going on there it's more than a fuse. Blows new fuses instantly when you hit the switch. Not looking to T/S that now but just mentioning it.

So, I drive it and all and it looks really good so I buy it. I go back to work where it sits for about six hours before my shift ends. I have a coworker follow me to my house in it while I ride my motorcycle that I drove in to work that day. Keep in mind during the test drive I never got it on the freeway. City streets and about the fastest I got it up to was around 60 mph. (speeding, I know!) The drive home from work is all freeway. About 7 miles into the trip the CEL comes on in the Jeep and the guy driving it says it started sputtering so he pulls over and shuts it off. I come back and check it out and it's idling fine but the CEL is still on. Temp and oil pressure gauges read normal. Oil level was good as were other fluids. So I drive it the rest of the way. At first I don't notice anything other than the light being on. But after a while it did start to cut out some, especially if I tried accelerating above 60 or 65 mph. It REALLY didn't like it when I romped on the gas to see what it would do. Hesitated, then surged, then hesitated and the CEL started flashing a big F-YOU at me. But I made it home.

The codes showed there had been misfires on all six cylinders. O2 sensor came up too I think but that's about it. After reading the forums and talking to a mechanic I decide first step is to change the plugs and coil rail. I'm hoping maybe they just put the wrong plugs in or gapped them wrong or something. Not so lucky. As I pull out the plugs they're exactly what the engine plate calls for (Champion Copper Plus) and are all gapped right. More importantly is apparently the previous owner (more likely a mechanic hired by him) had the same idea because they were all just about as clean as the ones I put in to replace them. I'd say those things were only a few days old if that. So, new plugs and coil obviously aren't going to fix the problem.

I had read bad CATs could cause this and to check it you could read the temp going in vs the temp out and the output should be at least 100 degrees hotter. Sure enough on the front CAT the input and output temperature flanges read about the same with my laser thermometer. Rear one read about 50 degrees hotter. However, after test driving the Jeep after changing the plugs I checked them again. At the time I checked them though it was sputtering pretty good even at idle. Sounded like at least one cylinder wasn't firing. When I first started out and until it got up to temperature it was running fine. Anyway, I couldn't get a decent temperature reading from the CATs while this was going on because both of them were fluctuating dramatically. I had started out reading around 500 degrees before the sputtering started. As I watched it would drop as low as 350 and sometimes go back up for a second then back down. Must have been due to the unburned fuel being shot into it during the misfires. I dunno.

That's basically where I'm at right now. Codes now show misfires on cylinders 1 and 6 but I imagine if I let it run long enough they'll all show misfires eventually just like before. I'm at a loss of what to do next. I'll probably have a muffler shop check the CATs themselves. I was on my back on the ground and may not have got good readings. I would appreciate any insight anyone can share on where to go from here.

Other items of note. My mechanic says the oil leak is just the oil pan gasket. That's good news, except they want nearly $400 in labor to change it out. Might just have one of my weekend mechanic friends do it for a few bucks and a case of beer. I can't even get plates put on this thing because there's no way I'll pass emissions with this going on. Battery looks new but the positive terminal looks like it's been chewed up by pliars a lot or something. I'm going to go through and check as many grounds and stuff as I can find and might dissect the battery cables. I have seen corrosion creep down the wires under the insulation until they become white inside but look good on the outside. From what I've read bad electrical could cause this. I don't expect I'm going to find anything there though.

I know I wrote a book here and I appreciate anyone who's read this far. I just know that when you're asking for help with an issue it's best to give as much information as possible. Thanks again. I look forward to reading your replies.

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Old 09-25-2011, 01:10 PM   #2
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"Sure enough on the front CAT the input and output temperature flanges read about the same with my laser thermometer. Rear one read about 50 degrees hotter." To clarify that statement a bit, both CATs read about the same temperature on the input. The rear CAT showed about 50 degrees hotter on the output than on the input while the front CAT showed little or no difference at all from input to output, which I'm told suggests it's blocked or otherwise not working properly.

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Old 09-25-2011, 01:16 PM   #3
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I had a rough idle and slight loss of acceleration on my '01 TJ and swapped the plugs, I bought the Forum suggested Autolite Double Platinums, husband installed then and the TJ ran WORSE than before I changed the plugs, went to Advance Auto and swapped the Autolites for the Bosch Platinum 4s (has 4 heads on the spark plug instead of one) and WOW!! It drives like a new vehicle. Better acceleration than before, and no more rough idle. I don't think the 4.0 motors like cheap spark plugs.
I didn't change my rail, just the plugs.
For the fog lights, it could be actual switch, mine went bad on my PT Cruiser and replaced it in about 15 minutes.
For the other stuff, sorry can't help with that, husband is the mechanic, good luck with it!
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Old 09-25-2011, 01:17 PM   #4
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Fog lights: your switch on the turn signal is broken common jeep thing.
Rear washer: make sure it's plugged in at the back of the hard top could have giggle loose or been disconnected for some reason.
Misfire: does your jeep start right away on a cold start? Mine didn't and it would misfire because of that.
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Old 09-25-2011, 03:13 PM   #5
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Once the codes are cleared and in today's case I took the Optima battery out of my truck and put it into the Jeep (same one fits in both fortunately) it drives great for a while after you do these things. Especially if you stay on city streets. But once she's warmed up and you get the RPM's to around 2800 or 3000, that's when the CEL comes on and she may or may not start sputtering at that point. After changing the battery and fixing a horribly deformed positive terminal (temp fix for now as I will simply replace the terminal) I drove it all over town. Well, about 7-8 miles. Only did about 2 miles on the freeway. Jeep did fine the whole time. Purred like a kitten. When I got back in the driveway and wasn't worried about it dying I put it in Park and drove the RPMs up to 3000 and crept it up to 3500. Then I did some revving from idle up to 3000 or so and then the CEL started flashing. Checked the codes and it only showed misfire on cylinder 6. Is it possible to get a bad connection on one plug when re-installing your coil rail? Somehow I don't think that's the case here. It's just that the problem isn't fixed, nor did I expect simply changing the battery would. Although it does seem a LITTLE better so far. Need to run it more to be sure. I'm just afraid to go too far from home in case it gets so bad it won't stay running. But I think I'll keep the code reader in the Jeep. Seems if I reset the codes I can at least temporarily get the problem to go away (notice I didn't say "fix the problem") long enough to limp home.

Neither front nor rear windshield washer works. Front wipers work but rear doesn't. Switch for rear feels like it's only a spring loaded temporary switch. Is that right or is it broken?

Thanks for the info about the turn signal switch. I may just run an aftermarket anyway. Those turn signal switch things can get expensive.
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Old 09-25-2011, 04:20 PM   #6
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UPDATE: OK, I finally just took the Jeep to Midas to have them check the CATs themselves. To my surprise they said both CATs are working properly. They said they read almost exactly 100 degrees hotter on the outputs of both CATs compared to their inputs. Well, I don't know how knowledgeable these guys are but I figure if they're NOT trying to sell you something chances are you don't need it.

So for now I'm off the catalytic converters. They said that if the O2 sensors were bad that shouldn't give a misfire code. Not sure I agree with that. But again I'm no expert.

Current codes are 300, 301, and 306. If I remember the chart right 300 is multiple misfires, 301 is misfires on cylinder #1, and 306 is misfires on cylinder #6. My next guess would be injectors but if it is the injectors why does the problem go away for a little while each time you reset the codes in the computer? That doesn't make sense to me. I'm thinking it has to be electrical somehow still. Either a bad sensor or something else that is controlled by and can be reset by the computer.
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Old 09-25-2011, 04:32 PM   #7
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Have you pulled the plugs to look at them? If you pull them, what brand and model # are they? How do they look?
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Old 09-25-2011, 05:59 PM   #8
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Have you replaced the plug wires or does that come with the coil?
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Old 09-25-2011, 08:41 PM   #9
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There are no spark plug wires or conventional ignition coil on a 2000 or newer TJ with the 4.0L engine. Instead, there are three ignition coils shared by the six spark plugs and they are mounted directly on top of the spark plugs on a one piece "coil rail" assembly.

It's called a DIS (distributorless ignition system) and it got rid of the need for a lot of parts that can cause problems like the distributor, distributor cap, rotor, ignition coil, and ignition wires.
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Old 10-16-2011, 09:53 PM   #10
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I feel your pain. I get the same cel on my 99 4.0 except i get a random misfire code and a misfire on cyl 2-6. I have changed plugs,wires, cap, rotor, and coil still have not found the prob.
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Old 10-16-2011, 09:54 PM   #11
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I was also told something about it having something to do with my valves. I duno i sick of throwing money at it. Im about to lite it in fire.
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Old 10-16-2011, 10:32 PM   #12
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Misfires are about the hardest thing to sort out because they are not the problem but rather the possible result of numerous issues.

The fact that you have soo many electrical gremlins would suggest to me that the jeep mat have even gotten flooded. If that is the case, you might be dealing with a mess.

Anyway, I would start at the basics. You need spark, fuel, and air to have ignition. If any of those things are missing, you will get a miss.

A few things you may not have thought of and some may sound dumb.

Load test the battery.

Listen to the injectors to see if they are firing. You can use a long flat head screw driver. Place the tip on the injector and put your ear on the handle. You will hear a continuous tapping sound. If you are getting a miss due to an injector it will be erratic. Obviously, you can't check this driving.

I am not real familiar with how the coils are set up, but it might be possible to load text them on a bench.

If I had to guess based on the other issues you are having, the coils got wet.

Find a friend with the same jeep. Swap his coils to your car and see if that solves the problem.
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Old 10-16-2011, 10:38 PM   #13
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Based on your symptoms and your temp measurements, I'd say it's the coil rail.
They short internally, hard to detect. Listen really close, you may even hear it "snapping" when hot.

Temps - warm it up to normal first - then run it at about 2000 R's for 2 or 3 minutes. You have to make it misfire to detect it. Go under and immediately check the inputs and outputs of the cats - 100 degrees in/out hotter is normal. But when the coil rail is shorting - one cat will be considerably hotter than the other. Like one might be 350, the other 700.
But - you have to run it hot for awhile for it to misfire and cause the temp differences.

Also you can aim the laser meter right at the exhaust ports from underneath - not easy, but possible.

If you do replace the coil rail, get a good one. There are many China knock-offs around now - $20-40 cheaper but you get to buy one every year. OEM was made by Standard Ignition - Car Quest, NAPA or dealer. Autochina and the rest are junk and last only a few months at best!
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Old 05-14-2012, 10:11 PM   #14
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Have had a very similar issue for the past 14 months.... Two mechanics, lots of cash, a fuel pump, coils, plugs, an ECM, valve spring replacement, and more sensors than I can shake a stick at.... Still P0300 and P0306 anytime I'm over 3000 rpm or 60 mph... Checking for evap leaks now, hoping to find a cracked manifold or something that may hold the key to an answer... Will post if I find something. Good luck!
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Old 05-15-2012, 11:45 AM   #15
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I seem to have this same problem as well. Jeep runs fine around town but sometimes when going 60-65 I lose power for a few seconds and it feels like hesitation. The hesitation seems to come and go. I would love to know the fix for this. Mine is an 05 with the 4.0 auto.
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Old 07-21-2012, 10:05 AM   #16
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I have the same problems with my 02 TJ. So far, have replaced the coil rail, two sets of spark plugs, throttle pos sensor, and the factory combustion cleaner and valve rotating procedure. Problem still there. Now the tech says the valve springs are too stiff and I should do a valve job at $850. No thanks. Still looking for the problem. BTW my Jeep only has 49K miles.
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Old 07-21-2012, 10:20 AM   #17
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A bad upstream 02 sensor can often cause an engine to run poorly from it tricking the computer into thinking the air/fuel mixture is too lean, causing the computer to make the air fuel mixture richer than it should be.
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Old 07-21-2012, 04:04 PM   #18
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So far there hasn't been any mention about FUEL pressure!

A slightly low pressure can cause your symptoms - too lean! There were at least 2 slightly different pumps/regulators made for the later 4.0's. One has about 5 lbs lower pressure. Even if the PO says he replaced it and the pressure is low, it may be the wrong one.

How do I know? Chalk that up to a stupid parts jockey!

Most have a Schrader Valve on the fuel rail to measure it, on those without the valve you need to install an adapter on the fuel rail. You can "borrow" a fuel pressure gauge from many parts houses. Or get your own on EBAY for cheap.

Unplug the upstream O2 Sensor - it will run rich unplugged - see if it runs better disconnected.
Bad O2 sensors can make the computer run too rich or lean, depending on how they fail. Disconnected it should cause it to run somewhat rich.
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Old 07-21-2012, 04:13 PM   #19
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Location for oil pressure switch for 2008 jeep wrangler?

can anybody tell me where the oil pressure switch is on a 2008 wrangler 4-door?
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Old 03-09-2013, 11:30 AM   #20
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Change the "Purge" valve yesterday 3/8/13. Didn't fix it.

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