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Old 02-17-2011, 02:59 PM   #1
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Misfire... Any ideas?

I'm a newbie here of sorts, I bought my Jeep back in early December, and have been hangin' out and reading quite a bit of info here and enjoying this site almost every day since. Unfortunately, my jeep has an eratic misfire, (it came with it when I bought it as an added bonus) and I haven't been able to pinpoint it so far. It is a '97 TJ, with the 2.5 4-cyl and 5-speed. Since getting it, I have replaced the coil, cap, rotor, wires, plugs (champion coppers @ .035), crankshaft position sensor, distributor(with new camshaft position sensor), throttle position sensor, and air charge temp sensor. I've removed and cleaned the air idle sensor in the throttle body. I even ran a can of sea foam through the gas, which is usually the 89-proof. None of these have made any real difference so far, and no codes have been set. The Champion spark plugs seemed to give it a surprising bit more power over the Autolites that were in it, and as a result have made the miss more pronounced (noticeable) when driving it. When you take the cap off of the distributor, the terminals inside have a greyish-white arc buildup, as if it's getting an uneven spark. I'm leaning toward the map sensor and oxygen sensors next, as I'm rapidly spiraling towards the bottom of my list of possibilities. My question is, do these items affect spark, fuel mixture, or both? Aside from this, this thing even for a 4-cyl seems to have more than enough power, and would have even more if I could just get this miss figured out. Anyone have any ideas on this one? I'm beginning to think the Jeep Gods are angry...

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Old 02-17-2011, 03:03 PM   #2
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Describe the miss fire, if it shutters in 4th or 5th gear when accellerating at 2,000-2,100 rpms no one has found a fix and I have accepted it as normal.

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Old 02-17-2011, 03:46 PM   #3
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When you say "erratic misfire", do you mean you are getting a code telling you there is a misfire on an OBD scanner (if so, what code) or are you assuming some power loss, shudder, bad idle, noise, etc.. is a misfire? If you are getting a single or multiple misfire code still after replacing all of those items, it is likely the 1997-2000 model valve spring issue that causes intermittent misfire issues and codes. The only fix for that issue is to replace your valve springs.

If you have an OBD, you can check the codes or go to an auto parts store and they will check for free..... Don't trust the dummy light on the dash. I have seen the check engine light disabled on a few vehicles.
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Old 02-17-2011, 04:26 PM   #4
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I know you stated that "no code have been set", so I would check and make sure the check engine light has not been disabled.
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Old 02-17-2011, 04:39 PM   #5
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Okay, the misfire is more of a surge, like an advance and retard of the spark would produce, mainly when accelerating or going up a hill. It runs pretty smooth cruising along with no acceleratioin or deceleration. The idle is smooth. There isn't really a 'shudder', it's more of an uneven power surge. Also, on deceleration, you can feel and hear the miss and uneven 'winding down' of engine rpm. Again, it's showing NO CODES either with the cel or obd scanner. It's been checked on two different obd scanners, as well. The one at my friend's garage is much better than mine, so I had him check it also. The cel light itsself does work, it just isn't showing any codes.
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Old 02-17-2011, 06:31 PM   #6
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So it's not really a thump thump thump misfire, it's more of a surge.

How old is the primary O2 sensor? -- The one(s) closest to the engine?

They get old and lazy (they are good for about 80,000 miles) but unless they fail completely they won't set a code. They can give a surge feeling - they just don't react fast enough.

They aren't all that expensive - stay away from the cheapies like Autochina sells, unless you want to replace it every year.

When you replace it you want to erase the computer memory to help it learn the new one. You have to disconnect the battery for at least 8 hours to completely erase it.
Scanners or a quick disconnect just erase codes, not the deep memory. There are no shortcuts.
Otherwise if you don't disconnect for at least 8 hours, it will take 100 or more miles driving to learn the new sensor - meanwhile you'll still have the surging. You'll think it didn't solve the issue.

Let us know how it turns out.
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Old 02-17-2011, 08:25 PM   #7
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Rrich, that's right. It's not a steady miss like a dead cylinder. I honestly don't know how old the primary oxygen sensor is. I've only had this one since early December. I did not know about the 8-hour rule for erasing the memory. I've erased the memory with my scanner, and my friend erased the memory with his, but this is interesting about the 'deep memory'. I'll unhook the battery and leave it overnight. I appreciate everyone's interest and thoughts on this one.
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Old 02-19-2011, 08:39 PM   #8
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Ok, I disconnected the battery terminal and left it overnight. Didn't get to drive it until this evening. The misfire is still exactly the same, no change. Next item on my list is to replace the oxygen sensors, then the map sensor. Anyone have any additional suggestions? I'm startin' to run out of options here. I've been reading other posts concerning gas octane ratings. I usually run 89 in this one. What do you think the chances are that this may be a contributor to my misfire? Your thoughts & suggestions are definately appreciated..

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