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Unread 03-21-2011, 08:21 AM   #1
Wood2km
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1998 ZJ 
 
Join Date: Feb 2011
Location: Clare, Michigan
Posts: 142
Rough idle, bogging down, misfire, no codes.

I have a 98 ZJ, with the 4.0. I just had a full tune up done, including plugs, wires, cap, and rotor, because it was running a little rough at idle, and, at times, seemed like it had no power. Fuel pressure was checked, and is good. Had the tune up done, and problem seems to not have changed really at all. Possibly a slight improvement. Only getting 16 mpg on the highway between 65-70. I don't expect 30 MPG, but I go easy on it pretty much all the time and it never goes above 16.5. Also, it will bog down sometimes, usually in 2nd or 3rd gear. I will just no longer be able to accelerate past a certain speed. Usually if I keep my foot on the gas and hold it steady, it will work its way through it and get back to normal acceleration. Does anyone have any ideas? My mechanic is stumped, and recommended that it goes to the dealership, even though he (and I) hates the dealership, because he doesn't really know what else to check, as there are no codes, despite the misfire. Any input is appreciated, even if it is something simple and seemingly obvious.

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Unread 03-22-2011, 07:51 AM   #2
Wood2km
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1998 ZJ 
 
Join Date: Feb 2011
Location: Clare, Michigan
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Does anyone have any ideas? Am I missing information that would be helpful? I really don't want to go to the dealership... They have not treated me well in the past, and obviously their prices aren't the best.
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Unread 03-22-2011, 08:14 AM   #3
Dodgeram1989
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1976 FSJ Cherokee 
 
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Location: Des Moines, Iowa
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Firing order, Coil.

No codes? Is there a misfire code or not?
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Unread 03-22-2011, 08:21 AM   #4
Wood2km
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No, there is no misfire code. And thanks for the ideas.
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Unread 03-22-2011, 08:22 AM   #5
Dodgeram1989
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Quote:
Originally Posted by Wood2km View Post
No, there is no misfire code. And thanks for the ideas.
When I swapped motors on my jeep I had 2 wires mixed up. It didnt set a misfire code but it ran like crap.
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Unread 03-24-2011, 02:42 PM   #6
Wood2km
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1998 ZJ 
 
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Location: Clare, Michigan
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Well, I had the jeep at the mechanic's all day today. I wanted him to give it one more shot, as I don't want to go to the dealership. He looked at it, checked the coil and firing order as recommended, and everything checked out good. He drove it with the scanner hooked up, and it did not miss or do anything strange for him.

Last night on my way home, I had something happen that has happened before, but not this bad. I was gradually accelerating, around 35 mph, when the rpms suddenly dropped to about 200. It did not stall, though. I took my foot off of the gas, and it went back up to idle. Applied pressure, and it began to accelerate normally again. Then it did it again. I repeated what I did before, and drove it the rest of the way home with no issues.
Is it possible that it could be the TPS or something? Does anyone have ideas?
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Unread 03-30-2011, 01:05 PM   #7
Wood2km
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1998 ZJ 
 
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Location: Clare, Michigan
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I'm not sure if anyone cares, but I am having the TPS changed on Monday, because I started having another issue, where when I have the cruise set, the rpms will surge up 100-200, and immediately back down. Searching for this led me to the TPS, so I'm hoping that this cures all the issues. I will update next week, in case someone else is having similar issues.
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Unread 03-30-2011, 03:29 PM   #8
JS97ZJ
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Has he eliminated the possibility that the cat might be clogged?
Check for vacuum leaks? Do a vacuum check. While the engine might not die it would cause performance problems like you have. It should get 21-25 hwy 17-18 around town, 16 city?
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Unread 03-30-2011, 03:34 PM   #9
funkyZJ
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What about the computer connections that I've read about here? The whole zip tie fix?
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Unread 03-30-2011, 04:10 PM   #10
JS97ZJ
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Quote:
Originally Posted by funkyZJ View Post
What about the computer connections that I've read about here? The whole zip tie fix?
It's getting to the point that it is starting to feel like an obsession that the PCM is the cure for all ills.
I think if I were doing this for a living, that ty-wrapping the connectors would be the first thing I would do.
Not knowing the history of the Jeep and how it was cared for, anything is possible. I've always taken good care of my baby, and it lasted 175k before it's first hiccup. It just so happened that it was the PCM. A process of elimination before throwing money at it has always been my way of troubleshooting. It could be an o2, weak coil, connection somewhere.
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Unread 03-30-2011, 04:15 PM   #11
wsar10
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have you physically pulled the NEW plugs and checked the gap on them ? or the ceramic isolators ? and BTW they should be OE copper core plugs, JEEPS DO NOT LIKE any toher kind (in my experience)
maybe check the plug wires witrh a multi-meter to see if there anything strange, You could probably see a diif. in ohm load through the wire if one was bad,

My jeep did this a while ago (lost power @55 or 65 mph) and I had a plug wire with broken insulation and it was arcing to the block under load.

The problem is that you did not do the tune up yourself....
just joking man !
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QUOTE=billzcat1;wsar10 is right on point on this oil thread - as usual. Anyone reading this would be well-served taking his advice.


-03 WJ laredo 4.0 120k a few minor mods
-93 XJ 5spd, 3" zone lift
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Unread 03-30-2011, 04:18 PM   #12
wsar10
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If youyr messing with the PCM make sure you have discharged the static from your body....... so many ppl overlook this or don't understand the importance of it.
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QUOTE=billzcat1;wsar10 is right on point on this oil thread - as usual. Anyone reading this would be well-served taking his advice.


-03 WJ laredo 4.0 120k a few minor mods
-93 XJ 5spd, 3" zone lift
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Unread 03-30-2011, 04:19 PM   #13
leadfootny
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I would look at O2 sensors. Mine would run like crap without codes, or would very intermittently set a misfire code. I did a full tune up to no avail. Replaced sensors and it was all better. I never got bad O2 codes. If your mechanic can chart the sensors behavior, he might be able to see if they are bad. Although, I would say it being a 98 and probably having over 100K, it might be time to change anyway.
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Unread 03-30-2011, 04:20 PM   #14
wsar10
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Quote:
Originally Posted by JS97ZJ View Post
It's getting to the point that it is starting to feel like an obsession that the PCM is the cure for all ills.
I think if I were doing this for a living, that ty-wrapping the connectors would be the first thing I would do.
Not knowing the history of the Jeep and how it was cared for, anything is possible. I've always taken good care of my baby, and it lasted 175k before it's first hiccup. It just so happened that it was the PCM. A process of elimination before throwing money at it has always been my way of troubleshooting. It could be an o2, weak coil, connection somewhere.
not only am I knoking on wood but i'm POUNDING on it...
but im @ 192k and nothing major yet............
I can trace EVERY oil cahnge and maintance since dec. of 97.
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QUOTE=billzcat1;wsar10 is right on point on this oil thread - as usual. Anyone reading this would be well-served taking his advice.


-03 WJ laredo 4.0 120k a few minor mods
-93 XJ 5spd, 3" zone lift
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Unread 03-30-2011, 04:32 PM   #15
wsar10
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Quote:
Originally Posted by leadfootny View Post
I would look at O2 sensors. Mine would run like crap without codes, or would very intermittently set a misfire code. I did a full tune up to no avail. Replaced sensors and it was all better. I never got bad O2 codes. If your mechanic can chart the sensors behavior, he might be able to see if they are bad. Although, I would say it being a 98 and probably having over 100K, it might be time to change anyway.
you can test an 02 sesnor before replacing it with a DIGITAL multi-meter or an oscilascope,
I have never done it , but in basic theory you could
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QUOTE=billzcat1;wsar10 is right on point on this oil thread - as usual. Anyone reading this would be well-served taking his advice.


-03 WJ laredo 4.0 120k a few minor mods
-93 XJ 5spd, 3" zone lift
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