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Unread 07-09-2013, 06:49 PM   #1
major722
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1996 ZJ 
 
Join Date: Jul 2013
Location: raleigh, North Carolina
Posts: 5
mis fire and CEL code 21 96 zj

ok guys, my jeep has been acting up since i bought it used from a friend. i rebuilt the top side of the enginewith all new gaskets,rotor, dizzy cap, wires, plugs, all of it. now my check engine light is coming on and when read with an obd2 its telling me i have a misfire in cyl 2 and 4, ive checked the connections and gapping the plugs look good and it appears to run fine wiith no noticeable misfire. my problem is that i am having this code 21 which i had read with an obd2 and was told that it was an oxygen sensor heater circuit short or somthing along those lines, the jeep has 152k miles so i replaced both oxy sensors. i still have the light. is this a wiring problem that i need to let a qualified mechanic take a look at? any help would be much apreciated!!

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Unread 07-10-2013, 04:10 AM   #2
Oldfrog
Web Wheeler
1997 ZJ 
 
Join Date: Feb 2008
Location: Louisiana/Texas
Posts: 4,229
Welcome to JF !

Ok....let's start from the bottom. You will spend a fortune letting a "qualified mechanic" try to fix your Jeep. Seriously. It wont get fixed, either.....I'm sorry to say. Not because I am any smarter than they are..( I'm NOT !) ...it's just because almost no one....not even the dealerships know how to fix most of these issues with any certainty. But they WILL charge you to experiment on them. Ask around....you'll see.

That said, most of us have been through what you are experiencing. For me personally, it was ( and still is) a LONG learning curve on these animals. They are finicky, they hate aftermarket sensors, ( especially o2 sensors) and they demand strong, clean electrical power throughout the entire circuit. The PCMs ( engine computers) pretty much all suck ( not as bad on the 5.2 I'm told) and if they dont, they probably eventually will, because the slightest little hiccup can fry part of their circuitry. In other words, they are as drama filled as 14 yr old school girls...and worse. Sorry for the bad news. Trust me, it gives me no pleasure to have to tell you all this. The good news is...there are some really good guys here and chances are, someone has had the exact same issues.

If you swapped the o2 sensors with anything other than genuine Mopar or NTK O2 sensors, you might as well go ahead and do it again....otherwise, you may never track down your issues.

Since you have a 5.2 and I'm more of a 4.0 guy, I'm going to let the more experienced 5.2 guys chime in. But until then, I'd start with your battery ( yeah I know it looks ok and starts just fine...'cuz they all say that)....but do yourself a favor and get it load tested. Jeeps HATE "marginal" batteries. You need to get used to that fact....sadly.

Then I'd take a look at all the grounding points, beginning with the cheap factory battery cables. Remove all grounds ( after removing the battery cables) , clean the contact area and re-install the grounds. I dont know where they all are on the 5.2, but I am assuming they have at least 3 locations, like the 4.0 does....and they are critical.

BTW, dont remove any sensor connections without FIRST disconnecting the battery cables. The PCM doesnt like that one bit.

More good news.....once you get a few common issues sorted out, you will love your ZJ !
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Unread 07-11-2013, 05:08 AM   #3
ZeeJay1997
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1997 ZJ 
 
Join Date: Dec 2008
Location: Sweet Home, Alabama
Posts: 6,889
you didnt mention a code for misfire. is this a pending code? misfires can be caused by lack of spark, fuel, or air.

i agree with the oldfrog that you should be sure you have a good charging system and PCM grounds first.
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Unread 07-11-2013, 07:02 PM   #4
homerx
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1993 ZJ 
 
Join Date: Sep 2011
Location: centereach, ny
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Did u drive 100 miles and reset the pcm (unpluge the battery for 30 min). If not it will still pull a code and check engine light. Also if its not runing rough or any hicup then ur fine and have no miss fire. Iv never had a miss fire on a 5.2 I couldn't feel. Also dose the obd2 pull a miss fire code?
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Unread 07-13-2013, 09:42 AM   #5
major722
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1996 ZJ 
 
Join Date: Jul 2013
Location: raleigh, North Carolina
Posts: 5
the obd2 code for the misfire was a p0302 and p0304 i havent driven 100 miles yet or left the battery unplugged that long, it still seems to be running well, i just got back from being away for a few days and wheni went to start my jeep it took about 3 or so long cranks before it would turn over and start. could that just be because it was sitting?
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Unread 07-13-2013, 09:49 AM   #6
ZeeJay1997
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Is it running well with CEL still on? Have you followed the advise you have been given?

Lots of stuff can cause long cranks. fuel pump, IAC, MAP, TPS, charging system.
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Unread 07-13-2013, 11:27 AM   #7
Billyjoebob007
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1997 ZJ 
 
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Location: Denver
Posts: 290
Quote:
Originally Posted by major722 View Post
the obd2 code for the misfire was a p0302 and p0304 i havent driven 100 miles yet or left the battery unplugged that long, it still seems to be running well, i just got back from being away for a few days and wheni went to start my jeep it took about 3 or so long cranks before it would turn over and start. could that just be because it was sitting?
Im having the exact same issue but for over a year now. CHeck engine light comes on and throws p301 misfire at #1 cylinder, but yet i detect no mis fire at all and she still runs fine, but recently within the past couple weeks when starting it starting take a lot more cranks before it starts up. Im thinking perhaps the crankcase sensor my be starting to go out, i dunno. Also, a mech said that the issue could be the camshaft sensor thats inside the distributor could be the issue. Heres meesage i got from him:

"a common cause of those codes on the 4.0 liter Jeep is a combination of the crank and cam sensors. When Jeep went to coil over plug ignition(the big *** assembly that bolts on over the plugs), they also started using the same little cam sensor synchronizer that always gave Ford such problems. That synchronizer can go to **** and cause the cam sensor to read incorrectly and count false misfires. Before the the switch to COP, it was the distributor that caused the same problem though you would generally get a rough idle. If yours has the dist, look for the shaft to be sloppy in the bushing."

"Same rule applies as to the 4.0 dist. and V8, Check the dist shaft for runout. The cam sensor is a wafer that sits between the dist housing and cap. These don't usually give us too much trouble and they're generally an all or nothing kind of thing. Still, if the dist shaft is okay, suspect the cam sensor. One more thing that can cause these codes on pretty much any application is valves. Carbon fouled or worn valve seats can cause an intermittent and random misfire. This always worse at idle though and you will feel it. One simple test for a worn or sunken valve seat is to run the engine at 2,000rpm for 15 seconds then release the throttle and watch for a misfire in the first 30 seconds after returning to idle. This is when the lifters will be pumped up and any valve(s) with excessive seat wear will tend to leak the most."

"Misfires can be weird. I've been in the business as a full time professional tech and as a master technician for most of the last 40 years. Just when I think I've seen it all and really have a handle on what can cause these misfires, I run into something new. Various degrees of valve damage and wear have been the cause behind many of these strange random misfire issues. Pinpointing the miss can be a real trick. I have lots of special equipment and generally just end up with the valve covers off, my leak down tester attached tapping the valve tips with a soft hammer and watching for the gauges to change. It's a real PITA."

Hope that helps. Im gonna replace the cam sensor, also know as the ignition pickup, and see if that helps the situation. Heres a pic:



Its under the cap n rotor.
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Unread 07-13-2013, 04:48 PM   #8
major722
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1996 ZJ 
 
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please let me know if that fixes it, ive heard that it can be a fix before
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Unread 07-13-2013, 06:57 PM   #9
Billyjoebob007
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1997 ZJ 
 
Join Date: Dec 2008
Location: Denver
Posts: 290
Quote:
Originally Posted by major722 View Post
please let me know if that fixes it, ive heard that it can be a fix before
Ok will do. I cleared the engine code warning out.I wont be drivin the JeeP until probably Mon though, so no update till then . When i started it , it did seem to run a lot smoother

*Welp, i run around through town for about 14 miles no engine light, when i went to get on the highway engine light came on - cylinder 1 misfire
Dag nabit !

Guess i will try and clean all the ground points next.
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Unread 07-19-2013, 01:24 PM   #10
major722
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1996 ZJ 
 
Join Date: Jul 2013
Location: raleigh, North Carolina
Posts: 5
so my check engine light is still on i did the rock on off thing and got 12,21,21,21,55 i wish i could figure out how to fix the 21 i know 12 is just cause i disconnected the battery
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Unread 07-19-2013, 01:49 PM   #11
Billyjoebob007
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1997 ZJ 
 
Join Date: Dec 2008
Location: Denver
Posts: 290
Ive just been informed that our issue is due to bad fuel injectors.

Im gonna go with a set of "upgraded" ones, about $140 a set.

As with issue in above post i also have this issue:

When on the highway and i floor it to say pass someone, me JeeP falls on its face, (And thats when it throws the mis fire code) like no power and hesitation/stuttering. What could possible causes of that be ? Happens at highways speeds only though. Say if im doing 40 and floor it, theres no problem at all.
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Unread 07-19-2013, 01:55 PM   #12
ZeeJay1997
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1997 ZJ 
 
Join Date: Dec 2008
Location: Sweet Home, Alabama
Posts: 6,889
Quote:
Originally Posted by major722 View Post
i wish i could figure out how to fix the 21
Since it flashed 21 3 times there are multiple failures of the 02 sensors. If you had the P code read by a scan tool rather than the MIL codes, you would know more about the problem.

Did you take the grounds apart and clean them like everyone told you to? One of them grounds the heater circuit for the O2 sensors. What kind of 02 sensors did you put on it?
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Unread 07-19-2013, 01:56 PM   #13
ZeeJay1997
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1997 ZJ 
 
Join Date: Dec 2008
Location: Sweet Home, Alabama
Posts: 6,889
Quote:
Originally Posted by Billyjoebob007 View Post
Ive just been informed that our issue is due to bad fuel injectors.

Im gonna go with a set of "upgraded" ones, about $140 a set.

As with issue in above post i also have this issue:

When on the highway and i floor it to say pass someone, me JeeP falls on its face, (And thats when it throws the mis fire code) like no power and hesitation/stuttering. What could possible causes of that be ? Happens at highways speeds only though. Say if im doing 40 and floor it, theres no problem at all.
please start your own thread, it gets too confusing for those of us who try to help.

i think your cat is clogged.
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Unread 07-19-2013, 11:02 PM   #14
ujoint
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2004 WJ 
 
Join Date: Sep 2009
Location: Hudson Valley, New York
Posts: 117
You could try swapping injectors around and see if the code changes to the Cylinder you swapped the bad injector with, May save you from buying injectors Also this does not usually throw a code unless it is real bad but open the Throttle plates and look down the intake at the bottom edges. if you see the gasket pulled in That will cause a Vacuum leak. common on the V-8s it also sucks up oil.

Trust your Mechanic. Fixing your car is how we make a living
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Unread 07-23-2013, 10:56 AM   #15
Billyjoebob007
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1997 ZJ 
 
Join Date: Dec 2008
Location: Denver
Posts: 290
Quote:
Originally Posted by ZeeJay1997 View Post
please start your own thread, it gets too confusing for those of us who try to help.

i think your cat is clogged.
I am trying to help. Anyways, i found my misfire code problemo: I had an accel coil and i switched that out Sunday with the stock coil, And now i no get misfire codes So perhaps op may have bad coil too, try swapping out the coil

Update* After a couple days the misfire code came back So i think it could be a bad injector,pcm,??? i dunno. I grow tired of this endless search for the issue.
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96 grand cherokee , check engine light , oxygen sensor

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