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Unread 07-27-2013, 04:41 AM   #1
MechanicTyler
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1998 ZJ 
 
Join Date: Jul 2013
Location: Battle Creek, Michigan
Posts: 15
4.0 Miss, Backfire, Misfire

Hello I have a 98 ZJ 4.0 4x4. the issue I have is that the jeep will not run correctly it will miss or backfire on occasion and has no acceleration if I were to press the throttle to the floor it would take of and then sound like it stalls but if I let off the gas it will stop and go back to idle if I slowly get on the gas it will move then take off a little but then will bog out the more gas I give it no power what so ever. List of replaced parts below.

Parts

Cap & Rotor
Coil Pack
Wires
Plugs (Champion Copper Core)
TPS (Autozone Part)
IAC
Crank Position Sensor
Head Gasket
Intake/Exhaust Gasket
Air filter
Timing Chain
Temperature Sensor
Fuel Pump/Regulator/Filter

I also tried re stabbing the distributor with no luck. Injectors are pulsating properly I believe fuel pressure is 30 to 40 and has a noticeable miss at Idle I have done the PCM trick as to use Di-Electric grease and zip ties to get a better connection. I have been told that the clock spring in the steering column shares a ground with the horn and TPS and that could be the problem my horn is very weak sounding. These symptoms were present off and on until I went mudding and stalled it out in a mud pit about half way up the doors in water and now the thing barely drives I had to replace the fuel pump as result of sucking mud and silt into the gas tank because it sat in the water so long with out getting towed out I would say 20 to 30 min and the jeep was more underwater on the passenger side but I drove it home with minor problems that was back in February

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Unread 07-27-2013, 04:59 AM   #2
MechanicTyler
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1998 ZJ 
 
Join Date: Jul 2013
Location: Battle Creek, Michigan
Posts: 15
I will try posting a vid of it running
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Unread 07-27-2013, 05:19 AM   #3
stubburnjdh
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1995 ZJ 
 
Join Date: Apr 2013
Location: Front Royal, Virginia
Posts: 9
Did you drain the gas when you changed the pump? I would check and clean all grounds. Bad grounds can give all sorts of problems.
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Unread 07-27-2013, 07:08 AM   #4
ZeeJay1997
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1997 ZJ 
 
Join Date: Dec 2008
Location: Sweet Home, Alabama
Posts: 6,340
Quote:
Originally Posted by MechanicTyler View Post
I believe fuel pressure is 30 to 40
Spec is 49 plus or minus 5 psi

have you tried disconnecting O2 sensors?
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Unread 07-27-2013, 07:58 AM   #5
dusterdude
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1996 ZJ 
 
Join Date: Jan 2012
Location: hopewell, va
Posts: 2,044
Im with z,fuel pressure isnt quite where it needs to b
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Unread 07-27-2013, 10:55 AM   #6
dylantheheeler
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1996 ZJ 
 
Join Date: Feb 2010
Location: Smithville, Texas
Posts: 246
You mentioned you zip tied PCM connections. Before you did that, did the wiggle test on the connections cause any change in idle? If so, the zip ties are only temporary, and very temporary at that. It will go back to normal because of the bad solder joints inside the PCM.


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Unread 07-27-2013, 09:39 PM   #7
Oldfrog
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1997 ZJ 
 
Join Date: Feb 2008
Location: Louisiana/Texas
Posts: 3,785
Quote:
Originally Posted by dylantheheeler View Post
You mentioned you zip tied PCM connections. Before you did that, did the wiggle test on the connections cause any change in idle? If so, the zip ties are only temporary, and very temporary at that. It will go back to normal because of the bad solder joints inside the PCM.
Not always, because a poster, "chalkboard", here on JF, found that his PCM connectors needed attention. Do a search on his name and you'll find his pics of what he did to cure his issues.

'Not saying you DON'T need a PCM, but before you replace it, you need to make sure there isnt something in your system that fried it ( and will fry a new one too), or it's just the connectors.

There is also TSB ref number 08-01-97 to consider. Date of issue: Feb 3, 1997. Jtec powertrain control module wiring harness packages.
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Cajun Recipes and Fishing tips

Words of wisdom, Posted by Zeejay:
"..it's not air, it is the opposite of air... suckage so to speak."

Stalling? Work through Zeejay's list
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Unread 07-27-2013, 10:28 PM   #8
MechanicTyler
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1998 ZJ 
 
Join Date: Jul 2013
Location: Battle Creek, Michigan
Posts: 15
Ok guys i have been working on it all day and I have pulled apart the pcm and checked for resistance on each pin, all were zero resistance i have not cleaned the grounds but when i replaced the fuel pump there was a metal band across the regulator on top and the airtex pump did not have one so i just installed figuring it didn't need it could that be the source of my pressure leak. there is also a hesitation when accelerating and it pops through the manifold between 1500 to 3500 rpms and seems like if i let off the gas it will speed up a little. sometimes there is a severe backfire through the muffler almost like it is fluctuating in fuel pressure i disconnected the upstream o2 when it does the idle fluctuation while warming up and it did stop the fluctuating. wiggle test on the PCM showed no problems i figured better safe than sorry though. i did clean the tank and put fresh gas in it. This Jeep only has 93,000 miles on it! also the set of autolite plugs i pulled out of it were bridged almost as if they melted a gap between the electrode and the bridge

Last edited by MechanicTyler; 07-28-2013 at 01:16 AM..
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Unread 07-29-2013, 08:25 AM   #9
MechanicTyler
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1998 ZJ 
 
Join Date: Jul 2013
Location: Battle Creek, Michigan
Posts: 15
Rechecked fuel pressure 29 PSI how with a new fuel pump but my gas gauge is acting funny now after 1/8 a tank it drops straight to E, but is running better with new head gasket and intake gasket, I can get up to speed if I slowly press on the gas but if I put the pedal to the metal it pops and bogs out also a really heavy gas smell as if I am not getting a hot enough spark to burn all fuel in the cylinders I am going to recheck the fuel system and get back with you guys have any of you ever had this popping or loss of power issue it also does it more right when I start it but when warmed up it acts like I said above.
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Unread 07-29-2013, 08:36 AM   #10
ZeeJay1997
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1997 ZJ 
 
Join Date: Dec 2008
Location: Sweet Home, Alabama
Posts: 6,340
Check the voltage to the pump. Also be doubly sure it's snapped together correctly.

Back pin across 12v and ground connections at the fuel tank and run your meter leads out where you can see the meter. Crank the vehicle, read the voltage. It should read what ever the battery reads. If it doesnt, check the ground to the pump and the hot to the pump individually. If it does, take the pump back to wherever you got it and get your money back. Then find a Bosche pump to put on it.
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Unread 07-29-2013, 05:30 PM   #11
MechanicTyler
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1998 ZJ 
 
Join Date: Jul 2013
Location: Battle Creek, Michigan
Posts: 15
It is an Airtex Pump and I hear they are crap and to go with a dealership pump or a Bosche pump like you said
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Unread 08-02-2013, 11:09 PM   #12
MechanicTyler
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1998 ZJ 
 
Join Date: Jul 2013
Location: Battle Creek, Michigan
Posts: 15
Update 12 volts across pump leads taking piece of junk back tomorrow and getting my money back also I am done getting gas at shell there is so much **** in the bottom of my tank its unreal after 4 months.
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Unread 08-02-2013, 11:26 PM   #13
Edwardmiller221
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Join Date: Apr 2011
Location: Plainfield, Il
Posts: 87
Mine did the same thing until I replaced my aftermarket crank sensor with an actual dealer part. I'm not kidding. Ran like Crap would work for a short time then get worse the longer running. I did think this would work, but research stated this is a known issue with this specific part. It was bucking and backfiring acting like out of time and like gas pressure was failing. Now runs like new.
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Unread 08-04-2013, 02:54 AM   #14
MechanicTyler
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1998 ZJ 
 
Join Date: Jul 2013
Location: Battle Creek, Michigan
Posts: 15
Update fuel pump replaced now runs like a champ don't ever buy Airtex fuel system parts.
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Unread 08-04-2013, 11:53 AM   #15
Oldfrog
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1997 ZJ 
 
Join Date: Feb 2008
Location: Louisiana/Texas
Posts: 3,785
Funny you say that. I've heard it before...but an airtex has been in mine for several years now. ( I didnt know any better at the time)
__________________
Cajun Recipes and Fishing tips

Words of wisdom, Posted by Zeejay:
"..it's not air, it is the opposite of air... suckage so to speak."

Stalling? Work through Zeejay's list
Oldfrog is offline   Reply With Quote




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