4.0 Random Misfire need help - JeepForum.com
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post #1 of 2 Old 04-28-2013, 12:10 PM Thread Starter
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1997 ZJ 
Join Date: Apr 2013
Location: Fayetteville
Posts: 1
4.0 Random Misfire need help

My ZJ had a really bad misfire a while back, idle was very rough ect. Did an oil change, air filter, plugs, wires, cap, rotor, dist, map sensor and upstream O2 sensor. This took care of the rough idle and horrible gas mileage, although i still have a random misfire on all cylinders, I'm unsure of what to do next.. any suggestions? I have heard of replacing the TPS and the CPS.. would this help?

MoparMan97ZJ is offline  
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post #2 of 2 Old 04-28-2013, 10:02 PM
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2007 WK 
Join Date: Feb 2008
Location: Louisiana/Texas
Posts: 5,092
I hope the sensors that you replaced were all Mopar sensors, otherwise, you're pi**ing in the wind. ( no Bosch O2 sensors for certain !)
I can assure you, that if you dont get a Mopar crank sensor, you'll have issues. But I'm not sure that's the problem just yet.

I would start with the ground connections. One near the oil dipstick, another at the coil ( this one is VERY important, as it's been said that this one can fry the PCM...although I cant confirm it.) Another at the fender well near the battery, and of course, the battery cable itself. You CANNOT tell if any of these ground points are bad by merely looking at them. You'll need to take them all off and get a wire brush after them to make them all shiny.

-Spark plugs...hopefully, you installed plain old champion copper tipped plugs...the cheap ones.

-Next, I'd pull the relays and fuses out of the PDC ( fuse panel under the hood) to clean up the contacts.
-Then the battery connections on the side of the PDC will need cleaning up.
-Just for grins, I'd clean the throttle body and the idle air controller.
-Check fuel pressure at the rail. Faulty fuel pumps are common.
- If that's good, run some Seafoam thru the fuel.

If it continues to stall, or misfire, then you'll need to :
1. Check the crank sensor. ( do a search for the procedure on this forum)
2. Next...( Gulp !) ....I'd wiggle the connectors on the PCM while it's idling. If you can make it stall, you'll either need to clean the PCM connector plugs ( rarely successful for very long) , or replace the PCM.
3. Of course there is also the occasional sensor that has a dirty connector somewhere, too.
4. Remember that Jeeps insist upon clean power ( 12v), good batteries and good alternators. They are temperamental little biotches.

2007 5.7 Hemi, Ltd. QD II -
1997 ZJ, 4,0 select trac, Up country, track lok.
Oldfrog is offline  

4.0 , misfires

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