4.0 Misfire Solution - JeepForum.com
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post #1 of 1 Old 06-05-2010, 02:02 PM Thread Starter
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1998 TJ Wrangler 
Join Date: Sep 2009
Location: So. Cal.
Posts: 56
4.0 Misfire Solution

P300, P304, P305 P306
I had these codes for over 2 years. I had to get it smogged (Cali) and scrambled to fix the misfrire. Spent all kinds of time and money troubleshooting it and was not sucessful. My scanner said the codes were happening at 2500 rpm and 55 mph. I tried plugs, wires, cap, rotor, CKP sensor, moved my injectors around, ran a compression test, Seafoam and almost every answer or solution I could find on this forum. I still wanna say thanks to all of you for your knowledge and ideas.

I ended up clearing the code, waiting for the monitors to all check in, got the emissions test and passed but the MIL still came back a day later.

About a week ago, it became tough to start and even stalled once. I had a failing fuel pump relay and it finally died in my driveway on on thursday. I put the horn relay in it's place and rolled to an auto parts store to buy a replacement. Yesterday the MIL went away on it's own. Go figure, I had put a pressure tester on the fuel rail months ago and never noticed anything weird.
I suppose the relay may have been shutting the pump off intermittently? The regulator was maintaining the pressure just high enought to fire some of the injectors? Wierd huh? Well anyway, I just wasnted to throw that out there. If you're having this problem, it might not hurt to swap your horn and fuel pump relays.

"Jeeps are cute, Barbie drives one" Is this supposed to be an insult? Barbie has an awesome car collection! A Mustang, a 'Vette, Kawasaki quad, a Fiat, an Escalade, a 57 Chevy Bel-air... I mean... Barbie's a gearhead!
PBNJTJ is offline  

P300 , P304 , P305 , P306 , misfiring , running rough

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