P0300, P0305, P0306 Misfires - JeepForum.com
  • 1 Post By jtec
  • 1 Post By Jerry Bransford
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post #1 of 6 Old 05-02-2021, 01:15 PM Thread Starter
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2003 TJ Wrangler 
Join Date: Jun 2017
Location: Orange County
Posts: 41
P0300, P0305, P0306 Misfires

Hey Guys,

Trying to track down a multi-cylinder misfire. I have a 2003 Rubicon 4.0 w/auto trans. I have posted before about this, but I thought I would start fresh.

I was able to finally check fuel pressure: 45 psi.
After 5 min, with engine off, and key on: 37 psi
After 45 min, with engine off and key on: 26 psi

I then drove it for 30 miles the MIL was on and it drove ok. I then got on the freeway and really pushed it, and then the engine started to misfire, and the MIL started to flash.
I turned it off and restarted it, which it then ran ok again, and drove home.

With the engine idling I hooked up the scanner and recorded the following live data:

ECT - 196 F
SHRTFT1 - 0.0%
LONGFT1 - 18.8%
SHRTFT2 - 0.8%
LONGFT2 - 14.8%
O2B1S1 - 0.400 V
O2B2S1 - 0.740 V

While watching the data live, both O2 voltages would fluctuate between 0.100 - 0.800 v
and both Short Fuel trims would fluctuate between -0.08 - 2.3 %
The Long Fuel Trims held steady at those values.

I have not changed any parts yet. The spark plugs were changed about 10k miles ago with Autolite XP-985.

Any help would be appreciated.

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post #2 of 6 Old 05-02-2021, 02:18 PM
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2014 JK Wrangler 
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Spec is 49.2psi, + or -2 psi SO...... pressure is CLOSE and no telling volume. That is a owner judgement call on pump.

With scanner Did you look at freeze frame for the codes? might be a clue to help.
Otherwise tell us any other codes?
Jeep is STOCK - just as it was made no added, modified, changed aftermarket BS?
ALSO I would ask about ECT - correct thermostat? Its in range but seams low.

18.8% correction, a misfire, a flashing CEL and questionable fuel pressure I would be thinking fuel pump..
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post #3 of 6 Old 05-03-2021, 05:37 PM Thread Starter
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The engine is completely stock. No mods.

I recently changed the thermostat about 2 months ago. Stant 195 degree.

I mistakenly reset the codes the first time it happened and lost the freeze frame. The freeze frame that is stored now is open loop and all fuel trims were 0%.

Those are the only codes. The first time it happened I had P300, P301, P303, P305, P306.
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post #4 of 6 Old 05-04-2021, 12:54 PM
Jerry Bransford
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2004 TJ Wrangler 
Join Date: Jan 2003
Location: Escondido, California
Posts: 72,227
Multiple misfires can be caused by a bad crankshaft position sensor, a bad O2 sensor, a bad pre-catalytic converter, or even the wrong technology spark plugs having been installed. With the newer waste-spark ignition system your 2003 has, platinum-tipped spark plugs with the platinum coating on just one side of the gap will cause misfires and a bad idle. Those are called "single-tipped" platinums and two of them that will cause that problem are the Autolite AP-985 and Champion 3034... platinums with the platinum coating on just one side of the gap. Spark plugs that are known to perform well with your 2003's waste-spark ignition system and not cause misfires include the Autolite XP-985, Autolite APP-985, and Champion 7412.

A bad upstream O2 sensor can cause a bad fuel mixture and the PCM is rarely smart enough to know if it's the O2 sensor causing it. If you replace your O2 sensors, make sure to ONLY install NTK O2 sensors and make sure to not use Bosch O2 sensors which cause problems.

Finally, a bad Crankshaft Position Sensor can cause this problem too. If you buy a replacement, avoid store brand sensors and try to go with a Mopar. Know too that the CPS sensor is different for an automatic transmission vs. manual transmission so make sure it's the correct model for your type of transmission.
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post #5 of 6 Old 06-11-2021, 05:05 PM Thread Starter
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I just wanted to give an update on this. I replaced the crankshaft sensor (Mopar) - no change. I then replaced all four O2 sensors (NTK) - no change. I then replaced the fuel pump (Delphi) - no change. So, I was reading some other post that suggested to disconnect the battery, which I did overnight. The next morning I went for a test drive, and at the 31 mile point the check engine light came on, but no flashing, and it was still running good. I checked the code(s) and it showed just P0432, which I never had before. This is indicating that the catalytic convertor is hosed.

The P0432 makes sense to me because since I was having the misfires and did drive it the first time it happened for about 10 miles in that condition, I probably damaged the convertor then. I was afraid of getting stranded, so I kept driving it. (Don't do this, turn off then restart, and the Jeep will run fine for awhile).

So, it appears my misfire is fixed, and now I will replace the cat as soon as it arrives. I wish I knew which component was the cause, but glad the issue seems to be resolved.
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post #6 of 6 Old 06-11-2021, 05:10 PM Thread Starter
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Also, I wanted to say after replacing the fuel pump, the pressure was 54 psi. I was at 45 psi before. I know the spec is 49 +- 2, so I'm above the spec, hopefully no issues will come up because of this.

And thanks for the suggestions from Jtec and Jerry - mucho gracias
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