2009 JKU P0304 Code + Rough idle, hesitation, bucking solution - JeepForum.com
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post #1 of 5 Old 02-02-2013, 03:03 PM Thread Starter
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2009 JK Wrangler 
Join Date: Feb 2013
Location: NY
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2009 JKU P0304 Code + Rough idle, hesitation, bucking solution

Hi everyone.

I'm copy/pasting my youtube video on what to look for when getting a misfire code. This misfire code could be several things, but for me it was bad plug wires. However, I replaced all wires and a new OEM coil because I had already bought them and thought 77,000 miles was a good time to do it while I was under the hood.

Here's what happened in chronological order. I hope it can help someone:

It's not my jeep, so I wasn't into what was going on with the maintenance, so I kind of ignored it.

It "became" my jeep because, well, it's my girlfriend's!

Although she had it regularly serviced (oil, etc.), the servicing never cleaned anything or really checked anything and cleaning agents were never used in the 77,000 miles its been running.

1. Problem stated when the CEL came on. Then it went off on its own. We thought it was bad gas, or something like it.

2. About a week later the Jeep went upstate NY and was fine until the CEL light came back on. Local garage pulled the plugs, said one was fouled, cleaned it and put it back in. If I was there we would replaced them but that didn't happen. Jeep ran fine.

3. A couple days later the CEL came back on and rough idle/hesitation/bucking.

4. We live in NYC so getting it serviced and off the island is difficult. I took the day off and bought some new plugs from Jeep dealership because there aren't many options in Manhattan (and I wanted stock parts). Replacing them on the street on a cold day when they've been in there for 77,000 miles is difficult. Make sure to get one of those angle things for your wrench and prepare for a couple bloody knuckles. I also put a can of Techron in. I did not know about sea foam at this point and probably would have used it had I known.

5. CEL was off, idle was better, but there was still occasional hesitation/bucking at low RPM. I thought maybe its clogged injectors since it never had any cleaning agents run through it.

6. A couple days later the Jeep was taken on the highway with low gas to hopefully clean it. All seemed good. Then the next day it was back to the same. I bought some throttle body cleaner (do NOT use carb cleaner as it is too harsh) and cleaned the throttle body. Idle was better, engine seemed smoother. Put another can of Techron in it and drove it back to the city. MPG went up to almost 16 highway so I thought things were improving.

Here's an awesome reference for cleaning the TB:

7. No CEL, occasional hesitation, etc, and, again, thought that it was just dirty. Jeep went to Long Island and had all fluids changed and a more thorough throttle body cleaning.

8. Jeep took a trip to New Hampshire and it was Ok on the highway. When it got to the destination it was back to the same. On the highway it was fine until the FDR when CEL flashed briefly, then was back on steady. Strangley, the MPG was up to 18 from 14.7!

9. When I replaced the plugs I also bought a coil and plug wires and was prepared to return them. Glad I did not!

10. Thinking that it was either the coil and wires or a plugged injector I decided today to change the coil/wires. Now watch the video and read the description. You will hear the misfire! BTW, the description is a shortened version of this.

So, after replacing plugs, wires and the coil the problem was solved! The Jeep runs as smooth as butter now and can pull away in third gear from a stop just like she used to. Sigh.

From doing a TON of research and reading everything on this awesome board this was my solution. I know that there can be a number of issues that produce a misfire, but this is stuff that just might help you if it is the same problem as I had. At the very least, cleaning your TB will improve your mileage and make the engine smoother, etc.

Good luck out there and drop me a line if you have questions or if it helps you in some way!

DirtCreature is offline  
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post #2 of 5 Old 02-02-2013, 04:04 PM
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Did you try just replacing the wires first? Coil may have been fine.
akjeepboy is offline  
post #3 of 5 Old 02-02-2013, 04:24 PM Thread Starter
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Originally Posted by akjeepboy View Post
Did you try just replacing the wires first? Coil may have been fine.
I did. Replaced the right bank (looking from the front of the engine bay) and the problem seemed to clear up.

However, after so many what seemed to be temporary fixes I also decided just to replace it anyway.

If I hadn't already bought the coil I would have replaced the rest of the wires and then waited to see what happened.

So, yeah, that's probably the right thing to do first, then see what happens.
DirtCreature is offline  
post #4 of 5 Old 02-02-2013, 04:39 PM
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It never hurts to replace them kinda parts, I had my cats replaced and had then replace my o2 sensors didn't need them but never hurts!
akjeepboy is offline  
post #5 of 5 Old 11-05-2013, 12:09 PM
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Join Date: Nov 2013
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P0303 and then 0304 after spark plugs change

Hi Guys

I just did a spark plug change (those write ups show it too easy not to do it... is a LIE... they should begin saying... COMPLICATED for short people with big hands...)

I changed all the plugs (even pull out all the pieces of the old plug broken on #5)

reinstalled all the plugs and then when I start the engine I got a p0303 whith a rough idle. I did what I read on all the JK forums available on Internet.

Pull out the plug and exchanged it with another one (I choose the #4 because is more accesible for me)

Cleared the P0303 code and start the engine, now I got a P0304.

Sadly I bought this plugs from the internet because is a bit difficult to find those in my country (not US, Panama Central America) and now I will need to order an extra plug. I can find the common Champion but not an NGK Iridium IX for the JK (not mentioning that today is holiday and all the spare part shops are closed, except for two which only sell japanesse/korean/chinesse car parts)

I will order 4 plugs to have some in case I need it.
lordj7x is offline  

misfire , p0304

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