P0303 engine code on 2008 Wrangler JKU

J U L I E

J U L I E

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Hi. My Jeep has over 223,000 miles on it. This has been a great Jeep! Recently, upon acceleration it started stuttering and misfiring, and I am pretty certain it gives a slight backfire when I mash the pedal. Check Engine light flashes, but does not stay on. It sounds like a John Deere tractor after that and at idle, lol. I have to keep it under about 45mph to keep it from stuttering and misfiring. I can drive it anywhere I want, as long as I don't hard accelerate and I drive like miss Daisy under 45mph, it runs totally fine.

We think we changed all parts that relate to the P0303 code:

Plugs, wires, coil pack, Fuel injectors.
In the last year, EGR valve, valve gaskets covers, head gaskets. Bank 1 up and downstream O2 sensors so far. I also have a new cat back, but did not do the Y pipe yet.

I have read one thread where there was a suggested bad lifter. I really hope that is not the case. Initially when we start it up, it idles very nice. I had always thought there was an infamous tick with a lifter going out. I have no tick at all in the engine. There is a weird new sound I hear upon start up, it is quick sound, I will see if I can attach the video on that. Then it sounds great, totally normal. As soon as we take it out for a test drive, same experience mentioned above. And then the CE flashes, but it does not stay on, and when we put the code reader to it , the P0303 code still comes up.

Poor thing, been such a good Jeep.

I need to get one more maiden voyage out of her before we bring it back into the OR for a LT swap. We are going to TN Jeep invasion in August and we won’t have time to do all that before we leave.

Anyone have experience with this? Appreciate your insight/suggestions.
 
With that many miles, I would also check the wiring harnesses before you start tearing down the engine. Over time, that engine compartment makes wire insulation brittle, or mice like to chew on things. My first thoughts would be just as you did, with plugs, wires, coil packs.

Does it only happen under load? Or can you rev the engine normally when it's in park?
 
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Only under load. We can rev it in park, and doesn't miss a beat, sounds perfect.

When we had everything apart for fuel injectors, we inspected wiring and replaced all the lume. It was so brittle lol. But everything looked great.
 
BLACKJKU said:
Do you remember what brand of spark plugs and wires you used?

Maybe try changing no 3's plug and wire with a plug and wire from another cylinder and see if the problem moves to the cylinder you put no 3's in.

Also don't know if you read any of these ideas or not.

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We ended up replacing the number 3 plug and wire twice just to make sure. Xccel wires, plugs are NKG

I did check find that other link initially, but could not find that damn link last night when we did our final test drive. I gave up looking again and decided to resort to sharing our journey lol. Thanks 😊
 
J U L I E said:
Only under load. We can rev it in park, and doesn't miss a beat, sounds perfect.
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This would lead me to believe it's either airflow choked somewhere and your loading up on fuel or a closed loop sensor is bad. You stated you replaced the O2 sensors already... Does the 3.8 have a mass air flow meter? I'm not real familiar with that engine.
 
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Another thing you might check is the underhood insulation coming loose over the air box and causing it to choke out the engine. It usually causes a P2174 CEL but maybe not all the time.
 
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tJKrider said:
This would lead me to believe it's either airflow choked somewhere and your loading up on fuel or a closed loop sensor is bad. You stated you replaced the O2 sensors already... Does the 3.8 have a mass air flow meter? I'm not real familiar with that engine.
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From what I have read, chrysler does not have a MAF, but they do have a MAP, but it is not necessary to clean them. We noticed the throttle body is pretty dirty. We are going to clean it. While I am sure it will make the engine happy, probably not the solution.
 
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J U L I E said:
From what I have read, chrysler does not have a MAF, but they do have a MAP, but it is not necessary to clean them. We noticed the throttle body is pretty dirty. We are going to clean it. While I am sure it will make the engine happy, probably not the solution.
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There are videos using Sea Foam to clean the TB's intake manifold, I know mine at 76,000 miles it had like tar stuff all over the inside of the intake manifold. I deleted my EGR valve i'm sure that's where it was coming from.
 
BLACKJKU said:
There are videos using Sea Foam to clean the TB's intake manifold, I know mine at 76,000 miles it had like tar stuff all over the inside of the intake manifold. I deleted my EGR valve i'm sure that's where it was coming from.
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We still have my EGR valve. But we do have that tar like substance in there.

20240427_124918.jpg
 
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