2011 jeep jk sputtering / backfire

T

THSPARKMAN

New member
Need Help. This has probably been asked before but I'm going to ask again so sorry if this problem has been addressed already. I have a 2011 Jeep Wrangler JKU w/ 3.8.
First let me start by telling you my problems.
* Sputtering, Struggling, Occasional Backfire, Occasional Red Lighting Bolt, Lack of Take off Power, Check Engine Light of course, just overall running / missing etc.
The most recent codes are as follows.......
PO305 Cylinder 5 Misfire and PO128 Coolant Thermostat (Coolant Temp Below Thermostat Regulating Temperature)
Second let me explain what I have already done
1. Replaced the Coil Pack w/new.
2. Replace the Thermostat w/new (twice).
3. Replaced the spark plugs and wires w/new.
4. Replaced the Coolant Temperature Sensor (twice).
5. Replaced the EGR Valve w/new.
6. Flushed the Coolant and Refilled w.new.
7. Oil Changed w/new.
My next thoughts / things to change would be the 02 Sensors (but not getting that code) or the Catalytic Convertor. I'm not a super mechanic by no means but am thinking that let's say the cat is plugged on the passenger side that it could be pushing back into Cylinder 5 (hardest plug to access) that is causing the misfire code. Just wanted to see if I could get some advice from those that are more experienced with this type of thing before I go and spend the money that it costs for a new Catalytic Convertor. Thank-you all in advance for your help and advice.
 
What did the plugs look like that came out? And more specifically what did the #5 plug look like? Diagnosis wise you need to start with the furthest upstream issue, the misfire, first. What brand plugs did you put in and how many misfire counts are you getting?
 
POOJEEP said:
What did the plugs look like that came out? And more specifically what did the #5 plug look like? Diagnosis wise you need to start with the furthest upstream issue, the misfire, first. What brand plugs did you put in and how many misfire counts are you getting?
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They looked terrible. It was definately time for a tuneup. Um Bosch Double Platinum Spark Plugs and Duralast Wire Set. I can't really remember if the #5 plug looked any worse than the others. They were all pretty bad.

When you ask for how many misfire counts am I getting. What do you mean? Sorry. On the code reader?
 
Do you have pics of the old plugs, that will be the most informative part to diagnosing. As to misfire counts I was referring to a diagnostic computer. Are you still getting misfires after the plug and wire I assume?

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If you have a code reader with real time monitoring, I would also check your long term fuel trim. Sounds like it might be running rich, which could support the oxygen sensor theory. How many miles on the O2 sensors and cat?

Could also be MAP or throttle body sensor, but that is less common.

Not sure if the coolant temp code is related or not.


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POOJEEP said:
Do you have pics of the old plugs, that will be the most informative part to diagnosing. As to misfire counts I was referring to a diagnostic computer. Are you still getting misfires after the plug and wire I assume?

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Don't have a picture. They were trashed a weeks ago. When I seen them and installed the new I just knew that was the problem and nope...........yes, still getting a misfire on cylinder 5 code. PO305. I think I will pull that plug this weekend to see what it looks like. Just frustrating and again I really don't want to drop $500 to $600 on a catalytic convertor if that is not the problem. If I knew for sure then yes for sure but I don't know.
 
Noble Woodsman said:
If you have a code reader with real time monitoring, I would also check your long term fuel trim. Sounds like it might be running rich, which could support the oxygen sensor theory. How many miles on the O2 sensors and cat?

Could also be MAP or throttle body sensor, but that is less common.

Not sure if the coolant temp code is related or not.


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Okay thanks. I am not sure if the 02 Sensors have ever been changed and my Jeep has 130K on it. Cat probably never changed either. I bought it used like 6 years ago or so and I know they haven't been changed in that time.
 
Yes, Bosch plugs are junk, go back to the OEM champions/NGK. The misfire will throw off your O2 sensors. You have to address the upstream problem with the misfire first.

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POOJEEP said:
Yes, Bosch plugs are junk, go back to the OEM champions/NGK. The misfire will throw off your O2 sensors. You have to address the upstream problem with the misfire first.

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Okay. A little development. After work yesterday I went to a local auto parts store and had them run an analysis with there tool and I got one more code than I got from my personal reader. The P0305 Cylinder 5 Misfire, P0128 Coolant Thermostat was still there but I received a new code of P0420 CATALYST SYSTEM EFFCIENCY BELOW THRESHOLD (BANK 1).

Just wanted to know with this new data is there any other suggestions.

So here is what I think I may do and would like your opinion.

1. Remove the Cheap Plug Wires and upgrade to a better quality (Autoparts store said they would give me credit to put towards a new set) What type do yall suggest?
2. Remove the new Cylinder 5 spark plug and inspect / replace if necessary. Is it okay to put a different type (Champion as per yalls suggestion) in one slot or do they all have to be replaced?
3. If there is still no improvement or change I will replace the 02 sensor on Bank 1 (Where ever that is) Drivers or Passenger? Also I have 2 on each bank......I guess I'll change the one closest to the motor.

What do yall think about this approach?
 
Are you missing any coolant? Head gasket leak can cause these issues. Have you swapped the fuel injectors? I would look a little further before throwing more money at it.
 
Was the oil milky at all? Does the oil cap have milky residue on it inside? Was the coolant discolored? Can you pressure test the coolant system? Can you do a leak down test on the cylinders? Can you test for exhaust gases in the coolant system. Honestly it sounds like you may be over your head diagnostic wise and you may not even have the tools to do it. Stop throwing parts at it.

Step 1 is to rule out possible head gasket. That is where the leak down and exhaust gases test comes in. Once that is done if ok.

Step 2 fix misfire. Factory Champion plugs. If misfire stays swap #5 wire with any other and see if misfire follows. O2 and cat codes may go away with misfire. Once solved.

Step 3 coolant temp code is usually thermostat. You have changed it twice. Possible water pump or plugged radiator or air bubble or leak or part of step 1.

You might have to pay someone to help diagnose. Please let us know what you find out as you go.

Worst case you may end up with a bad head gasket or something catastrophic but don’t worry. I know someone that just did a Hemi swap in a 2011 that has a good engine for cheap. Including cats and o2 sensors. [emoji16]


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A.J. said:
Was the oil milky at all? Does the oil cap have milky residue on it inside? Was the coolant discolored? Can you pressure test the coolant system? Can you do a leak down test on the cylinders? Can you test for exhaust gases in the coolant system. Honestly it sounds like you may be over your head diagnostic wise and you may not even have the tools to do it. Stop throwing parts at it.

Step 1 is to rule out possible head gasket. That is where the leak down and exhaust gases test comes in. Once that is done if ok.

Step 2 fix misfire. Factory Champion plugs. If misfire stays swap #5 wire with any other and see if misfire follows. O2 and cat codes may go away with misfire. Once solved.

Step 3 coolant temp code is usually thermostat. You have changed it twice. Possible water pump or plugged radiator or air bubble or leak or part of step 1.

You might have to pay someone to help diagnose. Please let us know what you find out as you go.

Worst case you may end up with a bad head gasket or something catastrophic but don’t worry. I know someone that just did a Hemi swap in a 2011 that has a good engine for cheap. Including cats and o2 sensors. [emoji16]


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Dang. I'm hoping it's not an engine swap. But you are right, most of your troubleshooting is above my skill level. I was hoping it was a plug or sensor. And to think when I started this thread I was worried about spending 500 to 600 on a new catalytic convertor........Oil Cap & Oil not milky, Coolant was a little discolored but not to bad, all of the leak test/pressure test (above head), I'm gonna put new/new plugs in (Champion this time) and upgrade the New / New plug wires and see what happens. Trying to get a hold of "MY GUY" that can do the other things but with this dang Coronavirus it's difficult.
 
You could swap a plug wire and see if the misfire moves before you replace plugs. If you switch the wire and it moves to that cylinder it is the wire.

FYI you can have the whole engine including cats o2’s and nearly new plugs for the $500-600 you mentioned.


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Last edited:
THSPARKMAN said:
If there is still no improvement or change I will replace the 02 sensor on Bank 1 (Where ever that is) Drivers or Passenger? Also I have 2 on each bank......I guess I'll change the one closest to the motor.

What do yall think about this approach?
Click to expand...

If you do decide to replace the O2 sensor, it is a good idea to change all of them at the same time. Ideally the cat would be changed at the same time too, but that's not always affordable. At 130k, if they are the originals, they will probably need replacing soon anyway.

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I just had the p305 code in my ‘08 with about 120k on it. Didn’t have the cat code, but I don’t know if they have the same codes. Whenever I would “accelerate” my engine light would start flashing. I did have almost all the same symptoms you are describing. Mine turned out to be a fuel injector (started with looking for a bad ground, then vacuum leak, and then plugs wires and coil pack). My 3.8 is definitely an oil burner, I think that played a part.


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littlebeas said:
I just had the p305 code in my ‘08 with about 120k on it. Didn’t have the cat code, but I don’t know if they have the same codes. Whenever I would “accelerate” my engine light would start flashing. I did have almost all the same symptoms you are describing. Mine turned out to be a fuel injector (started with looking for a bad ground, then vacuum leak, and then plugs wires and coil pack). My 3.8 is definitely an oil burner, I think that played a part.


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Okay. Thank-you. Me and a buddy are going to check/replace the fuel injector this week hopefully. I'll keep everyone updated. Just hope this is it.........getting tired of chasing this problem for sure.
 
Engine Replacement

Well. Ordered a brand new 3.8L. Engine is now out of the Jeep. Had to also order new exhaust manifold (showing signs of cracking) and a clutch kit cause mine was shot. In a couple of weeks the Jeep will have 0 miles on it and a 3 year unlimited mileage warranty. 135,000 to 0.
 
THSPARKMAN said:
Well. Ordered a brand new 3.8L. Engine is now out of the Jeep. Had to also order new exhaust manifold (showing signs of cracking) and a clutch kit cause mine was shot. In a couple of weeks the Jeep will have 0 miles on it and a 3 year unlimited mileage warranty. 135,000 to 0.
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Well technically it still has 135,000 miles on it but congrats on the new motor
 
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