Misfire

[kj.priamo]

I own a 2004 KJ with the 3.7L and it's been running fine until a few days ago I got a P0304 code and started feeling the misfire shaking the car badly. The only other codes I had prior to this was P0157 and P0138 that I assumed was linked to some sort of exhaust leak somewhere. I changed the spark plugs as they looked pretty bad, did the usual ring around the rosie with the coils to see if they were damaged but despite all this I still have the same misfire and am now left with the same 2 codes (P0157 and P0138). I was wondering what I should try replacing, checking, or doing next. TYIA

 

[lfhoward]

How is your coolant level? Possible head gasket leak into #4 comes to mind.

 

[kj.priamo]

How is your coolant level? Possible head gasket leak into #4 comes to mind.

Coolant level is fine. I don't have coolant leaking into the oil causing the head gasket to deteriorate nor do I have milky/white oil. I was thinking possibly fuel injectors or fuel filter, besides that the only other thing I'm thinking it could be is a bent/cracked piston

 

[ltd02]

Have you ever changed the O2 sensors?

 

[kj.priamo]

Have you ever changed the O2 sensors?

I have not. I want to order the right sensors corresponding to the codes but trying to find them online is a bit confusing. Hopefully this could maybe fix the problem due to the air to fuel ratio.

 

[ltd02]

Rock auto and get NTK

 

[kj.priamo]

Rock auto and get NTK

I now have a new code which is P0175. I’m left with P0175, P0138, and P0157

 

[ltd02]

How many miles on your liberty? Really good to get live data when chasing air/fuel issues.

 

[kj.priamo]

How many miles on your liberty? Really good to get live data when chasing air/fuel issues.

Only 155k. I don't have a good OBD scanner to see live data. Do you have any suggestions?

 

[ltd02]

Well if the o2 sensors are original, I'd probably change them all. They aren't too expensive and really have a finite lifetime. One acting up can affect the performance of others. You really can't tell which one is failing unless you can see the voltages. I think they're all narrow band o2 sensors.

 

[JeepKJ_Guy]

I had the same and I put in NGK plugs gaped at .040 and put in new Denso Ignition coils and fixed mine - even with bad O2 sensors throwing codes, then resolved most of the 18 ECM codes replacing sensors on the throttle body, TPS, etc - she runs fine, just the O2 sensors are throwing codes because of probable hard off road driving, she's got 193k, and I bought it used several months ago.

It's a 2003 KJ with the 3.7L, and never stalls out and has a smooth idle, and got it down to just 3 error codes out of 18, - 2 are from O2 sensors each rail, and EVAP code - those will be my next project, but she runs great!

 

[Ksat]

You may have a bad PCM. For some reason, #4 ignition coil control is the one that gets fried. Happened to me a few weeks ago.

I'm assuming you don't have a scope. Do you have a multimeter that can read frequency (via the test leads)?

 

[kj.priamo]

You may have a bad PCM. For some reason, #4 ignition coil control is the one that gets fried. Happened to me a few weeks ago.

I'm assuming you don't have a scope. Do you have a multimeter that can read frequency (via the test leads)?

Yes I have a multimeter. I changed the spark plugs which got rid of the P0304 code but the misfire still exists. I also changed the MAP sensor as my old one was split in half (somehow) but it seemed like that did nothing to fix the problem. It’s so strange that I’m left with P0175, P0175, P0138 and now an intermittent oil light.

 

[kj.priamo]

I haven’t changed my coils yet since they don’t seem to be throwing a code. Don’t want to dump money if it’s not going to fix the problem. Should I replace all 6 coils and hope for the best?

 

[JeepKJ_Guy]

I haven’t changed my coils yet since they don’t seem to be throwing a code. Don’t want to dump money if it’s not going to fix the problem. Should I replace all 6 coils and hope for the best?

I still would change the coils with Denso coils and use NGK plugs set at .040 just for openers to troubleshooting this issue first, and every coil/plug I replaced I started it up to make sure it ran, you have to start with a good base then tackle the other codes. - I went thru this, and if you have a good base then it could be MAF, Injectors, O2, and a whole host of sensors on the throttle body, I replaced all those, you need to have a good base to start from or it could be something more involved like:

These are the several factors that can cause the P0175, P0138 codes to appear.

• P0175
• A clogged or leaking fuel injector
• Fuel regulator failure
• A clogged or defective mass air-flow sensor
• Defective coolant temperature sensor
• Thermostat malfunction
• ECM requires reprogramming
• Unclean or faulty oxygen sensor
• Have vacuum leaks – PCV hoses, vacuum hoses, and the intake manifold gasket
• Issues with fuel delivery
• Inaccurate fuel pressure
• Leaking exhaust
  
  
• P0138 stands for “O2 Sensor Circuit High Voltage (Bank 1, Sensor 2)”
• The code is logged once the PCM detects that the voltage signal from the rear O2 sensor is too high for a given time.
• Although the P0138 code often appears due to a worn-out oxygen sensor, it’s not the only condition that can trigger this error. This diagnostic code can also appear when any of the related components fail or if there’s a fuel delivery issue.

On mine the O2 sensor wires are disconnected when I bought it, maybe that's why the previous owner did that was to get rid of the bad idle, I don't recommend as a permanent solution, I'm just throwing that out.


Also I disconnected the battery cables after each sensor replacement to reset the PCM

 

[Ksat]

Don't buy any ignition coils just yet. Ignore the O2 codes for now, especially if your miss occurs even when the engine is stone cold.

Swap #4 ignition coil with one from another cylinder, erase the codes, drive vehicle and see if the misfire changes cylinders. If no change, do the same with the 4's injector. If still no change, you'll need to use your multimeter to test for power and ground control signal going to both #4 ignition and injector. I can walk you through it if you get to that point.

If you have (or can rent) a compression tester, it wouldn't hurt to test #4 to see what it is. Floor the pedal and crank for a good 5 seconds to see what reading you get.

 

[kj.priamo]

Don't buy any ignition coils just yet. Ignore the O2 codes for now, especially if your miss occurs even when the engine is stone cold.

Swap #4 ignition coil with one from another cylinder, erase the codes, drive vehicle and see if the misfire changes cylinders. If no change, do the same with the 4's injector. If still no change, you'll need to use your multimeter to test for power and ground control signal going to both #4 ignition and injector. I can walk you through it if you get to that point.

If you have (or can rent) a compression tester, it wouldn't hurt to test #4 to see what it is. Floor the pedal and crank for a good 5 seconds to see what reading you get.

I ended up replacing all 6 coils to see if that'd fix the problem. Still the same issue. I put back the old coils and returned the new ones but I checked the brand new spark plug in cylinder 4 I put in and it was all black covered in carbon already. The only thing I'm thinking it could be now is either that piston/cylinder is done or the fuel injector on cylinder 4 is defective. I removed the battery and erased all the codes as well.

 

[kj.priamo]

I still would change the coils with Denso coils and use NGK plugs set at .040 just for openers to troubleshooting this issue first, and every coil/plug I replaced I started it up to make sure it ran, you have to start with a good base then tackle the other codes. - I went thru this, and if you have a good base then it could be MAF, Injectors, O2, and a whole host of sensors on the throttle body, I replaced all those, you need to have a good base to start from or it could be something more involved like:

These are the several factors that can cause the P0175, P0138 codes to appear.

• P0175
• A clogged or leaking fuel injector
• Fuel regulator failure
• A clogged or defective mass air-flow sensor
• Defective coolant temperature sensor
• Thermostat malfunction
• ECM requires reprogramming
• Unclean or faulty oxygen sensor
• Have vacuum leaks – PCV hoses, vacuum hoses, and the intake manifold gasket
• Issues with fuel delivery
• Inaccurate fuel pressure
• Leaking exhaust
  
  
• P0138 stands for “O2 Sensor Circuit High Voltage (Bank 1, Sensor 2)”
• The code is logged once the PCM detects that the voltage signal from the rear O2 sensor is too high for a given time.
• Although the P0138 code often appears due to a worn-out oxygen sensor, it’s not the only condition that can trigger this error. This diagnostic code can also appear when any of the related components fail or if there’s a fuel delivery issue.

On mine the O2 sensor wires are disconnected when I bought it, maybe that's why the previous owner did that was to get rid of the bad idle, I don't recommend as a permanent solution, I'm just throwing that out.


Also I disconnected the battery cables after each sensor replacement to reset the PCM

As @JeepKJ_Guy suggested. I'm now leaning towards possibly a clogged or leaking fuel injector since the spark plug on cylinder 4 was all covered in carbon despite the plug being a few days old.

 

[ltd02]

Good call. Leaky injector could cause your issues with a cylinder specific misfire and those O2 sensor codes. Could also have fouled your O2 sensors even if the injector clears or you replace the injector.

 

[JeepKJ_Guy]

@kj.priamo

You may want to try this easy way to clean all your injectors easily by this method below:

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I would pull the fuse to the fuel pump (front fuse box by radiator) - then run Seafoam straight thru, possibly 2 cans.

I haven't tried this but will, - back in the early 70's I used to use a can of liquid Gumout with a kit that connected straight into the carburetor, and plugged the gas line with a pencil.

Here is one good review from the video, - there are lots of similar videos there.

"Seafoam and this Seafoam Spray process just saved me a day changing out plenum gaskets on my Jeep .. and possibly a lot more! Jeep ran fine yesterday, started today and didn't sound quite right. I was half way between home and local auto parts store for another project when the check engine line popped up and beeped in intervals... which is different than when it vapor locks. Coded - [cylinder 3 misfire, intake manifold, plenum gasket, injectors] ... and I was a bit stressed. I knew I had Seafoam spray and a couple more cans for the fuel tank at home, so Iought 1 can of seafoam and plenum gasket set .. dumped the seafoam in and hobbled back, preparing myself to spend the rest of the day cussing the intake manifold. I dumped a second can of seafoam into my tank and got my husband to help with the spray process in this video. Jeep spit and sputterd and popped and made noises I never want to hear again but we got the can emptied.... let it sit for about 15 minutes and held my breath while I started it up. I still can't believe it, but it sounded perfectly normal [holy $hit!]. I checked codes with my trail dash 2... zero codes! Drove it around for about 15 minutes at varied speeds and had zero issues.... and still zero codes!I am never getting cheap gas again and will seafoam every tank. I am so friggin appreciative of @SeaFoamOfficial ! Thank you!!!"