Rough Idle or Missfire After Trying 4 Hole Fuel Injectors & P0431 Code

[Matt14]

I have a rough idle or missfire that I am trying to trace down. You can only hear it really and the RPMs aren't affected. It sounds more like a minor missfire to me but without any misfire codes.

The issue started when I decided to give 4 hole fuel injectors a try, big regret, I fell into the trap that they would be a good mod. Anyway, once I swapped those out, it started. I then put the original Mopar fuel injectors back in after cleaning them and replacing the O rings, but the same exact issue was present. I even took them back out again and replaced the O Rings again and the filters just to make sure. I can feel the injectors firing, so I think they are working as they should.

After a month or two with just dealing with this, I started getting a P0431 Code. I believe this is being caused by the rough idle and not the cause of it, but please let me know if I am wrong. I don't really want to replace O2 sensors or even the catalytic converters until this issue is fixed so I don't burn through the new ones.

I have cleaned everything in the throttle body, replaced the throttle body gasket, and done all the other things I have heard about for rough idle.

The spark plugs are only around 1-year-old and are Autolite Iridiums and I have checked the gaping recently, but could check again. I also replaced the coil rail without that fixing it as well.

Last time I did a compression test, everything was good there too. I can try this again to see if anything changed.

I have done some live testing but don't really see anything corelating with the slight missfire.

Since I also had some transmission issues, I was trying to sort out that seemed electrical. I replaced the PCM with a Wrangler Fix one, but that didn't help the idle.

I took it to a shop, and they didn't find anything either and said to try the coil rail.

Thoughts on next things to try? Also, do you think my thinking right about the code? Or should I try addressing that in hopes it fixes the other things?

Thank you for your help!

 

[Link]

That code appears to be one set when the downstream O2 sensor detects the catalytic converter isn’t functioning correctly. That sensor doesn’t affect running….it just monitors the converter. I’d say it’s likely the converters are clogging up. A clogged converter will cause running issues as well.

 

[Painfultruth]

The post converter sensor does provide feedback on the converter efficiency, but that feedback is used for regulating the A/F, check your fuel trims.

 

[Link]

The after cat O2 sensors aren't used to regulate engine running. You can take them off and, while the computer will throw a code since it's expecting them to send data, it will not affect how the engine runs.

You can take the vehicle to a muffler shop which can test the catalytic converter. But, unless the OP is in CA, new catalytic converters aren't very expensive.

 

[Painfultruth]

The after cat O2 sensors aren't used to regulate engine running. You can take them off and, while the computer will throw a code since it's expecting them to send data, it will not affect how the engine runs.

You can take the vehicle to a muffler shop which can test the catalytic converter. But, unless the OP is in CA, new catalytic converters aren't very expensive.

That’s odd because the hardware documentation states otherwise.


“The first function is to measure catalyst efficiency to meet OBD II requirements. If the catalytic converter is working properly, the oxygen content of the exhaust gases at the converter outlet fluctuates significantly less than at the converter inlet. The PCM compares the switching rates of both downstream and upstream O2 sensors under specific operating conditions to determine if the catalyst is functioning properly. Any time the upstream to downstream switching ratio exceeds a calibrated value, a catalyst efficiency fault will be stored.


The second function is downstream fuel control. This function adjusts the upstream O2 Goal Voltage within the range of operation of the upstream O2 sensor. The upstream Goal Voltage is used to ensure long catalytic converter life by allowing the PCM to control the amount of air and fuel that is supplied to the catalytic converter.


Before 1996, the Goal Voltage was a pre-programmed fixed value based upon where it was believed the catalyst was most efficient. While the upstream O2 sensor input was used to maintain the 14.7:1 air/fuel ratio, variations in engines, exhaust systems and catalytic converter ageing can cause this ratio to be less than ideal for a given vehicle. If gases leaving the catalyst contain too much oxygen, the mixture is too lean. The PCM responds by raising the upstream O2 Goal Voltage. This increases fuel quantity and reduces excess oxygen. Conversely, if the gasses leaving the catalyst do not contain enough oxygen, the PCM lowers the upstream O2 Goal Voltage. This reduces fuel quantity and increases excess oxygen. This function is active only during downstream closed loop operation.”

Yeah you can eliminate the down stream sensors and the vehicle will run but the PCM will always be in open loop. There is no error correction without the down stream sensors, otherwise in a closed loop state the downstream sensors will have an impact on A/F to the system.

 

[macleanflood]

You can also measure before and after the cat with an IR thermometer.

-Mac

 

[cb4017]

Can't help with the O2 sensors but I can say that I did the 4-hole injectors on my 2001 XJ.

I don't think there was any noticeable benefit in power or economy but it ran great and I never had any problems with it.

I take that back. I did tear some injector O-rings until I figured out the trick. Good thing I had some spares on hand.

 

[Link]

That’s odd because the hardware documentation states otherwise.


“The first function is to measure catalyst efficiency to meet OBD II requirements. If the catalytic converter is working properly, the oxygen content of the exhaust gases at the converter outlet fluctuates significantly less than at the converter inlet. The PCM compares the switching rates of both downstream and upstream O2 sensors under specific operating conditions to determine if the catalyst is functioning properly. Any time the upstream to downstream switching ratio exceeds a calibrated value, a catalyst efficiency fault will be stored.


The second function is downstream fuel control. This function adjusts the upstream O2 Goal Voltage within the range of operation of the upstream O2 sensor. The upstream Goal Voltage is used to ensure long catalytic converter life by allowing the PCM to control the amount of air and fuel that is supplied to the catalytic converter.


Before 1996, the Goal Voltage was a pre-programmed fixed value based upon where it was believed the catalyst was most efficient. While the upstream O2 sensor input was used to maintain the 14.7:1 air/fuel ratio, variations in engines, exhaust systems and catalytic converter ageing can cause this ratio to be less than ideal for a given vehicle. If gases leaving the catalyst contain too much oxygen, the mixture is too lean. The PCM responds by raising the upstream O2 Goal Voltage. This increases fuel quantity and reduces excess oxygen. Conversely, if the gasses leaving the catalyst do not contain enough oxygen, the PCM lowers the upstream O2 Goal Voltage. This reduces fuel quantity and increases excess oxygen. This function is active only during downstream closed loop operation.”

Yeah you can eliminate the down stream sensors and the vehicle will run but the PCM will always be in open loop. There is no error correction without the down stream sensors, otherwise in a closed loop state the downstream sensors will have an impact on A/F to the system.

No it won’t. It doesn’t need the downstream sensors to go into closed loop operation. I ran mine without downstream sensors for awhile. Plugs in their place. Same MPG as with them. Their sole job is to monitor the catalyst. You do not need them for the engine to run properly but if they are not there you will get codes set in the PCM because the PCM isn’t receiving the expected signal.

 

[Link]

Can't help with the O2 sensors but I can say that I did the 4-hole injectors on my 2001 XJ.

I don't think there was any noticeable benefit in power or economy but it ran great and I never had any problems with it.

I take that back. I did tear some injector O-rings until I figured out the trick. Good thing I had some spares on hand.

Grease on the orings makes them easy to install but do not use silicone grease. Silicone can damage your O2 sensors.

 

[Matt14]

Okay thank you everyone. Here are some videos and pictures of my fuel trims and O2 sensor voltage testing live data. I think my Long Term Fuel Trim is a little low, but not sure what it need to be. Also my bank 1 O2 sensor voltage seems to not oscillate like the others are. Can y'all take a look and let me know your thoughts?

Here are the fuel trims at idle and then I rev up to 3000 RPM and then let off.

Here are the O2 sensor voltage readings idle and then I rev up to 3000 RPM and then let off just like the other test.

Here are a few screenshots from the O2 Senors Voltage scan:

Here is a screenshot from the fuel trim scan:

 

[Matt14]

You can also measure before and after the cat with an IR thermometer.

-Mac

Thank you Mac, I just did this test, and it is about 100 degrees hotter on the outlet than the inlet, so if I understand this test correctly, that means the Catalytic Converters are working correctly and the issue is likely with something else?

 

[Link]

Replace the downstream sensor on the affected bank. If you still have the code then you need to look at the catalytic converters.

 

[Matt14]

Replaced the downstream sensors on the affected bank a few months ago, and unfortunately, nothing has changed. The rough idle isn't horrible, so I have just been living with it, but I need to fix.

It sounds like I should look into replacing the catalytic converters. I'm just wary to replace them since if they end up not being the problem, I would think the rough idle, if it continued, would ruin the new catalytic converters.

The rough idle immediately started after I replaced the original fuel injectors and didn't progressively get worse over time, which is part of why I am hesitant on the catalytic converters. However, when I put the original fuel injectors back in it didn't get better.

 

[macleanflood]

We t through and read your original post.

Have you followed the procedure to clean the IAC?

Thread 'How to clean your IAC (Idle Air Controller) valve'

Jan 2, 2016

If you're experiencing idle issues on your Jeep Wrangler TJ the most likely culprit (and the first thing you should check) is your IAC (Idle Air Controller). Often times the IAC gets very gunked up causing the Jeep to idle improperly. A simple cleaning is usually all it takes to get it running as good as new.

If you end up needing to replace your IAC valve (they do fail on occasion), you can find the part numbers (and the links to purchase them on Amazon) below:

2005-2006 IAC Valve (Part # 4861552AC)
1997-2004 IAC Valve (Part # 4874373AB)

There is no need at all...

• Chris
• iac idle idle air controller idle valve
• Replies: 191
• Forum: TJ How-to Guides

• 

I'd start there. Then if that doesn't help I'd get a can of Seafoam. Start the Jeep. Pull the hose off the brake booster and dip it in the can. Run the throttle to keep it running. Smoke should come out the exhaust...and everywhere else you might have an exhaust or intake leak. Video...GoPro or a cell phone helps pinpointing the issues later.

I'm suspecting you might have a loose intake. Possibly exacerbated my wiggling fuel injectors around. Might want to just throw a wrench on the bolts and see if they're tight.

-Mac

 

[Link]

Replaced the downstream sensors on the affected bank a few months ago, and unfortunately, nothing has changed. The rough idle isn't horrible, so I have just been living with it, but I need to fix.

It sounds like I should look into replacing the catalytic converters. I'm just wary to replace them since if they end up not being the problem, I would think the rough idle, if it continued, would ruin the new catalytic converters.

The rough idle immediately started after I replaced the original fuel injectors and didn't progressively get worse over time, which is part of why I am hesitant on the catalytic converters. However, when I put the original fuel injectors back in it didn't get better.

It’s been a while since your original post. Have you had the catalytic converters tested yet as suggested earlier? You can even do it yourself if you buy the back pressure gauge for about $40 or so.

We are guessing at what might be wrong. You need solid test data.