Jeep TJ cylinder 3 misfire, please help me diagnose

[Jeepinaround]

To recap the video, we have an intermittent cylinder 3 misfire causing rough idle and stuttering / hesitation at low RPMs. Once we pass 2,000RPM, the truck drives perfectly. So far, we have checked the following:
Replaced all 6 fuel injectors with OEM
Checked all spark plugs, all gap within range installed 2 years ago
Checked coil for physical signs of damage – did not functionally test
No vacuum leaks
Engine compression – all within spec, changed throttle position sensor, cleaned idle air controller, replaced
Unplugged 02 sensors, problem did not go away
Fuel pressure test – results in video, is this normal?
Fuel pump is 2 years old – it drives fine at high RPM, does that rule out pump?

Thank you for watching, appreciate all the help in advance

 

[JPNinPA]

First always state year and model, engine plus any relevant changes like engine mods in this case. Thought you had a 2015 JK???

2. stop changing things until you diagnose the issue. By changing sensors and plugs... you could be introducing new issues.

3. post up the code(s) you are getting.

4. explain exactly when it happens and when it doesn’t. Below 2000 rpm is important. Does it happen when starting cold? Or only warm or hot? Does it stop after time or only 2k rpm?

5. post the results if the test you did. Plugs gap and pic of condition. Cylinder pressures of each cylinder.
How do you know you don’t have a vac leak? What rpm do you idle at?

Comment on the fuel pressure video- that should be stable at 50.
The fact it gets steady at higher rpm shows it is unlikely a pump, or filter issue. Usually they cause a drop in pressure.
Check each injector connector and cable. Use a screwdriver os stethoscope on the injectors to verify switching.


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[JPNinPA]

After some thought and no additional info: injector, injector wiring (constant flow) or vac/exhaust leak at the cylinder or fuel pump regulator or regulator vac line.


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[JPNinPA]

Any luck?


Regards,
JPNinPA

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[Jeepinaround]

After some thought and no additional info: injector, injector wiring (constant flow) or vac/exhaust leak at the cylinder or fuel pump regulator or regulator vac line.


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Thank you for the comment! I have not been able to fiddle with the rig due to work, but I ordered the factory heat shield for the third injector. Hopefully it does the trick. (https://www.amazon.com/gp/product/B...eb7261cc99bc96bae10b2373b94da3&language=en_US)
The injectors are all brand new. If it was the wiring, I don't think it would be an intermittent issue. My best guess is the cats are clogged causing excessive heat in engine compartment leading to vapor lock issue. I think I may just end up replacing the coil and also all 4 oxygen sensors just cause the ones on there are over 250,000 miles old. Let me know if you think of anything else and thank you so much for the help!

 

[JPNinPA]

Well I gave my advice.

You do what you want.

FYI the heat shield is to protect against vapor lock when starting after a long heat soak.


Regards,
JPNinPA

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[Gennybro]

Well I gave my advice.

You do what you want.

FYI the heat shield is to protect against vapor lock when starting after a long heat soak.


Regards,
JPNinPA

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I sense a spiritual irregularity, must be that time of the month. LOL Ya I get it too.

 

[Gennybro]

Well I gave my advice.

You do what you want.

FYI the heat shield is to protect against vapor lock when starting after a long heat soak.


Regards,
JPNinPA

Sent using TapTalk

I sense a spiritual irregularity, must be that time of the month. LOL Yeah, I get it too.

 

[Gennybro]

Oh $hit I think mine is starting. LOL. This is gonna be fun.

 

[Gennybro]

OK, Back to the issue, before the one with issues interupted Jpn sounds like he knows what he is talking about.

 

[JPNinPA]

I sense a spiritual irregularity, must be that time of the month. LOL Yeah, I get it too.

No. I have nothing new to add. If I had more info from what I asked and suggested testing I’d have a more specific answers. The OP does not have to follow my or anyones advise if they don’t want.


The OP has ideas he should follow. Its times like this we learn.

Thanks for your concern and support.


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[Gennybro]

$hit I think you scared away the sponser to this thread, darn i thought they was a nice person

 

[Jeepinaround]

Totally appreciative of your post. Will be trying exactly what you said after I try the the injector 3 heat shield. Thanks for your help!

 

[Jeepinaround]

After some thought and no additional info: injector, injector wiring (constant flow) or vac/exhaust leak at the cylinder or fuel pump regulator or regulator vac line.


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Regarding the regulator vac line, I don't have a vac line directly connecting to the regulator. I have the vac line connecting to the two rollover valves on either side of the gas tank. Is that what you are talking about? If not, where would the fuel pressure regulator vacuum line be?

Thanks!

 

[JPNinPA]

What year jeep?
Older had fuel pressure regulators near or at the end of the fuel rail. Newer models have the regulator at the tank/furl pump.
What year?


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[Jeepinaround]

What year jeep?
Older had fuel pressure regulators near or at the end of the fuel rail. Newer models have the regulator at the tank/furl pump.
What year?


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My regulator is at the pump, its a 2001.

 

[JPNinPA]

Yes 2001 is on top. 2004 and 2005 is incorporated with the pump.

Bosch pumps are the best. I don’t know if any regulator is better than another.


If you could reach the pump without dropping it all, pull the vac line and check if gas got into the line. Gas in the vac line will reek havoc on the regulator operation.

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[Jeepinaround]

Yes 2001 is on top. 2004 and 2005 is incorporated with the pump.

Bosch pumps are the best. I don’t know if any regulator is better than another.


If you could reach the pump without dropping it all, pull the vac line and check if gas got into the line. Gas in the vac line will reek havoc on the regulator operation.

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My fuel pressure regulator on top of the pump does not have a vacuum line. Just two connections, 1 for electrical harness and 1 for the fuel output. I have the vacuum lines from the EVAP system that go into the 2 rollover valves on either end of the gas tank. Which would you like me to pull?

Thanks for your help!

 

[JPNinPA]

Well its more likely the regulator than evap causing the shaky fuel pressure until 2000rpm. With this any cylinder 3 small exhaust or vac leak, valve...may result in the misfire on cylinder #3

Strange your regulator look like this?
Post a pic.



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[Jeepinaround]

Well its more likely the regulator than evap causing the shaky fuel pressure until 2000rpm. With this any cylinder 3 small exhaust or vac leak, valve...may result in the misfire on cylinder #3

Strange your regulator look like this?
Post a pic.

View attachment 23581

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Yes, my regulator looks like that. The only output is the fuel out as pictured. Where is the vac line in that image?

Also, I will be doing some injector noid testing to test your suggestion of faulty injector wires.

Lastly, based on your experience, is the fluctuating gauge at the fuel rail normal?

Thanks for all of your help!