Jeep TJ cylinder 3 misfire, please help me diagnose

No the fluctuating gauge isn't normal. I thought I commented earlier. It should be steady from idle up to rev limiter. A quick rev might make it dip and spike a little but it should be a smooth settle not erratic.

The electrical connection you mentioned above on the regulator is why I asked if yours is the same as the pic.


Regards,
JPNinPA

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JPNinPA said:
No the fluctuating gauge isn't normal. I thought I commented earlier. It should be steady from idle up to rev limiter. A quick rev might make it dip and spike a little but it should be a smooth settle not erratic.

The electrical connection you mentioned above on the regulator is why I asked if yours is the same as the pic.


Regards,
JPNinPA

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Click to expand...

Since the regulator is part of the fuel pump as an assembly, I said that it (the entire unit) only has two connections. More importantly, there is no vacuum line. Any idea why? Do these vehicles just not have a vacuum line to the fuel pressure regulator?
 

I can't answer why there is no vac line. And, I don't have the field service manual for years inc 2001. I guess it is more like the 2003-2006 than the earlier models. I did find 98 to 02 has different emissions than 97 and from 2003-2006. So I'd have to review how it is set up to determine if it can cause an issue.

Regardless the fuel pressure regulator regulates the pressure to 49.2 +/-5psi and it's usually constant as mentioned. If the pump was bad or filter clogged it wouldn't reach 49psi. However if the regulator fully failed the pressure would be 8-10 psi higher. This is what happens when the vac line is removed from the older models. So a failing regulator can be sporadic. At 2K there may be enough flow to overcome the fluctuation.

The only other reason I could think of to cause the fluctuating pressure is an open injector either bad or faulty wiring.

Why only under 2000 rpm? Below 2k rpm your consumption is much less than either the fluctuation or the leak.




Regards,
JPNinPA

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I'm just going back to be sure I didn't miss any details.

Are you throwing codes for misfire? What are all the codes by number?

In your video you mention checking plugs and cylinder pressure. Also using cheap fuel pump.
Did you write down gap numbers you could share? What brand and part number are the plugs?
What were the cylinder compression readings?

What brand fuel pump? As noted Bosch is a favored pump due to its reliability.

You can test the water temp sensor by measuring resistance as temp rises. Though the issue would likely go away at a temp not rpm.

Regards,
JPNinPA

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JPNinPA said:
I'm just going back to be sure I didn't miss any details.

Are you throwing codes for misfire? What are all the codes by number?

In your video you mention checking plugs and cylinder pressure. Also using cheap fuel pump.
Did you write down gap numbers you could share? What brand and part number are the plugs?
What were the cylinder compression readings?

What brand fuel pump? As noted Bosch is a favored pump due to its reliability.

You can test the water temp sensor by measuring resistance as temp rises. Though the issue would likely go away at a temp not rpm.

Regards,
JPNinPA

Sent using TapTalk
Click to expand...

Thank you for your response!

Throwing P0303 Code - cylinder 3 misfire. Nothing else.
Plugs are champion 7034 and gap was about .035 on all 6. No fouling or gas smell on any of them.
Coil is the original coil, with 256k miles on it.
Compression was all 150 +/- 10 among all 6 cylinders.

Regarding fuel pump, replaced it 2 years ago with one that cost $70 from rock auto. Do not know the brand. I am not able to find a bosch fuel pump for sale for my vehicle. I am willing to change out the entire fuel pump and also the coil pack since its relatively old. I found the mopar coil pack but am not able to find the bosch fuel pump.

Thanks so much,
 

Id start with just the regulator. You need constant fuel pressure.

Also be sure there are no vac leaks at cylinder #3. Missing or extra o-ring on injector. Intake not lining up or gasket leak. Spray some intake cleaner around #3 intake, plug and injector while running. Listen for s change in idle.
I could be wrong but I don’t think spark is the issue. There are some good u-tube videos of testing the rail.

OE
https://www.boschautoparts.com/en/auto/fuel-pumps/fuel-pump-assemblies?partId=67655

Part Number:
67655
This part compatible with:
2001 Jeep Wrangler L6 4.0L
1997 Jeep Wrangler 4.0L L6
1998 Jeep Wrangler 4.0L L6


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Last edited:
JPNinPA said:
Id start with just the regulator. You need constant fuel pressure.

Also be sure there are no vac leaks at cylinder #3. Missing or extra o-ring on injector. Intake not lining up or gasket leak. Spray some intake cleaner around #3 intake, plug and injector while running. Listen for s change in idle.
I could be wrong but I don’t think spark is the issue. There are some good u-tube videos of testing the rail.

OE
https://www.boschautoparts.com/en/auto/fuel-pumps/fuel-pump-assemblies?partId=67655

Part Number:
67655
This part compatible with:
2001 Jeep Wrangler L6 4.0L
1997 Jeep Wrangler 4.0L L6
1998 Jeep Wrangler 4.0L L6


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Click to expand...

Thank you for all of your help. Yes, I found that link too, but they no longer make the fuel pump. (it is obsolete).

Will try all the other suggestions soon.
 
JPNinPA said:
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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Replaced the coil and put the injector 3 heat shield on and no difference.

I notice when i place my finger over the injectors while the engine is running, that the number 3 injector's pulses do not feel as strong as the rest. I am wondering if it is the wiring. Going to be doing a noid light test shortly to see if I can isolate the wiring as an issue. If not, what should I do next?
 
Noid testing is good. You could try swapping #3and # 5 injector. To see if the problem follows.
However, the Fuel pressure regulator is likely the basic issue.
You could replace the entire assembly but the regulator is causing the fuel issues.


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JPNinPA said:
Noid testing is good. You could try swapping #3and # 5 injector. To see if the problem follows.
However, the Fuel pressure regulator is likely the basic issue.
You could replace the entire assembly but the regulator is causing the fuel issues.

Noid light testing was perfectly normal. Swapped injectors still cylinder 3 misfire. Would clogged pre-cats cause this issue? Jeep seems to be hiccuping or burping from exhaust with the misfires..Like if you place a sheet of paper under exhaust the flow is very turbulent.

Also unplugged all 4 oxygen sensors today and it didn't change anything.

Should I unbolt the cats and see what happens?
Click to expand...
 
No cats would not be just #3

Unscrewing the O2 above the cats will do the same as opening the cats.


Regards,
JPNinPA

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JPNinPA said:
No cats would not be just #3

Unscrewing the O2 above the cats will do the same as opening the cats.


Regards,
JPNinPA

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Click to expand...

Hmm so it made no difference that I unplugged all 4 cats. What should I try next? I am at a loss here.
 
Did you take care of the fuel pressure? Either entire unit or regulator?
When swapping injectors you verified o-rings are on, nothing that could cause a leak and proper seating of the injector?




So, You unplugged the wires but did not remove the O2 from above the cats?
I don't think it's a blocked CAT. Blocked cats would rough running and stalling. Not just number 3 misfire. You mentioned blocked cats so by removing the sensor you allow exhaust pressure escape and not build.






Regards,
JPNinPA

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JPNinPA said:
Did you take care of the fuel pressure? Either entire unit or regulator?
When swapping injectors you verified o-rings are on, nothing that could cause a leak and proper seating of the injector?




So, You unplugged the wires but did not remove the O2 from above the cats?
I don't think it's a blocked CAT. Blocked cats would rough running and stalling. Not just number 3 misfire. You mentioned blocked cats so by removing the sensor you allow exhaust pressure escape and not build.






Regards,
JPNinPA

Sent using TapTalk
Click to expand...

I haven't taken care of the fuel pressure yet. Had a mechanic friend take a look and he said its normal for the pressure to bounce like that at idle because there is no fuel return. In any case, I will replace the pump soon just to have that piece of mind.

Today I held a piece of paper against the exhaust and noticed it sucks in immediately after the pumps of air. I think it may be a burned or damaged exhaust valve. Meh. In the process, I also found a vacuum leak. So I guess it's time to replace the entire head with a reman. Anything else I should do while I am at it?
 

Damaged exhaust valve should have shown up in the compression test.
You noted they were all 150 which is really good compression. So were they all 150 +/- 15

Where was the vacuum leak and how did you find it?

You shouldn’t have to replace a head for a vac leak.


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JPNinPA said:
Damaged exhaust valve should have shown up in the compression test.
You noted they were all 150 which is really good compression. So were they all 150 +/- 15

Where was the vacuum leak and how did you find it?

You shouldn’t have to replace a head for a vac leak.


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Apparently you can have a normal compression test with a burnt valve. The only real way to diagnose valve problems is by leak down testing, which I will perform if time permits.

I found the vacuum leak on the connection from the rear PCV vent to the intake manifold. I don't think this is what is causing my repeated cylinder 3 misfire, probably just an indolent but clinically silent issue.
 
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