P304 cylinder 4 misfire

[Jpross]

I am getting a cylinder 4 misfire code and i am stumped. Everytime i clear it, it comes back once the system's readiness check goes back online. There is no noticable miss.

What i've tried...
Swap spark plugs
Swap injector 2 and 4
Install heat sheild under injectors
Swapped coil pack
Checked compression, all at right about 160 psi

Still i get the miss on the same cylinder.

The engine ran fine since janury when i installed it until about 2 months ago when it started.

Engine is 4.7 stroker ls valve train ported polished tuppy head borla headers,deleted precats along with diablo tune to handle the deleted precats. It runs strong all the way to redline. 2006 jeep rubicon. I only use 93 and high zinc oil.


What should i check next? I figure if the compression is equal for all cylinders there is no cam issue. What should i check next?

[Frankenstien]

Harness to coil pack for that cylinder.

[Retlaw01XJ]

Along the same line of thought, check wiring to the injector. You can test injector signal with a noid light (test light).

[Jpross]

Harness to coil pack for that cylinder.

Wouldn't that cause a miss on two cylinders? That is if there were a wiring issue going into the coil pack.

The coil pack has three total coils one for two cylinders.

[SilverXJ]

Wouldn't that cause a miss on two cylinders? That is if there were a wiring issue going into the coil pack.

That is correct. If it wiring to the coil rail problem he would have a misfire on #3 and #4

[Jpross]

Is it likely there is a valve or cam issue even if the compression looks good? Or is it pretty much an election issue at this point?

[Jpross]

Not sure if it is related but yesterday when i pulled into the driveway the jeep started sputtering and went lean off the wideband chart when i was idling. I keep an eye on the wideband normally and it stays within 14.7 range other than this one time. I think it may have been that i just ran the tank to nearly empty and stopped to fill up and a gas station i dont normally go to. Maybe i had a little water in the tank and towards the end it pulled some? Idk idont think it is related but thought i would throw it out there.

Last time the code was sent was during partial throttle accelerating from around 2k to just under 3k. I'm thinking about replacing the upstream o2 sensors with new dealership versions just in case... I've had issues in the past with parts store versions. Only thing with this is if they were going bad i would think i would get multiple cylinders misfiring and the wideband would reflect an issue.

[Jpross]

Along the same line of thought, check wiring to the injector. You can test injector signal with a noid light (test light).

I checked the injector's harness with the noid light on sunday and it is flashing just fine.

[Jpross]

This morning I replaced the two upstream o2 sensors with mopar replacements.. still same code, although the old ones looked like they have seen better days, so it can only help. I talked to a guy at a shop that saw the exact same issue in his 2006 wrangler and said he had tried all the same things, he ended up buying a new pcm and it fixed the problem... so today I drove an hour to get a new new pcm from the only dealer that had one. Talking with several mopar mechanics, they all said it sounds electrical, so keeping my fingers crossed that I didn't just waste 800 plus bucks on this new pcm.

[Jpross]

New pcm finally got it reflashed and drove it this morning to grocery store and cel!

[Retlaw01XJ]

I get occasional misfires too. Usually set when it's cold.... in my case it runs rough (lean) for the first minute untill it goes into closed loop.
I just reset the cel which does get annoying!

But I did do a little research back then and posted this in another thread, maybe something will help you:

>>Found a nice article on OBDII misfires:
http://www.underhoodservice.com/tech-fe ... i-engines/


Some highlights:
>>>Most engine control systems with OBD II monitor the speed of the crankshaft between cylinder firings to detect misfires. <<<
>>>Misfires can be caused by one of three things: fuel mixture problems (too rich or too lean, or not enough fuel), compression problems, or ignition faults.<<<
>>>A much more common cause of fuel-related misfires on fuel injected engines is too much air and not enough fuel (lean misfire).....
>>>Vacuum leaks have the most effect on the air/fuel mixture at idle, and less of an effect at higher engine speeds. If you suspect a vacuum leak, look at the fuel trim values on your scan tool when the engine is idling, and again at 2,500 rpm. If the engine is running lean only at idle, it has a vacuum leak. If it is running lean at all speeds, the problem is in the fuel supply or control system (weak pump, leaky regulator, bad sensor inputs to the PCM, etc.).<<<

PCM:>>>Lean misfire also may be caused by a bad fuel injector or a bad injector driver circuit in the PCM.<<<

>>> Compression misfires will occur if a cylinder fails to hold compression because of a leaky head gasket, bent valve, burned exhaust valve or broken valve spring. Low compression also can cause misfires if a cam lobe has rounded off.

These types of problems also can be found with a vacuum gauge. A flickering vacuum reading typically indicates a cylinder with a compression problem. You can follow up with a power balance test, a compression test and/or a leak down test to further isolate the fault.<<<

Best to read the whole article. Sounds like solid advise.
I still recommend putting a vacuum gauge on there as a first step.... it's easy and cheap.

[Jpross]

I placed a vacuum gauge on the break booster line and I am seeing slightly low reading at idle compared to what the tool says it should be. It is reading about 17 inHg, but that could be characteristics of the build. Blip of throttle drops it to 0 and back up to about 21 rests at 17. Rpm above 1000 and up holds at 20.

[Jpross]

After rereading the article that Retlaw01xj provided, I wonder if it's an issue with the pcv valve.

If it was letting too much air through at idle, it would create an overall lean condition. The connection point for the pcv tube on the intake happens to be right in front of cylinder 4. So maybe cylinder 4 is being more affected by this?

[Jpross]

I'm starting to think there has to be something mechanical. I now have access to the misfire counts and i can watch them climb. The climb is fastest with no load at 1200rpm and goes away with load. What would this indicate? And i can now feel/hear the misses. If this was still electrical i dont think it would go away with load.
Cyc : count
1 : 20
2 :15
3 : 28
4 : 1005
5 : 1
6 : 2

Any thoughts?

[Retlaw01XJ]

More good reading:
http://www.aa1car.com/library/misfire.htm

From what I've been reading, misfires are caused by: an ignition problem, a fuel mixture/injector problem, or low compression.
You seem to have ruled out ignition (swapped spark plugs, #2 firing ok).... unless there's a problem with your coil rail and #4 isn't getting good spark.
Lots of misfires caused by lean conditions, primary cause being vacuum leaks. Maybe take a closer look there. You swapped injectors, which rules that out.
Mechanical problems would be low compression, bad cam or a valve leaking. You had good compression. When you checked vacuum did you take it for a test drive? a flickering/bouncing needle could point to a valve problem and loss of compression.
The article mentions testing with an O-scope. That might be a next step for you.
I can't explain your 1200 rpm/no load misfire unless that's related to a lean condition (vac leak). Under load it's adding fuel so maybe it fires ok. At least you narrowed down when the misfire is occurring.