Another Misfire thread!

[Dnny44]

Hey guys I know recently there has been quite a few discussions on misfires. I have a 2004 Expy 5.4 with a misfire on cylinder 5 as far as I can tell from my OBD II Reader. It seems to be mostly triggered between 40-55 mph between 1200-1500 rpm at pretty much any level of incline. I have replaced the COP and now fuel injector. It seemed to run fine for about 20 mins on the new injector but it is now back lol. I've had this issue for a while and had already done a spark plug replacement all the way around about 6000 miles ago (140,000 on originals). Could it be a bad plug was put in because when they were replaced it didn't seem to fix the problem. Most of the other posts that I have seen reference random misfires and not one on a specific cylinder. Any help is appreciated as always.

[EDIT] Also forgot to mention that I did finally get the truck to throw a code for a misfire on cylinder #5 and random misfires as well. It only happened when I was hauling a 5000 lbs trailer through Tennessee and went to pass a semi on a 65 degree incline at highway speeds. The truck threw the first two codes when the truck geared down. When I did this again under similar circumstance it only threw the cylinder specific code for #5. I am assuming that the #5 misfire occurring too often could have triggered random misfires while the truck was trying to make up the torque. (Nice to be able to read codes while going down the road lol)

 

[1955moose]

I've heard of shady shops not replacing number 4 and 8 because of their difficulty. If you can you might want to take a compression reading of that cylinder to be sure you don't have a valve or piston problem. Hopefully it's something simpler.


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[Dnny44]

All the plugs were done by myself and my cousin who is a mechanic. Really hoping its not a piston problem. My cousin seems to think it may be a vacuum leak issue somewhere but would that cause a misfire on a specific cylinder? I would think that if it even did cause a misfire that it would be random if it was something to do with the vacuum system? Hopefully its just the plug, however still open to other ideas?

 

[Skauber]

vacuum leak causes lean codes (P0171 and P0174) and usually a random misfire if it's bad enough. That can however in certain cases give a specific cylinder as well, if for instance one cylinder has more misfires than others due to weak plug or something, which makes the PCM pinpoint it with a code.

 

[Dnny44]

Its specific all the time though, I can read the sensors while driving the truck and the only one that ever gets triggered is cylinder #5

 

[1955moose]

You might want to check injector wiring on number 5. Use a noid light to be sure your getting signal from computer. Also try swapping a cop to another cylinder. See if your reading changes to another cylinder. Is your SUV running rough? Or is it just under load. A single injector could possibly be weak or partially plugged. You can pull it and replace that one, after confirming you have signal to it. The fact that you keep getting that one cylinder coming up. I would do all the elimination steps including compression of that bank.


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[1955moose]

One more thing, while spark plug is out replace with a new one. I know you probably would anyway, but what the heck, thought I'd mention it.


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[jdecker]

Have you tried swapping the cop with another to see if the misfire follows? I've had coils that were bad out of the box

 

[1955moose]

Didn't I say that?


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[Dnny44]

So update! Put in a new spark plug on cylinder 5 and . . . no change. Next up I suppose is a compression test and see what that yields. Other Ideas that I have been given are:

• something caught in fuel rail between cylinder 5 & 6
• crankshift position sensor?
• cams and/ or cam phasers
• Burnt Cylinder/Bad head/other issue involving pulling the engine

Still up for any ideas. Leaving tomorrow on a post Christmas Trip to Louisiana for a few days, guess I'm taking the backup Silverado :-(. Planning pulling the spark plug back out upon return and test to see that its actually sparking and make sure its not bad cables going to the coil.