Misfire, but no Misfire?

[John Stephan]

I bought my Exp'n used with a major engine issue. The timing chain guide on bank 1 was gone. Needless to say I disassembled most of the engine and replaced the necessary components from VCT solenoids, phasers, all the way down to the lash adjusters and roller rockers. Initial startup was smooth and the first drive had some hesitation. With so many unknowns I decided to change the plugs and COPs, luckily without issue.

After all of this, I was still experiencing a slight misfire that felt like a single cylinder only in park or neutral at around a 1500 rpm range. Forscan has some limited Mode 6 tests it would run but I wasn't really finding any solid misfire data. I decided to pull the plugs again to see if there was anything obvious. What I found was Cylinder 7 had a wet plug. I finally had a direction. I found some information about 2005 5.4L fords with injector issues. I replaced just the one injector that exhibited signs of failure, but that didn't seem to fix the problem. I still have a slight misfire. I cleaned the throttle body, MAF, checked for vacuum leaks both with brake fluid and spraying water nothing seemed to help. I don't have an ability to do a cylinder contribution test, but cranking in clear flood mode doesn't give any audible clues to low compression.

I keep looking at data on Forscan and all of the odd information I am seeing is on bank 2. Both O2 on bank 1 look good. The upstream is switching and the downstream is steady. On bank 2 the upstream doesn't look perfect, but it does switch. Every time I feel the misfire at a base idle, the downstream O2 voltage drops quickly showing lean but recovers until another misfire event can be felt. The STFT does have some spikes in reaction to the lean condition. But nothing regarding the fuel trims is alarming.

I don't have any codes, active or pending. My fuel trims don't look horrible, but they are trending to lean. I did put 3 quarts of Seafoam into the tank at the last fill up. My mileage over the last couple of tanks has gone from 14.8, 15.3, 15.8 to 16.1.

I recently started using Torque Pro more just for ease of accessibility. I setup a misfire page to see which cylinder might trigger something. On my last 5 or so drive cycles, it detects 1 or 2 misfires and they usually occur at startup, which tells me nothing.

I'm leaning towards 2 options. The upstream O2 on bank2 occasionally responds erratically, but is still switching. Can that cause a fueling issue leading to a random misfire due to a lean condition? Could it be all sooted up from the fouled #7 and just not reading well?

Should I still be chasing a bad injector knowing this is an '05 with the issues they had with injectors?

 

[1955moose]

How bout checking that cylinder for a clean hot spark? It's possible that one cop spring ain't making good connection. The wet plug on #7 was gas, not oil? Your getting too far ahead of your diagnostic with other technical stuff. Your misfire is that cylinder not firing that sparkplug. Pull that plug, disarm the ignition, either by pulling CPS, or fuse, and do an old school compression test. See if it's at least 130 psi. Report your findings.

Sent from my N9131 using Tapatalk

 

[John Stephan]

I've thought about that. I was planning to check compression but these engines need a special adapter. It's not expensive, just another tool to buy.

I've thought about finding someone with a scanner that can just run a relative compression or injector leak down.

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[TobyU]

Are you feeling the miss at idle in gear with foot on brake all or most of the time?
If so remove connector from #7 to confirm it's the one missing as it shouldn't make a difference but all the other ones will be solid miss when connector removed.

You should be seeing mode 6 misfire counts for 7 if it is misfiring.

Put another new plug in as a simple drop of tilting of socket can damage and plugs are cheap.

When its out check cylinder for basic operation of valves.
Stick finger in hole or get 7 inch piece of 3/8 rubber line and stick into plug thread hole 1/2 inch, or use something like wood dowel rounded on end like brook stick that will not fit into plug threads but will press against hole to seal off most air.
You are looking for blow blow blow blow when someone cranks it.
Never a sucking in.

I had a 4.6 with rocker follower that popped off and intake not opening. In this situation you get some blow but hard sucking in through plug hole. It should just blow blow blow.
easier with engines you can get finger in plug hole.


Try these two things.