Weird Intermittent Misfire

[boy&hisdogs]

Good news! It's fixed!

I pulled spark plugs #6 (misfire cylinder) and #4 (known good, zero misses) and looked at them side by side. #4 was a nice looking toasty marshmallow, and #6 was dark and wet. The exhaust had a strong fuel smell so no surprise on the wetness of the bad plug. In this picture, you can see #4 on the left and #6 on the right.

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I moved plug #6 to cylinder #4 and the misfire followed it. I then replaced it with a new plug and let the truck run for about 15 minutes with no misses. Throughout the week I saw a few misses coming from #5 here and there, not enough to trigger a code or notice without the scanner but since I have extra plugs I'll replace that one too if it persists.

I feel like I just won the lottery! I'm so happy that it was just a bad plug, a $5 part that takes 5 minutes to replace. These plugs are only about a year and a half old and shouldn't be acting up this soon but oh well, way she goes sometimes. If anyone is wondering, these are the AC Delco Iridium plugs, normally I only buy NGK but I heard a rumor that these were made by NGK so I figured I'd chance it. We'll see how the rest hold up.

 

[Road Trip]

Good news! It's fixed!

I pulled spark plugs #6 (misfire cylinder) and #4 (known good, zero misses) and looked at them side by side. #4 was a nice looking toasty marshmallow, and #6 was dark and wet. The exhaust had a strong fuel smell so no surprise on the wetness of the bad plug. In this picture, you can see #4 on the left and #6 on the right.


I moved plug #6 to cylinder #4 and the misfire followed it. I then replaced it with a new plug and let the truck run for about 15 minutes with no misses. Throughout the week I saw a few misses coming from #5 here and there, not enough to trigger a code or notice without the scanner but since I have extra plugs I'll replace that one too if it persists.

I feel like I just won the lottery! I'm so happy that it was just a bad plug, a $5 part that takes 5 minutes to replace. These plugs are only about a year and a half old and shouldn't be acting up this soon but oh well, way she goes sometimes. If anyone is wondering, these are the AC Delco Iridium plugs, normally I only buy NGK but I heard a rumor that these were made by NGK so I figured I'd chance it. We'll see how the rest hold up.

Alright boy&hisdogs,

Appreciate the quality photo and happy to hear that you have regained control over this problem.

Now to find out if the spark plug replaced was the perpetrator of the fault (bad plug) or was it
a good plug that was the victim of an weepy injector that is incontinent overnight and flooding
this plug every morning. IF the problem is fixed, it will remain so. IF the problem is still there,
how long it takes to return will tell us if the problem is a subtle one or a more obvious failure.

Remember, as an analytic troubleshooter your first task is to develop a specific problem definition, which you did.
(#6 cylinder misfires first start every morning, eventually clears, and runs fine the remainder of the day.)

When we make a parts change and it corrects the behavior of the system, we have to be able to
explain why the changed part cured the symptom stated by our problem description.

For example, I could declare that due to some unseen manufacturing error the heat range of this
particular plug was too cold / out of specification, and that this led to excessive conductive deposits
on the porcelain insulator that led directly to the cold start misfiring. (ie: Spark plug as perpetrator.)

And if the engine continues to run smoothly for an extended period of time, and you pull the new plug
and it is now coloring the same as the other 7 cylinders, it is indeed fixed.

However, if a month later the morning misfire returns, it is again #6, and pulling the same two plugs
we see the same failure footprint, then we will switch our problem description to the spark plug is
the victim, and the perpetrator is the associated fuel injector feeding this cylinder. Does this make sense?

Don't get me wrong, I am not saying that you didn't fix your truck. For I 100% believe that it is
now starting & running well post plug replacement. Fingers crossed that after sitting overnight it
starts perfectly tomorrow morning. And the next day. And the day after. For thanks to your
detailed observations the 1st start in the morning was bad. But the remainder of the engine starts
during the rest of the day were fine?

****

But let's document a couple of things so that we can be specific about how long the fix lasts just
in case we have to return to this:

First thing is to write down the odometer reading when you changed this plug.

The second thing is to also top off the engine oil, and check it often. And when the
engine requires another quart, let's again write down the mileage, for it's important to know
how many miles per quart of oil your engine is consuming. (500 miles per quart? 2000 miles per quart?
Makes a difference in determining if corrective measures need to be performed. And it could be
a valuable hint. For example, if the oil consumption is a low 2000 miles/qt, then if #6 plug fouls again
I'm leaning towards the weepy injector. But if it's drinking 400 miles/qt, then maybe you are sucking oil
up from the intake manifold valley? No matter what, knowing you miles/qt of oil consumption is
always good to know.

****

Again, congrats on being able to change the behavior of your misfiring engine on the 1st attempt. (!)
Now to see if the fix sticks or do we have to take the repair effort to the next level. Time will tell.

I look forward to your next status update.

Good stuff.

 

[boy&hisdogs]

Alright boy&hisdogs,

Appreciate the quality photo and happy to hear that you have regained control over this problem.

Now to find out if the spark plug replaced was the perpetrator of the fault (bad plug) or was a
good plug that was the victim of an weepy injector that is incontinent overnight and flooding
this plug every morning. IF the problem is fixed, it will remain so. IF the problem is still there,
how long it takes to return will tell us if the problem is a subtle one or a more obvious failure.

Remember, as an analytic troubleshooter your first task is to develop a specific problem definition, which you did.
(#6 cylinder misfires first start every morning, eventually clears, and runs fine the remainder of the day.)

When we make a parts change and it corrects the behavior of the system, we have to be able to
explain why the changed part cured the symptom stated by our problem description.

For example, I could declare that for some manufacturing error the heat range of this particular plug
was too cold, out of specification, and that this led to excessive conductive deposits on the porcelain
insulator that led directly to the cold start misfiring. (ie: Spark plug as perpetrator.) And if the engine
continues to run smoothly for an extended period of time, and you pull the new plug and it is now
coloring the same as the other 7 cylinders, it is indeed fixed.

However, if a month later the morning misfire returns, it is again #6, and pulling the same two plugs
we see the same failure footprint, then we will switch our problem description to the spark plug is
the victim, and the perpetrator is the associated fuel injector feeding this cylinder. Does this make sense?

Don't get me wrong, I am not saying that you didn't fix your truck. For I 100% believe that it is
now starting & running well post plug replacement. Fingers crossed that after sitting overnight it
starts perfectly tomorrow morning. And the next day. And the day after. For thanks to your
detailed observations the 1st start in the morning was bad. But the remainder of the engine starts
during the rest of the day were fine?

****

But let's document a couple of things so that we can be specific about how long the fix lasts just
in case we have to return to this:

First thing is to write down the odometer reading when you changed this plug.

The second thing is to also top off the engine oil, and check it often. And when the
engine requires another quart, let's again write down the mileage, for it's important to know
how many miles per quart of oil your engine is consuming. (500 miles per quart? 2000 miles per quart?
Makes a difference in determining if corrective measures need to be performed. And it could be
a valuable hint. For example, if the oil consumption is a low 2000 miles/qt, then if #6 plug fouls again
I'm leaning towards the weepy injector. But if it's drinking 400 miles/qt, then maybe you are sucking oil
up from the intake manifold valley? No matter what, knowing you miles/qt of oil consumption is
always good to know.

****

Again, congrats on being able to change the behavior of your misfiring engine on the 1st attempt. (!)
Now to see if the fix sticks or do we have to take the repair effort to the next level. Time will tell.

I look forward to your next status update.

Good stuff.

Again, I really appreciate the detailed response! And yes, your perpetrator/victim analogy makes perfect sense.

For the record, the current mileage is 212,069. The oil is full, I changed it pretty recently. The typical oil burn for this truck is mild, about 1qt every couple thousand miles. I typically change it every 3,000 miles or every few months, whichever comes first, high mileage conventional and a new filter every change.

Initially, it would misfire once in the morning, and be fine for the rest of the day. After a while, it started missing most frequently after any cold start, and missing less frequently (but still a consistent misfire) while warm.

I just hope I didn't celebrate too soon.

 

[Road Trip]

For the record, the current mileage is 212,069. The oil is full, I changed it pretty recently. The typical oil burn for this truck is mild, about 1qt every couple thousand miles. I typically change it every 3,000 miles or every few months, whichever comes first, high mileage conventional and a new filter every change.

That level of oil consumption is encouraging.

Initially, it would misfire once in the morning, and be fine for the rest of the day. After a while, it started missing most frequently after any cold start, and missing less frequently (but still a consistent misfire) while warm.

I just hope I didn't celebrate too soon.

No way. You have plenty to celebrate. Changing the behavior on the 1st part tried reinforced that we are on
the right track. FWIW there are countless stories of folks swapping 3/4/5+ parts and not being able to change the
behavior of what they are working on one iota...and singing the parts cannon blues. :0)

This is also good practice in carefully observing how the spark plugs are coloring, and using that data to guide
you where to go next. (If necessary.) Plus the fact that your scan tool identified the #6 cylinder as misfiring,
and sure enough when you pulled the plug it had correctly directed you to the wet/fouled plug. This gives us more
confidence in the accuracy in your live data collection. (There have been instances elsewhere in the forum where
the VCM/scan tool combo misidentified the misfiring cylinder(s) to the owner.)

****

Thinking a bit more about your situation, if the #6 injector is a bit sticky/varnished/dirty due to the heat cycles
since replacement compounded by running non top-tier fuel (?) ...possibly we can restore proper operation by
running some Techron through your fuel system. If you haven't already done so, please start doing this asap.
(See attached.)

Who knows? Between the new plug and some injector cleaning *maybe* this is all your setup needs to succeed.
And we'll stop here for the evening. I'll await further status updates before discussing stuff like the Injector
Balance Test, Fuel System Pressure Test, or pointing you towards L31MaxExpress's intake manifold leak testing
using a smoke generator.

Because a fresh plug and a little ungumming of the works make be all it takes.

Best of luck --