Weird Intermittent Misfire

[boy&hisdogs]

The truck is a '98 K1500, 5.7, 4l60e. Engine is stock, had a bunch of repairs done a couple years ago (new stock cam, injectors, timing chain, almost all ignition parts replaced, fuel pump, 02 sensors, and a bunch of other little things. This was all done by a reputable shop. The truck has ran great since then, including some long out of state trips as well as daily driving.


For the past week, every morning when I start my truck I immediately get the check engine light for a misfire on #6. I can feel the truck missing and running rough. I clear the code when I start driving and the misfire goes away for the rest of the day, the truck runs smooth. Next morning, same song and dance.

Has anyone seen this before?

 

[L31MaxExpress]

One of the symptoms I just chased on my 383 in my Express and found leaking intake gaskets.

 

[1997]

my 97 had similar problem, turned out to be 1 of the poppets (#1 in fact) in spider would "stick".

 

[boy&hisdogs]

I found something interesting just now!

I've been paying attention to how it drives and it runs roughest (constant misfire) after a cold start, then runs much better under throttle while driving around (still missing but way less) and then starts to get a little rougher if I just sit and idle (already warm).

Just now I went and put a multimeter on the battery... six volts!

I immediately went to start the truck and it fired up fine (still missing) but the multimeter only read 11.5 volts with the truck running. I let it run for a minute and then shut the truck off. 10 volts on the battery now. I drove the truck three times today. To work in the morning, went out for lunch, then home. Each drive was short and low speed/throttle (5 mins through the neighborhood). Last drive was about 3 hours ago. The battery is about a year and a half old.

WTF? Maybe it's just my Harbor Freight multimeter... but something ain't right here. If the battery really was that far dead it should not have started normally. I should add that the battery did sit for a few months in the winter when I was out with a broken ankle and leg and not driving, but it has been running fine since then, until this all started a week or two ago.

And then the low charging voltage... maybe it's because the battery was so low, but it really should have been much higher than that while running. Lately I have been noticing a slight whine coming from behind the dash, like when a pully is starting to go bad. Coincidence? Perhaps, but perhaps not.

 

[fancyTBI]

Runs better cold - getting more fuel and as it enters closed loop it runs worse. Is it actually going into closed loop?

 

[boy&hisdogs]

Runs better cold - getting more fuel and as it enters closed loop it runs worse. Is it actually going into closed loop?

It runs better warm and under throttle. Cold idle is the worst.

Yes it hits closed loop, I tried it just now and it only took about a minute.

 

[boy&hisdogs]

Well I dug out my Klein multimeter and it says the battery has 12.5 volts with the truck off... and now it won't start. It did this to me the other day, I tried to hot start it after it was only off for a few minutes and it wouldn't start. I let it sit for a bit then it started fine.

EDIT: I think my HF multimeter is wonky. I checked it on my wife's car battery, it says 9v. I checked it on my truck after running it for a few minutes... it says 9v. I then set it aside and got the Klein multimeter, the Klein says 12.5v on my battery a few seconds later.

So when I checked initially and the HF multimeter said 6 volts, it was wrong, but still low.

 

[fancyTBI]

It runs better warm and under throttle. Cold idle is the worst.

Yes it hits closed loop, I tried it just now and it only took about a minute.

My apologies. I mis-read your post.

Good on the closed loop.

 

[Road Trip]

For the past week, every morning when I start my truck I immediately get the check engine light for a misfire on #6. I can feel the truck missing and running rough. I clear the code when I start driving and the misfire goes away for the rest of the day, the truck runs smooth. Next morning, same song and dance.

Good, specific problem description. If this truck was mine, I would use the spark plugs to see if I could
get a clue from what's going on in the combustion chamber.

I'd pull the #6 plug, and also pull a plug from a cylinder showing zero misfires. I'd then compare them both.
IF the #6 plug was much darker/carboned up as compared to the 'known-good' plug, then I would inspect the
plug for any cracks anywhere, but especially the insulator.

If the plug shows no physical damage, then I'd head down the dribbly injector troubleshooting path identified by
@1997. If the misfiring plug #6 is either lean or oily, then I'd go down the leaky intake manifold path that
@L31MaxExpress is on.

If you would like confirmation of a deeper diagnosis (failing injector or leaky intake) and at the same time
remove the possibility that it's just a cracked/marginal spark plug, you could buy -1- new plug prior to
pulling the plugs outlined above, and do this:

* Pull both bad and good plugs. Take side-by-side photo for us.
* Install new plug into misfire-free cylinder.
* Take 'known-good' plug from misfire-free cylinder and install that into the intermittently misfiring #6 cylinder.
* Drive vehicle until symptoms return.
* Pull both plugs and re-examine. If failure footprint repeats, we have eliminated the spark plug as a possibility and now we have solid data to support going deeper.

****

To summarize, via the spark plugs we can get the no-kidding failure info straight from the combustion chamber.
Quite possibly the plugs will confirm the P0306 (or misfire count) that is being called out?
And we will use a 'known good' spark plug from a misfire-free cylinder to compare/contrast the #6 plug condition.
And moving the new plug into the misfire-free cylinder will allow us to contain any performance variable that the new plug represents.
And the misfire-free cylinder gives us a 'known good, flown good' spark plug as a constant to test the suspect cylinder.

And please share the photos. The fact that you are already instrumented for this misfire is a plus. And it will be
interesting to see if it takes 2 days, 2 weeks, or 2 months for a noticeable misfire to come back.

I realize that this seems like a lot of extra steps. But for the price of a single new spark plug and your
attention to detail, we can eliminate the engine being a victim of a bad plug, and give us more failure info before
we pass judgement on your recently replaced injectors and intake manifold gasket.

EDIT: On the TBI system, zero fuel pressure is held against the injectors once power is removed. On the other
hand, not only is the Vortec fuel delivery system designed to hold pressure on the rails between trips, the VCM
actually takes the additional step of powering the fuel pump for 2 additional seconds *after* Key OFF.
(In an attempt to have a round in the chamber so to speak, especially for those drivers who don't let the 2-second
Key ON prime complete prior to starting.)

So a dribbly injector literally has all night with a slow seep/weep to dribble the fuel out. And this would also help to
explain why it works fine during the day -- there's not enough time for the marginal injector to drain the fuel rail down.

I have yet to do this, but it would be interesting to see just how well, or how poorly my chore truck holds pressure
overnight. In other words, if you had a fuel pressure gauge on your truck, and it read full pressure when you shut
if off at night, then you came back in the morning and the pressure was gone, and then you discovered that the
#6 plug was all carboned up, then the facts would really line up that you have a bad injector.

Imagine replacing the injector, and now the pressure stays overnight at/near the same pressure at the end of the
previous shutdown. (Checked *prior* to inserting the key the next morning.) And the #6 plug stays as clean as
it's brothers & sisters. It would be good to know if this is a valid theory. (If anyone reading this has performed
an overnight pressure test on their Vortec fuel delivery system, please add your insight here. TIA!)

Food for troubleshooting thought.

Best of luck --

 

[boy&hisdogs]

Good, specific problem description. If this truck was mine, I would use the spark plugs to see if I could
get a clue from what's going on in the combustion chamber.

I'd pull the #6 plug, and also pull a plug from a cylinder showing zero misfires. I'd then compare them both.
IF the #6 plug was much darker/carboned up as compared to the 'known-good' plug, then I would inspect the
plug for any cracks anywhere, but especially the insulator.

If the plug shows no physical damage, then I'd head down the dribbly injector troubleshooting path identified by
@1997. If the misfiring plug #6 is either lean or oily, then I'd go down the leaky intake manifold path that
@L31MaxExpress is on.

If you would like confirmation of a deeper diagnosis (failing injector or leaky intake) and at the same time
remove the possibility that it's just a cracked/marginal spark plug, you could buy -1- new plug prior to
pulling the plugs outlined above, and do this:

* Pull both plugs. Take side-by-side photo for us.
* Install new plug into misfire-free cylinder.
* Take 'known-good' plug from misfire-free cylinder and install that into the #6 cylinder.
* Drive vehicle until symptoms return.
* Pull both plugs and re-examine. If failure footprint repeats, we have eliminated the spark plug as a possibility and now we have solid data to support going deeper.

****

To summarize, via the spark plugs we can get the no-kidding info straight from the combustion chamber.
Quite possibly the plugs will confirm the P0306 (or misfire count) that is being called out?
And we will use a 'known good' spark plug from a misfire-free cylinder to compare/contrast the #6 plug condition.
And moving the new plug into the misfire-free cylinder will allow us to contain any variable that the new plug represents.
And the misfire-free cylinder gives us a 'known good, flown good' spark plug as a constant to test the suspect cylinder.

And please share the photos. The fact that you are already instrumented for this misfire is a plus. And it will be
interesting to see if it takes 2 days, 2 weeks, or 2 months for a noticeable misfire to come back.

I realize that this seems like a lot of extra steps. But for the price of a single new spark plug and your
attention to detail, we can eliminate the engine being a victim of a bad plug, and give us more failure info before
we pass judgement on your recently replaced injectors and intake manifold gasket.

EDIT: On the TBI system, zero fuel pressure is held once power is removed. On the other hand, not only is the
Vortec fuel delivery system designed to hold pressure between trips, the VCM actually primes the fuel pump
for 2 seconds *after* Key OFF. (In an attempt to have a round in the chamber so to speak, especially for those
drivers who don't let the 2-second Key ON prime occur prior to starting.) So a dribbly injector literally has all
night with a slow seep/weep to dribble the fuel out. And this would also help to explain why it works find during
the day -- there's not enough time for the marginal injector to drain the fuel rail down.

I have yet to do this, but it would be interesting to see just how well, or how poorly my chore truck holds pressure
overnight. In other words, if you had a fuel pressure gauge on your truck, and it read full pressure when you shut
if off at night, then you came back in the morning and the pressure was gone, and then you discovered that the
#6 plug was all carboned up, then the facts would really line up that you have a bad injector.

Imagine replacing the injector, and now the pressure stays overnight at/near the same pressure checked *prior* to
inserting the key. And the #6 plug stays as clean as it's brothers & sisters. It would be good to know if this is a
valid theory. (If anyone has performed an overnight pressure test on their Vortec fuel delivery system, please add
your insight here. TIA!)

Food for troubleshooting thought.

Best of luck --

I ran and got a new battery since it was still under warranty but no luck. The truck did start up a little nicer and the misfire lessened as it warmed up but the miss was still very clearly there. I was hoping for an easy fix, but oh well.

I'm out of daylight now but the spark plug experiment will be my next step tomorrow. I also ordered a four-pack of the same ones that are in there now so I can swap them in as needed. I really appreciate your detailed response and everyone else that has contributed so far.