Possible misfire, or battery issue, or gremlins.

[thesameguy]

I am going to take it for a drive here in a sec to generate that data, but here is some other information:

It looks like the CMP value is dynamic - so I have 4 degrees when cold, but it zeroes out once warmed up... like, within 3-4 minutes of idling.

I realized Tech 2 has a misfire logging screen, and from there it appears the major misfiring cylinders are 3 and 6. An occasional misfire from #4. They have pretty high counts when cold. Within 3-4 minutes of idling they drop to zero.

I looked at the data log from earlier, and fuel trims got smaller and smaller - they started at double-digits after about 25 minutes of running they dropped to LTFT -4% and STFT 0%. After sitting all day and firing it up for a second this evening (to double check the CMP value) LTFT was ~4% and STFT was ~3%. I also noticed that the severity of the rough idle was pretty low this evening, and seemed to fade much more quickly - although the CEL and AIRBAG light did come back on. Argh.

I also checked the O2 sensor logging and it looks good. Nice swing from both banks, and post-cat sensors hovering around 300mv, nice and solid.

With the misfire data I crawled under the hood to quadruple check everything. Still nothing obvious. Taylor plug wires look excellent. I did not pull plugs, but I will probably do that tomorrow. Delco cap is snug in place - I did not inspect the rotor. The recently replaced Delco coil looks fine. The PO installed a Napa ignition module, but I'm not sure that's relevant here.

Leaky injectors seem a real possibility here - that would explain why it's a problem only after sitting a while, why the CEL sets only if you sit there idling, and why the problem is not a problem within just a few minutes of driving. Maybe fuel is collecting in #3 & #6 when sitting and it burns off really quickly. I think the entire intake manifold has to come off to replace them, yeah?

I cleared the DTCs & I'm going to go put some mixed miles on it and see what I can see.

Downside: The real difficulty here is that the problem completely disappear within a minute or two of driving & when warm. Really annoying.
Upside: It's dark now, so I can see if the headlight flicker has changed at all.



I appreciate all your thoughts - hopefully I am not taking up too much of your time.

 

[thesameguy]

Okay, let's discuss cylinder #3....

At idle, LTFT creeps up to -12%, and STFT up to -2%
At 40-50mph cruise, LTFT is -8% and STFT between 0% and +3%
At 65mph cruise, LTFT is -4% and STFT -1% or 0%

Injector pulse is between 3.6ms and 4.1 on both banks at idle

But, telling: At idle or *very* light throttle cylinder #3 is misfiring like a **** - no activity on any other cylinders. With a little throttle the misfires zero out.

Edit: Since the firing order is 1-8-4-3-6 I am wondering if maybe the cold idle misfire detection was confusing #3 misfires as #6.... on the road, zero misfire activity on #6. I will run this test again tomorrow when it's cold again.

Sounds like maybe a leaky injector on #3, but maybe a bad plug? I will yank it tomorrow and see what it looks like.

I bought this Tech 2 to help do, well, anything on my Saab because there aren't dealers to do it anymore... really happy to have it right now!

 

[thesameguy]

The #3 plug is pretty brown, and when stone cold there may have been some dampness on it - nothing I could smell, but a little shimmery. #1 still looks pretty new.

The plugs are NGK Iridiums. I have no idea why I went with Iridiums - I typically only use them on very modern cars as they are super sensitive to combustion chamber issues. I cleaned up the threads and used a soft brush on #3, regapped it (it was barely off - and you generally aren't supposed to gap iridium plugs, but whatever) then swapped #1 and #3 and will do some more logging tonight to see if the problem moves or changes. I had to move the truck from one driveway to the other and it was still pretty shaky, so at least I know the problem is still there.

 

[thesameguy]

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Test drive this evening:

Same crap startup, fading with just a little heat. But, the misfire shifted from #3 to #1 - it was not as dramatic, with single-digit misfire counts vs double-digit but still only affecting the one cylinder.

Fuel trims were slightly less negative -

At idle, LTFT is -10%, and STFT up to -2%
At 40-50mph cruise, LTFT is -7% and STFT between -1% and +1%
At 65mph cruise, LTFT is -4% and STFT -1% or 0%

So, bad plugs. Still don't know what I was thinking when I ordered these iridium plugs, but ~6k is a really bad life. I probably don't drive the truck hard enough or often enough to keep them in good shape. In any case, time for replacement!

(And, *no* explanation for the airbag light, or headlight flicker, but this test seems pretty conclusive as far as the drivability issues)

 

[thesameguy]

File this under no explanation: I replaced the NGK iridiums with Delco coppers last week. I have not put a large number of miles on the truck, but in several test drives the misfire has not returned, and neither has the AIRBAG light. I cannot explain that interaction at all, but apparently there is one. Anecdotally, in the one nighttime drive I have done the head light and dash light flickering seems to have disappeared as well.

It's my understanding that Iridium is just really hard and doesn't degrade easily, but that copper is a superior conductor. To completely grasp at straws, perhaps the truck has a weak battery or weak alternator, and the additional drain from less conductive plugs in poor shape was enough to upset the larger electrical system? Seems crazy, but what else could it be? I'm taking it on a 1200 mile road trip this next weekend so we'll see if anything changes in that time.