OtherCarASaab

Ok, I've posted a few times with inquiries, but this one is going to need the assistance of those who have really troubleshot these engines/computers.
'06 Yukon XL 5.3 with Flex Fuel capability
(I've never used E85, don't know for sure about prior owners, but don't think they did, 2nd owner I bought from pretty sure didn't)

Main issue is constant misfire. P0300 (random cyl) and P0301 (cyl 1) only codes I've gotten for last 4k miles, but get P0300 almost every time I start the car. This misfire is felt at idle, but always there, getting O2 sensor reading jumping no matter the engine load or speed, so un-ignited air is going by. Previously got Evap Emissions code a few times, but not since changed vent solenoid.

Tested/replaced:
Compression ok - 175 all 8 cylinders
Battery ok, ~750 CCA
new (OE) plugs, wires, coil pack on 1, intake manifold and gasket, knock sensors, cam sensor, oil change (2x since issue started), air filter.
Did fuel injector cleaner, and have moved injectors around, misfire stays at 1, injectors all good and clean.
Repl Evap vent solenoid (per note above)
*unrelated, I think/hope* - rebuilt transmission about 2k miles before this all started.

Newest observed symptoms:
occasional other electrical issues, turned key off and car alarm has gone off when opening driver's door (~4 times over last month, start the car 6x+ a day), or other odd lights on, etc.
Can tell by feel upon startup if the idle is going to be rough 'all day' or not. Some days rougher than others, have not been able to change based on retstarting car (cold or warm engine, tried restarting at red lights to no change). CEL does not go on every time I start the car, but can tell on startup if it will blow a code or not that day. Will run better for a few days, then worse for a few days, sometimes seems to get better the day after filling up the tank, but not always, and this isn't a consistent variable, I think sometimes gets better without. (and only fill up every couple of weeks)

Misfire details: If you're still reading, here's more in-depth details of the misfire.
Watched the misfire closely with a Tech2 (mechanic has, I only have cheapy code reader).
Misfire on 1 is near constant, on 8 is roughly 20% as often, hence reason thought it might be cam sensor as they're next to each other in firing sequence. Alas, still there after replacing.
Idle is low, ~520-570, all the time.
Idle feels rough, car shakes, and when misfire hits harder, can dim car lights, etc, as revs dip, but engine has never completely stalled and died.


So the questions - what to check/test now?
Grounding on computer or other parts? Individual pin testing on computer? Computer swap?
What should I be trying here? I've swapped enough componets I feel pretty comfortable that the information the computer is getting is good.
So it seems like fuel and air are good, Spark I'm expecting is good if getting signal properly to ignite.
Getting weary of this issue, but really want to fix it now while the garage to work in is still warm (I'm in Wisconsin, no heater in my detached garage).

Any and al thoughts are appreciated!

kevinkpk

If you feel it misfire, and you get a 300 code, it is a misfire. What does #1 plug look like after you get 301? Do the rest of the plugs look white? If it is just this one cylinder, could be a head gasket leaking at around that cylinder. Since you did a compression check, that seems unlikley.You say the O2 sensors are sporatic, is it both sides upstream, or just on the #1 side? Doubting it is the sensor, rather something causing the sensor to be sporatic. The upstream sensor does play into fuel schedule. That is an inexspensive item to rule out, replace it.

OtherCarASaab

in all my haste, I didn't ask the mechanic specifically about the plugs, but when noting the compression test, etc, he didn't mention them looking bad or discolored. (had been ~3k miles after plugs swapped that I went back in and had compression test done). He was very thorough in explaining the results in everything else (readings for compression, misfire, etc) so I expected they're fine if he didn't mention it.

Both upstream O2 sensors are giving the poor readings... about every 4th reading jumps from the typical 0.7v down to 0.2v or lower (refreshing 2x a second on my cheap code-reader). Bank 1 is jumping more often, which seemed consistent with it misfiring on that side more often per misfire count reader on Tech2 scanner at mechanic.
I would expect the misfire to be more 'spread out' and not consistently on cylinder 1 and 8 only if it was really O2 readings causing it to adjust fuel too much.

I haven't gotten the P0301 code in probably the last 50 codes (2 weeks), it's always been P0300... occaionally earlier on I would get cylinder more often, largely went away (but not quite completely) after doing the intake manifold and gasket (that had been leaking a bit as observed from a smoke test).


I might go to swapping the O2 sensors soon, but at $200 just in parts for the pair, I was hoping to see if there is any additional testing I can perform before going back to the 'swap parts and see what happens' troubleshooting method.
(I'm nearly $2k into trying to fix this misfire issue, so getting weary of the 'nickel and dime' part replacement method, it's adding up on me)

kevinkpk

My experience with misfire has always been fuel related. Is it possible for you to swap #1 injector with say #3? See if the problems stays or follows. I know you replaced the injectors, but if possible try that.

OtherCarASaab

did indeed perform injector 'swap'
1 to 2 and others, misfire stayed at cylinder 1 per misfire counter - but misfire on cylinder 8 remained prevalent enough that I still got P0300 code next time the code blew.
Therefore, pretty sure it's not the injector itself.

in2pro

You bring up grounding, I would check the engine ground wire just to be sure it is sound....