2000 s10, 4.3L:
i get intermittent codes for lean on both banks and a misfire on cyl 5... a rough idle comes and goes as well.
the codes come and go but seemingly do have a pattern to them: fuel fillups trigger the symptoms, as do warmer ambient temps. the problems/codes virtually vanished with the arrival of cold temps but a couple days of higher temps brought the symptoms right back.
fuel pressure checks out, leak down test was normal, threw a new fuel filter in for good measure.
i did a few diy smoke tests with a cigar... i did not cap off the pcv (not sure if i needed to do this or not?)... to my knowledge i have no vacuum leaks...

i know it is overdue for spark plugs, but i would not think 'lean on both banks' would be a symptom of plugs needing replaced...
i have no clue where to go from here. i have run injector cleaners thru and cleaned the maf and throttle body as well.
any suggestions?

 

what is the fuel trim at idle and at 2500 rpm
swap #5 plug with another cylinder and see if the misfire follows the plug.

 

my answers are going to suck, i just want to say i appreciate your assistance in advance on this one...

my fuel trims are constantly changing, especially at idle.
i drive it like a grandma, feathering the throttle. doing so prevents it from ever running too rough. generally it idles very rough on warmer days and throws the codes.

these are the fuel trims i observed on a day with temps cold enough to not throw the codes but to still idle slightly rougher than it should:
after a few miles of mixed city/highway driving and feathering throttle:
LT on 1 @ idle = variable between +24 and +15
LT on 2 @ idle = variable between +14 and +8
ST on 1 @ idle = variable between +6 and 0
ST on 2 @ idle = variable between +1 and -1

then after a consecutive short trip, i drove up a steep incline and put it in 'park' :
LT on 1 @ idle = +11
LT on 2 @ idle = +8.5
ST on 1 @ idle = variable between +1 and +2
ST on 2 @ idle = 0

then in park i revved to 2500 for 30 secs:
LT on 1 @ 2500rpm for 30 secs = variable between 0 and +1.5
LT on 2 @ 2500rpm for 30 secs = variable between 0 and +1.5
ST on 1 @ 2500rpm for 30 secs = about +13
ST on 2 @ 2500rpm for 30 secs = about +13

then i let it idle again:
LT on 1 @ idle = +11
LT on 2 @ idle = +8.5
ST on 1 @ idle = variable between +1 and +2
ST on 2 @ idle = 0

then in park i revved to 2500 for 45 secs:
LT on 1 @ 2500rpm for 45 secs = variable between +5 and +7
LT on 2 @ 2500rpm for 45 secs = variable between +5 and +7
ST on 1 @ 2500rpm for 45 secs = about +10
ST on 2 @ 2500rpm for 45 secs = about +10

then i let it idle again:
LT on 1 @ idle = +10
LT on 2 @ idle = +6
ST on 1 @ idle = variable between 0 and +3
ST on 2 @ idle = 0

then in park i revved to 2400 for 45 secs:
LT on 1 @ 2400rpm for 45 secs = variable between +11 and +13
LT on 2 @ 2400rpm for 45 secs = variable between +11 and +13
ST on 1 @ 2400rpm for 45 secs = variable between +1 and +3
ST on 2 @ 2400rpm for 45 secs = variable between +1 and +3

then i let it idle again:
LT on 1 @ idle = +10
LT on 2 @ idle = +6
ST on 1 @ idle = variable between 0 and +3
ST on 2 @ idle = 0

I have no clue how to interpret this fuel trim data on this vehicle.
I am stuck on the 'chicken vs egg relationship' between the lean codes, the misfire code, the older plugs, and my inability to find a vac leak via cigar smoke... Unfortunately we do not want to pull the plugs fearing they are seized and will break... they have 130k miles.
i am thinking if the plugs were too far gone that i would be getting Rich codes with misfires, not Lean codes with misfires... but i am no expert, especially with this fuel system's setup.

i previously forgot to note that i lightly cleaned the IAC, just enough to ensure it wasnt the culprit.

 

any reading over +-\10% is a concern. anything over 24% is the adaptive limit of the system.
lean or adding fuel (+fuel trim)..... at idle but normal at 2500 is indicative of a vacuum leak.

 

right on. i have smoke tested it a few times with a cigar... i never capped off the pcv though. are you familiar with a 4.3L? do i need to cap the pcv on a smoke test?
i know in at least two of my smoke tests i could hear the pcv opening after i had blown enough air/smoke in...

my other guesses are leading me to the purge valve... i guess it can get stuck open and cause lean/misfire. oddly it never throws any evap codes. i suppose a smoke test would not catch a stuck open purge valve and subsequent vac leak... i believe it would also concur with my symptoms of running rough after fillups and higher temps due to the extra fuel vapors from the rising ambient temps...

 

if the purge solenoid was open you would have a hard start after refueling. easy to check...remove the l ine from the solenoid and start the vehicle and check for vacuum at the valve.

check for vac leaks with brake cleaner...spray over the intake or possible leak points with the engine r unning...if the idle speed changes that is the leak source. brake cleaner is flammable so don't spray over an ignition source.

I also use a mechanic stethoscope to find vac leaks...pull the end off the stethoscope tube and you can hear air leaks well

 

i checked the purge valve, it was closed.
still no headway on finding a vac leak either.
this thing is throwing me for a real loop...

i suppose there are a few things like the brake booster and dash controls (and maybe the EGR too?) where I have no way of checking for/pin-pointing a vac leak without tearing stuff apart...

do you think capping off the pcv is essential for a thorough smoke test on this one?

 

I suppose I left this one open...
New vacuum hoses and spark plugs cured it. Fuel pump died about 6 months later.
One rotten vacuum hose was kinking itself closed at certain temperatures, hence my odd temperature notes.