mountainmanjoe

Ouch you drove with misfires for two years. It's not very good for your engine and exhaust system.
Yes EVAP issues can cause misfire. Your fuel trims are all over the place and I wouldn't be surprised if you're leaking vacuum from the EVAP system.
The proper way to test is with a bi-directional diagnostic scan tool, but you can probably just hotwire the solenoids to manually turn the vent & purge valves on and off. Make sure air flow is either blocked or allowed. I wouldn't be surprised if the charcoal canister and valves have gone bad after 15 years.

MitchM

ouch is right. i a big part of my frustration is that chevy dealer refused to look at 3500 van in Nov. 2021 when I drove it in for my scheduled 'wait for it' appointment , a time while it was actually showing the MIL P 0300 code per my OBDII (it's now showing a confirmed 446 and 'pending' p0300. ) (That 11/21 visit to chevy dealer was after 3rd CkPs replace and relearn, from an authorized chevy garage didn't fix it. it coded p0300 after about a month of no codes following that 3rd CkPs. At nov 2021 appointment , the dealer refused to look at any of my OBDII data. though I drove it in hot and coding the P0300, they just turned it off , didn't put the scan tool on it. dealer then let 3500 sit on their lot telling me periodically 1/ sorry we don't have a lift for it , it's tied up w/ an rv . 3 weeks later I took my van a way away, still coding p0300 after 20 min of highway hi speed running. dealer set another appointment, a 'wait for it' early dec. Brought it back for this 'scheduled appointment 12/6 , dealer staff drove it around for an hour and ' couldn't get it to code. ' put in a new CPS thinking new CPS needed to 'match' new CkPS. so it immediately started throwing p0300 AND p336 . dealer told me ' just drive it around for a couple months. . . are any particular PID graphs of interest for the new 446 Code ?

mountainmanjoe

Never go to the dealership for service.

there are lots of interesting graphs, but the quickest path to diagnosing and repairing the issue is rolling up your sleeves and getting your hands dirty.

MitchM

Thanks Joe as usual. there are 3 pretty good independent mechanics nearby and the p 446 is a new wrinkle.

MitchM

07 chevy 3500 express with rv conversion with "P0446 confirmed. "EVAP vent system performance. " ( since 8/29/22. ) took to independent mechanic w pro scan tool on Friday who looked at freeze frame, did vent command, found 'vent worked audibly on and off. ' he noticed FT pressure sensor voltage was reading erratically w key on engine off. ( voltages KOnEOff varied 1.5 to 2.8 volts intermittently. )

per my OBDII Actron bosch , The recent intermittent random P 0455 'EVAP control system' (confirmed) DTC s are always accompanied with a 'pending' P0300 ' random cylinder misfire. the observed wrong variations in voltage for the fuel tank pressure sensor circuit does not throw any FTP sensor codes such as P0451 to P 0454 code--- 'fuel tank pressure sensor intermittent' , or ' FTP sensor low or high voltage . '

if this Fuel Tank pressure sensor is going bad intermittently bad, doesn't that intermittent 'bad' FTP sensor failure screw up the fuel air mix that the PCM computes, then the PCM intermittently sends a wrong mix to the injectors ? (possibly causing an intermittent p 0300 )

there are no drivability issues, just a current intermittent P0446 accompanied by a 'pending' intermittent random p0300 misfire.
the intermittent P0446' usually occurs on startup of an engine which sat overnight, now that we have cold morning air temps ( 43-55 F. ) (the p0300 does not ever occur 'by itself' any more. )

all ideas welcome . (PS my chevy dealer quotes a 2 week wait for an aappointment, refused to look at an independent mechanic's data (or any of my 5 recordings, recorded right after the MIL appeared, ) wants to 'drive it themself' , and can only diagnose it 'when it's coding.' which is really difficult ---since it's an intermittent DTC. )

mountainmanjoe

I'm confused why your indie mechanic didn't finish the job and fix your problem.

No, fuel tank pressure has no effect on fuel trims.
Make sure all the vent/purge/vacuum tubes are clear and not rotted or leaky. Best practice would be to find leaks with smoke machine.
Replace the pressure sensor as necessary.

MitchM

My 3500 express has the roadtrek rv modification and so it’s gross weight 9600 # is over my mechanic s limit for his garage lift. Did not want to put on ramps or jacks. more news later, found an excellent truck shop that has it now w all the freeze frames and a $$$$$ scan tool and scope.

MitchM

chevy 3500 express 2007 . 11/20- code 455- evap leak, rx: new gas cap. from 11/20 til now: random p 0300 s , at 1st once every 3 months, then from 6/21 to 6/22, every time I drove at 60 + mph up a hill in closed loop. 7/22: replaced all grounds . P 0300 went away for a while then came back randomly as 'pending', accompanied by evap system codes. 10/22: new fuel tank pressure sensor replacement at dealer, (based on erratic electrical readings from f t p s vs. vacuum. vent control opened and shut properly according to their tests, but the 3500 showed no f t p s - specific codes such as p 0451 FTP S performance. No p 0452 low voltage or vent control codes were showing as confirmed or pending at the dealer's but we replaced the F TP s any way . )

a week after after new f t p s by dealer, 2 new evap system codes were showing, but only intermittently . codes P 446, 'vent system performance confirmed' and 'p 449 vent control solenoid confirmed .' NOTE these codes never showed at the chevy dealer's. (Codes were read from my OBDII reader. ) it appears to me this 3500 has an intermittent vent control solenoid problem that won't necessarily show up when I drive it to the dealer (and lose more 1/2 days of work. ) (my electrical engineering buddy says many solenoids are notorious for intermittent failures . )

questions:
1. what's the standard time , labor and parts charge to replace the vent control solenoid in a 3500 ?

your answers will control whether I schedule a 'wait for it' appointment and lose only one 1/2 day of work for my self , or a 'drop it off and pick it up later ' appointment, which is 2 1/2 days of work for both me and the long suffering spouse. (please don't suggest d I y because I physically can't do mechanical work on any vehicle any more due to an injury. basically all I can do nowadays is run amp and voltage tests with my Fluke. )

all thoughts welcome. (including whether intermittent evap system failures can cause random cylinder misfires. )

mountainmanjoe

You don't know what the issue is.
Changing a solenoid is quick, but testing and diagnosing an unknown problem is an evolving process.
It might be as quick as replacing a vacuum hose. I'm might be as complicated as dropping the fuel tanks.

It's like going to a doctor, without doing any exams, and asking "I have a pain in abdomen. How long will it take for you to cure me?"

MitchM

I am assuming the p 0446 and 449 codes are a logical starting place for chevy dealer to run tests on their expensive scan tool without trying to drive it around for hours and hope the code 0449 appears again. I assume the 0449 is still in the $ dealer -accessible memory .

the gm 2015 tech bulletin on 0446 outlines all the items to check. This Vent control solenoid that fails intermittently and throws the code right after a fuel fill up is currently the prime suspect and first on the list to check.