MitchM

re: GM dealer scanner capability :

My recently-purchased Chiltons manual on p. 6-6 says " many more DTCs are available to dealers w proprietary scan tools...' so I'm thinking a GM dealer w a proprietary scan 06 scanner may have the capability of reading a lot more of the data necessary to diagnose a really tricky intermittent p0300 cylinder misfire.


i have spent a small fortune over 8 months unsuccessfully throwing 'recommended parts' at my chevy to fix a p0300 random cylinder misfire that only shows the MIL dashboard light intermittently. (parts include new ac delco points, plugs, coilpacks, tuneup, air filter, fuel filter, MAF, b1s2 o 2 sensor, injector cleaning, even a new gas cap..) the parts replacements were done by 2 different local mechanic shops who have used their scanners on my cold engine, at a time it was not showing the MIL on the dash but had p0300 as the stored code. (the only stored code is the p0300.) (the MIL light wasn't on when the van was in the 2 shops. so when the shops put their scanners on , they weren't reading freeze frames or live data. no shop wanted their mechanic to drive around for hours waiting for t the mil check engine lite to go on so they could attach their scanners . )

1/ can a gm dealer w propietary GM software diagnose more specific codes when the MIL lite is OFF and NOT showing on the dash at the time they put their gm scanner on it ?
2/ or do I have to just wait til it's throwing the MIl check engine on the dash, use my fixd to see it's coding p300 , and then drive it straight to a GM dealer while it's hot and throwing the code?

thanks in advance for for any thoughts.

MitchM

GOOD NEWS ! this van has had 2 new crankcase position sensors in the last 8 months w 2 'rlerns. each time, the p0300 went away for a couple weeks . the constant p0300 has continued for a week or 2. today it now started throwing a specific P0336 Code: Sensor A of CrankCsaePosition Sensor is:not getting or sending a good signal to/from ECU. . there are 3 wires in the "sensor A," (hall effect.) I drove the van 'hot' and it coded right away into my OBDII.

this 07 chevy 3500 is now at a chevy dealer. the diagnosis of the _0336 includes ruling out all of the following :to check:

bad/ broken reluctor

reluctor ring stripped/ dislodged from its stationary location

reluctor ring missing teeth or debris lodged in ring

wiring harness chafing causing intermittent short

an old GM service bulletin says to check for witness marks on the CkCPos sensor. and if so a shim is required . (" ONLY shim if there are witness marks of sensor 'scraping' or rubbing. sensor should not be scraping or rubbing..") my ealer says they don't have these shims anymore and since the CkCPos Sens is brand new it sure should nt have witness marks on it...

my FIXD subscription warns that all of the above things should be checked BEFORE throwing a new CkCasPos Sensor in.


more once the final diagnosis is done....

MitchM

UPDATE: there is a new thread posted a few days ago on oxygen sensor replacements on the chevys pre- 2011. the thread claims that chevy mandates a computer re-flash after an oxygen sensor replacement in a pre-2011 engine. (mine is a 2007; my p 0300 s and p 0336 s were present after a downstream bank 0 2 sensor was replaced 6 months ago. i know the mechanic did not 're flash ' the computer because he's an independent mechanic and does not have the software to re flash the e c u ( or p c m ) . (but his equipment did pull the p 0300 and p 0336 codes. )

i am now thinking ll of the $$$$ i spent on various fixes for the p 0300 may have been due to the 1s independent mechanic's not re flashing the computer after the recent o 2 sensor replacement. an independent mechanic would not know to re flash the computer , or even be able to do this for lack of the proprietary software. . (I am assuming a 'relearn' of CkC Position sensor is not the same as a 're flash ' of the P C M or e c u. ) is that correct ? (. the independent shop is a 2 mile walk from our house. .I love my chevy dealer but it's 22 miles one way and 1/2 day off work for me to get the van there and arrange for rides to and from the dealer. we're a chevy family, my dad bought our 1st family, chevy the impala in the 50 s and has been loyal GM ever since... total of 9 GM s since then. this is my 2nd 3500 express van...) )

mountainmanjoe

any good mechanic should have the tools to perform basic diagnostics on GM vehicles. They widely available, and not that expensive. I've never heard of an O2 sensor relearn. I know you can manually reset fuel trims, but it's not strictly necessary. Why don't you just call the shops in your area and see what they can do.

MitchM

thanks , i took your advice and now have an appt at the chevy dealer (again.)

1/ independent mechanic #1-- 2 miles away. , installed new o 2 sensor , new crankcase position sensor aug of 2021. $400. bill. brought back 2x still throwing codes. relearned 2x at $100 each. relearn fixed temporarily then it started coding intermittently again. it still threw intermittent codes . said he couldnt do any more because van was showing intermittent p 0300 p0336 codes but not coding when it was in the shop, said 'take to a chevy specialist.'
2/ got an appointment w independent chevy specialist #2 for oct. 15 the earliest. he cleaned the e c u connections and said the cKposS installed by #1 was an off brand not oem, installed an oem ckCPos S. $550 bill. van stopped throwing codes for a few weeks then threw codes intermittently. sensor was re learned, codes went away then came back . this shop doesn't have capacity to re flash e c u (which was not done at #1 shop after o 2 and ckKPos sensor replacement. ) ecu software is likely 07 , mechanic said a dealer can re flash e c u especially since both cKposS and downstream o 2 sensor were recently replaced. (both downstream o 2 sensors are pretty new at this stage. )
3/ got appointment w good chevy dealer 22 miles away. by 11/3 van now throwing constant p0300 and p0336 codes. got an appointment for 11/15 , e c u was re flashed and updated w new software. van was still coding p 330 and 336 constantly now. but the 'truck lift' was tied up w another vehicle so t hey couldn't diagnose any more without van being on the truck lift. i now have a diagnostic appt for 12/20.... stay tuned.

MitchM

it turned out to be a stretched timing chain. 3500 was still throwing p0300 s even with new CKP and CMP sensors, finally started with a P336 code 'more than 3 sec delay' in CKP signal..

tech2

i would have thought the vehicle would have a built in rev limiter in the pcm programming. my 07 cobalt did.

MitchM

o7 express van, 56 000 miles. ( 2 coding p430 o 2 sensors replaced, 1 within last 9 months. regular oil changes every 3000 miles, excellent gas for hi-volume gas stations, tuneup, points plugs MAF, CrankKPos sensor and CamMPos sens both replaced. ) p0300 random cylinder misfire code only at highway speed and always when going up a hill at 50 mph plus. i heard from a dealer that 'at 80 ooo miles or 10 years the cat con will need to be replaced and he's also telling me I need a new timing chain '. "because a clogged cat con can cause the ecu to send wrong signals of how much fuel to put into the injectors resulting in random misfires. . "
so : can my clogged cat con, that is not yet throwing any of the the obdii i cat con codes, be so clogged it's resulting in my p0300 random misfires ? can that be an explanation for why I only get a p0300 at highway speeds with hot engine ?
all thoughts welcome. (PS I can replace a cat con myself --- a lot cheaper than going to a dealer for a full timing chain remove/ replace which dealers quote as drop the oil pan, remove all kinds of parts, access timing cover, 12 hours labor and $900 of parts . ) my ACTRON v data (which dealer doesn't want to look at ) suggests my upstreams are fine but my downstream is going wild with fluctuations rich-lean . (I haven't run temp or pressure difference tests on cat con to look for more than 50 degrees temp difference between inlet and outlet as cat con is not OBDII coding. . . ) do I replace cat con and if that doesn't fix the p0300 only then go for the $2k of dealer timing chain replacement work which I certainly am not skilled enough to do myself. . . ) suggestions? all thoughts welcome with thanks in advance. I'm at 9 months and $1500 of work done to try to cure this p0300 and make my van drivable and reliable for work as an e m t .

kevinkpk

Clogged cat can cause it. You should take the cat out, straight pipe, see if that cures it.

William Kisselstein

The way these vans are made it's tough to just remove the cats. At least on the 97-02 the cat is not a separate part from the downpipe.

It might be better to buy some downpipes at a junkyard and swap those on, and run no cats or mufflers. You're only going to run it long enough to check if there's a clog. That way you didn't cut up your exhaust if there's nothing wrong with it.