trying to fix a p0300 random misfire - 07 chevy express 3500 vortex 8 cyl

I'm here because I'm trying to get an idea on curing my persistent P0300 random cylinder misfire on my 07 chevy 3500 express van . w/ 57,000 miles on it. (oil changed religiously every 3000 miles since new, trans fluid always kept topped up and was changed at 50,000 miles. never low on oil or trans fluid, I check every couple weeks. . ) DTC p0300 w steady CE light was initially diagnosed by my cheap little FIXD in 6/21. took it straight to local shop #1 who diagnosed a b1 S2 o 2 sensor (driver side downstream ) by its code (tho my FIXD did not ever show that code. ) 1st shop replaced that b1S2. no joy. #1 shop then did a tuneup with new points, plugs, wires, ' professional 'injector clean.' ' no joy. #1 shop cleaned then replaced a new MAF, cleaned throttle body pos sensor . no joy . shop #1 then installed new CkPos sensor and relearned it: the DTC p0300 disappeared and CE steady light light went out for about 2 months then reappeared Labor day weekend 2021. Made appointment w/ 2nd garage who cleaned ECU ground, and claimed #1's one's CkPos sensor was not ac delco OEM, so replaced #1's CKPos w an oem AC Delco, and relearned it. the steady CE light and P0300 went out but after 300 miles, the p0300 reappeared 1st week of nov. Shop#2 did a relearn CKpos sensor (for the 3rd time, at 100$ a pop. ) CE light and p0300 went away for a month then no joy . thanksgiving weekend , drove van hot w CE light and P0300 showing per instructions of shop#3 the chevy dealer who refused to put its scan 6 onto the van when it was hot and coding. would not look at my Actron 48 frames open at idle rpm 897 . (My Actron was showing occasional odd STFT for b1S2 (the 'new one' ) as occasionally ' "NA". ) sat on dealer lot fr 3 weeks, they relearned CKpos s . no joy. Chevy dealer replaced cam pos sensor and 'relearned. no joy. now 12/15 tells me I need a timing chain replacement because they see a 'P0334: ' ( CKpos/CPS mismatch more than 5 sec. ) I ran the van home 12/15 w my Actron on -- no trace of a P0334, just a p0300 as usual w usual symptoms of steady P0300 and MIL steady, w occasional flashing CE light when I accelerate up a hill at speeds more than 50 mph. so the repairs for the past 9 months --from 3 different garages-- throwing parts at my van including tuneups, upstream B1 o2 sensor, new CKpos Sensor, (relearned 2x) new cam pos sensor, (w/ relearn ) are about $1400 . and I still have p0300 random cylinder misfire. chevy dealer is now telling me I need timing chain replacement because they claimed to see a p0334 cam pos/ CKpos sensor 'more than 5 sec 'mismatch. ' ( I don't think any garage put a scope on it when van was hot and coding ( let alone use the diagnostics suggested in scanner's book at which I just bought to see where the spark is occurring in each cylinder relative to TDC. my actron is showing IGN advance from 2 degrees to 18 degrees at idle rpm 22 % load -- which I find odd. ) my symptoms are: steady MIL , DTC p0300 only. ( w my Actron, I cant replicate the P0334 the chevy dealer most recently claimed to justify a $1300 timing chain replacement. To say I've lost faith in chevy dealer is an understatement. ) current drivability: still no missing no knock no loss of power, no hard start, no shake or noise. steady MIL and p0300. when going up a hill at more than 50 mph, the CE light starts to flash for a few seconds but stops flashing when I leave off the gas or top the hill. all CE lights go off as I coast down a hill. then the steady one comes on and turns to flashing as I accelerate up a hill . I don't know enough about STFT and LTST s to make sense of those numbers so I'm studying scanner's book. I'm about to check the cat con temperatures at intake side and output side to see what temperature difference I get ( but actron is not throwing any cat con codes. ) Can my distributor timing be totally off ? (Fortunately I live 6 miles from Danner's garage, that's where I'm headed next. ) . All thoughts welcome ( and if scanner wants this puzzle for the final exam in his engine diagnostics class he's welcome to all my actron data. . ) .

It's important to first identify the misfiring cylinder(s) and then go after the cause....Did you or anyone had a look at mode six data, freeze frame data, historic misfire counters........? I'd start there. A flashing CEL under load may point to an ignition issue , live data on a test drive can help to differentiate between fuel delivery and spar k. Do you actually feel the misfire?

Btw, you wouldn't find points on a '07 Chevy Express.

I'd take those receipts with you and take it to Danner Automotive.

Maybe let him know his brother might want a "silver bullet" or "rabbit hole" story as my guess is unless you need a timing chain job or Catalytic Converter you probably won't spend more than a $200-$400 tops.

Something smells amiss here. Definitely take all the receipts with you as a good history is always helpful to a mechanic that's diagnosing a issue such as this.

I find it odd that you said it was running hot while climbing a hill but that's a cheap uView head gasket test kit widely available on Amazon.com.


If you feel Paul aka scannerdanner has helped you enough then you can always go thru his Amazon storefront & support him & the free videos he puts out while letting him earn a little for his time. Sure they get paid in views for their time from Amazon but the algorithm has to keep being fed with YouTube.

YouTube thinks they should be paid more than those creating the content hence the reason I'm sure scannerdanner has his own site now as hopefully it cuts the middleman out some. Keep in mind his son has to still do the videos, editing, etc to be able to make any money.


My guess is it's something that's not going to be to awful hard to find. Just keep in mind that you need to stay focused on what makes sense vs going down rabbit holes

Now I need to find my Scannerdanner book so I can read again as I try to read it regularly such as at least every other month.

IF I did strictly diagnostics all the time I probably wouldn't need to read it all the time. I do keys, immo, programming, electrical, carpentry, plumbing, hvac etc, etc... I like to stay well versed in a few trades as it comes in handy when you least expect it especially on stuff at home when the wife says "this or that broke".

thanks for the thoughts. reading paul's book has helped me try to figure out the best course of action here. I'm going to try some cat con catalytic converter cleaner to see if that changes the status of the DTC and turns off the steady code p0300 when the engine's under max load going up hills. more later , and I'll be sure to post the results. thanks !

I’d have another shop do a crank relearn. Start there if you have no scanner to watch

I've had had 2 new crank pos sensors, 1 was non ac delco and bad, 1 was ac delco, total 3 cKposS relearns and 1 cam pos Sens w relearn. no joy.

What about checking the timing chain yourself? Sometimes you can get on the alternator nut with a socket and breaker bar and slowly move back and forth to feel the slop, if that doesn't work you will have to go to the crankshaft pulley bolt

update: came by Danner's shop 7/23 with a p0446 'evap vent system performance and the p 0300 as usual. Danner's commanded the vent with a scan tool; saw it worked audibly on and off. But they noticed the fuel tank pressure sensor had erratic voltage 1.5 volt to 2.8 volt. (with key on engine off. ) note there were NO F T P Sensor voltage error specific codes like P0452, 453 or 454 -- low, high or intermittent FTP sensor voltage. Danner's also noted that the evap vent solenoid control appeared to work but needed to be checked. (no specific P0449 code 'vent control solenoid circuit' for that showing, either. ) chevy gave me an appointment for early October. (I needed a heavy duty lift appointment due to 3500 's weight not fitting at Danner's. ) I noticed that with cool autumn weather, I was getting a p 0300with cold engine open loop, as soon as I started the engine, and also a P 0446. When I pulled over a minute or so later to clear off the p 0300 and p 0446, the p 0300 went off permanently. for th rest of the trip. so by early fall, the p 0300 and p 0446 were coming on early in cold mornings, could be shut off, and would not reappear the rest of the day.
by early October I had the p 0300 on cold start and now also a confirmed p 0449 vent control solenoid malfunction (not old p 0446. ) . in early oct. chevy took the 3500 in for service ; replace f t p sensor and check p 0449. (The gas tank on the 3500 needs to be dropped to access and replace the f t p s , cost $115 for the sensor but 3 hrs labor to drop the tank. ) along with the f t p s replacement, also put in a new vent control solenoid, all parts GM OEM. After all this work was done, I ran a bottle of GM Techron thru the fuel system. mechanics tell me the vent control solenoids fail but it may take a while to throw the confirmed P0446 EVAP vent system or specific p 0449 vent solenoid control circuit problem code.

many thanks to Danner's for the good work on the erratic f t p sensor voltage. I have had only 2 p0300 s since this work was finished in early oct. 2022.

so, let's say there is occasional erratic f t p s signal voltage from a failing f t pressure sensor or bad vent control solenoid. doesn't that erratic signal from f t p s into the e c u' affect the e c u's calculation of what fuel/air to send to the injectors? can a bad fuel/air mix calculation from the e c u then cause a p 0300 ? what freeze frame data would be most helpful ?

update: came by Danner's shop 7/23 with a p0446 'evap vent system performance and the p 0300 as usual. Danner's commanded the vent with a scan tool; saw it worked audibly on and off. But they noticed the fuel tank pressure sensor had erratic voltage 1.5 volt to 2.8 volt. (with key on engine off. ) note there were NO F T P Sensor voltage error specific codes like P0452, 453 or 454 -- low, high or intermittent FTP sensor voltage. Danner's also noted that the evap vent solenoid control appeared to work but needed to be checked. (no specific P0449 code 'vent control solenoid circuit' for that showing, either. ) chevy gave me an appointment for early October. (I needed a heavy duty lift appointment due to 3500 's weight not fitting at Danner's. ) I noticed that with cool autumn weather, I was getting a p 0300with cold engine open loop, as soon as I started the engine, and also a P 0446. When I pulled over a minute or so later to clear off the p 0300 and p 0446, the p 0300 went off permanently. for th rest of the trip. so by early fall, the p 0300 and p 0446 were coming on early in cold mornings, could be shut off, and would not reappear the rest of the day.
by early October I had the p 0300 on cold start and now also a confirmed p 0449 vent control solenoid malfunction (not old p 0446. ) . in early oct. chevy took the 3500 in for service ; replace f t p sensor and check p 0449. (The gas tank on the 3500 needs to be dropped to access and replace the f t p s , cost $115 for the sensor but 3 hrs labor to drop the tank. ) along with the f t p s replacement, also put in a new vent control solenoid, all parts GM OEM. After all this work was done, I ran a bottle of GM Techron thru the fuel system. mechanics tell me the vent control solenoids fail but it may take a while to throw the confirmed P0446 EVAP vent system or specific p 0449 vent solenoid control circuit problem code.

many thanks to Danner's for the good work on the erratic f t p sensor voltage. I have had only 2 p0300 s since this work was finished in early oct. 2022.

so, let's say there is occasional erratic f t p s signal voltage from a failing f t pressure sensor or bad vent control solenoid. doesn't that erratic signal from f t p s into the e c u' affect the e c u's calculation of what fuel/air to send to the injectors? can a bad fuel/air mix calculation from the e c u then cause a p 0300 ? what freeze frame data would be most helpful ?