I've been rocked on and off by a P0300 code after doing all the above, typical repairs and maintainance. No MIL, then, on a second ignition on, car running, minute later, on goes the MIL lite and bingo, P0300. Anyone, I've atatched a Tech 2 freeze frame file of todays events. Any and all assistance would be appreciated. Regards n Happy Holidays!

 

So, P0300 indicates Multiple Random misfire, Is there a felt misfire in the engine or is it just in the data? What is your Cam sensor Retard Value and have you replaced the Crank sensor recently? you also mentioned maintenance parts, Did you use an ACDelco/Delphi Cap, Rotor, and a decent set of Plug wires? What is the condition of the spark plugs?

There is an infinite amount of variables that can cause a misfire, Including vacuum and intake leaks also, but since this occurred after maintenance then it seems likely to be related to those parts.


Here is swartltkk's thread on diagnostic testing for it

https://blazerforum.com/forum/diagno...4-5-6-a-35153/

 

Over the past year, for the build, I have been chasing codes after every parts replace and adjustment. Cam retard, spot on, crank sensor replace and relearn, done with OEM AC Delco parts. I performed a smoke test 3 times. No discernable vacuum leaks. Thanks for taking the time, to respond, Greatly appreciated. The quest continues.....

 

Going back to a bit you missed

Can you feel a misfire? start there, if you cant feel a misfire and you are getting false returns then you need to start looking at live sensor data from the Knock sensor, Crank sensor, and the Cam sensor when the condition sets. as in raw data what those particular sensors are seeing at that time, those freezeframes show it happened at about 1,600 RPM at fairly low speed, i have to ask how you got it into 4th while at such low speeds as if its an Automatic Tranny/4L60E then you should barely be into 3rd, as far as testing vacuum leaks cap off the Plenum cover spigots with vacuum caps to disconnect all vacuum components from the motor to test the harder to check feeds such as the HVAC system, 4WD If equipped, there is a vacuum canister in the drivers side fender that can crack too and cause a hard to trace leak.

Cam sensor Retard is within +/- 2 degrees of 0, Crank sensor AND a CASE relearn done, no vacuum leaks and good plugs, this is going to get weird.


Spider injectors, you have CPFI or MPFI injection on that truck? i forget when the changeover year was.

 

No discernable misfires at all, engine quiet no noise all the way through idle, snap throttle, standing on it, launches like a saturn five. No shake rattle roll, can place a champagne glass on plenum, won't lose a drop. No snap crackle or pops. Fresh charcoal canister installed. No vacuum leaks that anyone can see. Had multiple people assist with that. Shift points seem to be fine! Smooth, no apparent problems. It is spider injection, new injectors replaced. Checked fuel system with tech 2, using fuel pressure gauge and GM protocol, all checked out fine! Fuel Pressure on the money. Compression fine all cylinders. Complete Tune Ups, all fresh best parts.

Checked calibration, last couple of updates, not done for this car? Both emissions related....Obviously, I'm not picking up on some issue....Once again, thanks for your expertise! Would be nice if I could roll this, right over to you.

 

man that actually has me stumped. only thing i can think of is sensor voltages or voltage/ground dropout & intermittence on either crank or cam wiring, and if all of that fails start pin testing the PCM via backprobing, will need to raincheck on the pinouts for a bit as i would have to dig them up and im not sure if alldata has the right ones.

also, you mentioned emissions related issues? what type(s) and how did you discover them.
Unfortunately i do not believe our Trucks are capable of showing us something like cylinder power balance/contribution. since we've looked through all of the real technical stuff lets go back to the basics
Plug wires fully seated
plugs in good shape
poor compression or carbon buildup on the intake runners
worn out distributor gear/camshaft side distributor drive gear
improperly spaced cam sensor/junk stuck on the reluctor
check your grounds, all of them back of the block on either passenger or drivers side head, cant remember which. Frame, Block, and core support grounds coming off your main battery lead, etc.
if you happen to be in florida you actually could roll it over to me lol.

 

The calibrations were checked at GM AC Delco against records online there! There were 2 calibrations never updated as pulled off the Blazer PCM by my scanner, specifically using Tech 2 data. Both, as described, were emissions related. I'm going to get the car up on a lift, to get better look at the wiring. One other thing that has me spooked, 3 times after attaching my scanners one Autel, the other a GM MDI 2, with key off, drained the battery down to a no start! Never, ever happened before!
So great to have your thoughts on this!
I'm in So. California, would be great to meet up one day. Where in Florida are you? I used to live in Coral Gables, outside Miami, many years ago. All the best to you, I will definitely keep you posted on the next part of this discovery process!

 

Southern 813 Area, a pretty long ways away to drive eh? Something is not reading right, you said you used all Delco parts so im not quite ready to call it on Quality control for those parts just yet. Check over the wiring thoroughly, check your voltage drop between sensor and frame/body ground, and in general make sure you have the right voltage where you should have voltage, and a ground where you aught to have a ground.

 

Car runs great, damn P0300 code v difficult to get a grip on it. Originally this SOB limped into the driveway, barely, thus began this year and a half effort to get this through "Cally" smog/ emmissions approval. Rats had eaten through various wires n such. Unravelling the mystery continues. I'm still convinced its wiring/ grounds, pcm. I'll let u know!

 

oof, Yeah i feel that with the smog system being a right pain, my rig was originally sold new in North california before making its way up to Indiana and then down to florida where it remains now

Pretty big thing, REPLACE THE SMOG PUMP CHECKVALVES
ASAP.

If you dont you will be in for a world of pain like i nearly was. check spoiler for images of the cleanup of a Passenger side valve blowout when you get lucky.