Chasing a P0300

[George B]

I can see live data through OBD adapter and app. No codes for individual coils and when I unplug them one at a time I can hear the cylinder go down. Do better scanners read better? I know they can dive deeper but I was hoping that I can see enough through a phone app to solve this.

You want to be able to look at a misfire count for each cylinder. there may be several doing it.

 

[Cornelius]

So maybe one coil or injector could be missing enough to set random misfire but not enough to set code for individual cylinder?

 

[J Mac]

I had the dreaded P0300 on my 04 Denali. Took it to 3 shops and since it was random, they couldn't figure it out. Dealership recommended a new engine...it actually says that on the ticket! Ended up being coil packs. Once I looked at those as a possible issue...I noticed the truck had 3 different types/brands of coil packs so I just replaced all 8. Never had a P0300 again.

 

[mpope2]

What’s the casting numbers on the heads? Maybe valve/seat issue that changes with temp?

 

[Doubeleive]

ya I have ran into the same thing before, replaced all the coil packs problem solved. It was a gamble but it worked in my case

 

[Cornelius]

UPDATE

I had some time to keep messing with this yesterday and removed the injector rail to replaced the injector o rings and run some carb cleaner through them. I found a video online that used a tire valve stem with the core removed and after drilling a tiny hole in the cap the carb cleaner straw fits in and secured by epoxy. The injector top clicks right in to the end of the valve stem. Genius. I powered the injector with a 9 volt battery. Anyway, I did find that 2 or 3 injectors didn't spray quite as forcefully as the others, but when i was done they all worked great. Reinstalled everything, and yes it does run smoother, but still got flashing cel around 65-70 when backing off the gas. Also checked fuel pressure. 48 ign on not running, 52 running. Seems a bit low but it always starts right up, never had long crank problems. The spec is 55-60 so could lower pressure be causing a lean issue? But I would have thought low pressure would lead to issues happening on WOT or up hills, and times of demand, not coasting. Any and all opinions welcome!

Not sure if these fuel pumps work until they don't, or if it's possible that mine is getting weak and that's actually the problem. I hate to just fire the parts cannon at this. Dropping the tank isn't all that hard but most will agree that it's on the list of PITA jobs, and worse if the result is not different.

Done so far:
Plugs
Wires
Fuel pressure regulator
Cleaned injectors and new o rings
Cleaned TB
Checked and fixed vacuum lines
checked grounds
Fuel pressure seems low at 48 not running 52 running
NO OTHER CODES EXCEPT P0300

 

[exp500]

From what little bit you have diagnosed, looks like a fuel pump. Try finding fuel trims, short term and long term. Also do misfire counts on each cylinder. Swap two coils and see if misfire follows coil.

 

[exp500]

I should add- find the name of your app, go to their web page and find the instructions. Some are quite detailed. I don't have apple so can't help there. Make sure you have the GM specific codes loaded if thats an option.

 

[Jason in DLH]

I wonder if you have a leaking head gasket?

 

[Cornelius]

Misfires are happening at 1, 5, 6, 8. Swapping coils makes no difference. I thought if I had a spark problem I would get a p030x specific to the cylinder not random p0300. I haven’t seen any signs of failing head gaskets. Other than this p0300 it runs great. No coolant loss, no coolant in oil, no gas smell in oil, normal oil usage (and rear main seal is seeping), normal exhaust smell, color and appearance.