P0300 code driving me nuts!

Yukon25006.0 · January 1, 2017

The truck:

2000 GMC Yukon XL 2500 6.0 litre

Bullydog gas tuner ran on performance

Spectre cold air intake

Factory exhaust (now has hollowed cats :/ )

What has been done:

I've changed plugs, wires and coil packs

I've cleaned the Throttle body, MAF sensor

Every time I get gas I add injector cleaner and octane booster

Run only 94 octane

Cleaned the MAF and throttle body yesterday

Just cleaned the FUel pressure regulator and it did not have fuel in the vacuum line

Hollowed out the cats (everywhere I read someone says cats will throw the random misfire code)

If you have made it this far thank you for sticking with me but I wanted to let everyone know what has been done...

Since I bought the truck 6 months ago it has always had the p0300 code but only flashed when at highway speeds (100-120km/h) and only when maintaining that speed, or if I put it to the boards, but only flashed a few times and then stopped,

In the past month it has been flashing from 60km/h and up normally at 2000rpm and doesn't stop flashing till under 60

Now it has NO whatsoever signs of a actual misfire! It doesn't shake jump jerk or anything, just a light flashing at me. So today I hollow the second cat that the previous owner didnt and I took the fuel pressure regulator off and cleaned the screen and used a different regulator from another 6.0 I have which was a month old part. I did miss the last little o ring when 1st re installed which cause rough ideal and choking out once touching the pedal, I then installed properly and the truck is running mint! Better throttle reasponse and take off after today's mission

And during take off no flashing light and stayed off for 5 min which included stop signs and aggressive take offs, once I maintained 100km/h for 30 seconds the light started flashing with no actual felt misfire, and it remained flashing till I was under 2000rpm. I scanned it and now not only do I have the p0300 but also P0108 (MAP high pressure) as well as P1637 (PCM code) and U1026 (not sure on that one)

So I didn't know if the MAP and PCM was caused from when I did not have the little o ring on the regulator and trying to run it but more so I'm looking for more ideas on the misfire code, next step I guess is clean injectors... thing that I don't get is I have no O2 sensor codes with the hollowed cats

Anyways hopefully I've given enough detail that someone can help

Jennabear · January 10, 2017

P1637 is an alternator code, and the MAP sensor is an issue that should be addressed before doing any further P0300 diagnostics.

Have you been clearing the code after each time you try a fix?

johnnybravo · June 5, 2017

Hello all,

Was wondering if there is anyone that could help me with a problem ive been having for over a year now, the P0300 (random missfire) code flashing at 70mph....not 65, not 69 / not when ir if i punch it hard from lught to light just when i hit 70 and above. Ive replaced everything, below is what ive replaced, it stared with bad knock sensors from washing the motor using some spray degreaser then hoseing it off, as i learnned quickly the knock sensors lay in the valey, water got in and next thing i knew i got a code for the bad knock sensors so i decided /to replace them. Since i was at 115,600 miles at the time i figured id go ahead and replace a lot of the old with new since i was doing the work anyways, but now i have the P0300 code at high speeds but no lose of power, no hick ups, it has great power, idles great, starts right up, never stalls. I checked and cleanned off all the ground locations, ground wires all inspected and fine, All work/new parts have 5,000 miles on or less now, all parts are OEM, not generic auto parts stuff. I checked all hoses and found no leaks, smoke test w/no leaks found, fuel pressure fine, oil pressure fine also, i can tow or haul with no problems, no lack of power and other then the flashing ses light cant find anything wrong. Here is list of new OEM parts i installed:

Fuel Pump -Oem

Fuel Filter- oem

Map sensor- oem

2 knock sensors -oem

Knock sensor harnest -oem

EGR-oem

Mass air sensor-oem

Idle air control valve -oem

Fuel pressure regulator -oem

Fuel pressure regulator vacuum hose -oem

PCV valve-oem / hose-oem

Teal (updared version) intake gasket-oem

Teal (updated version )throttle body gasket - oem

Intake bolts (with garment-oem)

Acdelco plugs -oem gap 0.44

AcDelco plug wires -oem

Coik pack -oem

Fuel injectors -oem

And has stock air filter system on

Many other parts installed (not related) as well like belts, alternator, starter, etc

Ive read a lot of forums that mention the crank relearn , but when i went to the dealership the service manager said he never heard of such a service for this issue then started grilling me on why i think i need it and said they would have to do a full range of test that could be between $250-$400 but would not just do a relearn.

I live in Va (woodbridge 22193), and was wondering, is there any of you out there that are in my area or within resonable driving distance that can help me with doing a relearn on my 02' gmc sierra 2500HD 4x4 6.0? Id pay of course, this is driving me crazy and although ive learnned to just stay below 70mph it becomes fustrating and boaring to drive in the slow lane all the time. I dont run hard anyway but there are times (like most of us) i want or need to pass someone but then that DAM light starts flashing and i just start thinking something is going to break. Id really appreciate some help guys, there has to be somebody that can help me with this relearn thing or maybe knows somebody in my area.....please!. Maybe it wont fix the issue...IDK, but if it dosnt then i xan atleast eliminate it as a possibility and move on. Iv read it doesn't take long to do, just have to have the right type of scanner, i tryed to get a tech at the other dealership (chevy) to do it as a side thing but he had no intrest, if yhere is anyone that is willing to help or knows of someone that can help please let me know.

Thanking all in advance for any help with this matter.