2006 Silverado P0300

[dj_bosse]

Have a 2006 Silverado 2500HD 6.0 with ~49,000 miles. Runs great until I hit ~70mph, and then it throws a P0300 random misfire code. It doesn't seem to bog the engine down (doesn't stutter/hesitate or anything). Pulled out my code reader to read live data and it's misfiring on cylinders 1, 5, 6, & 8. Replaced all spark plugs and wires and the problem persists. I find it hard to believe I'd have 4 coils go bad at once so I kind of ruled those out. From research, it seems a lot of people have had multiple random cylinder misfires like this on the 6.0s and seems like everyone had a different solution ranging from cats to O2s to crankshaft position sensor etc. Was wondering if anyone had some guidance on how I can pinpoint the issue without throwing money at parts or taking it to a shop.

 

Engine light flashes a P0300 once I hit 70 mph and occasionally stays on for a P0300 (one is pending and one is current). Also just threw an evap system code for the first time (P0446), not sure if related.

[Formulabruce]

Check your air cleaner ( gauge) . Its on the air cleaner box. You may need a new element.

I would also take off the big air intake hose and clean the throttle body extremely well, both sides of the plate.

This has to be perfectly clean.

 Mine will do a P0446 if I overfill the gas tank sometimes. Good luck !

[idnative1948]

Wow, 49K that is low! I had the same thing on my '05 with 102K so changed plugs/wires and was good for a couple of weeks. Took it back to my shop buddy and he told me I still had miss-fire on three cylinders. Long story short, he told me to replace all of the coil packs, which I did and problem solved. Just back from a 2144 mile trip towing the TT and never a blip.

[riverbanks]

You could swap them coils, quickly,write it down,where they went,see if misfires follow

[dj_bosse]

I had thought about swapping coils around but then I thought it would be very strange if 4 coils failed at the same exact time. Still something I can do, I just thought it'd be unlikely for them all to fail concurrently and at such a low mileage. I'll likely get around to swapping them this weekend and will try to report back with any findings (nowhere really close I can get up to 75mph so won't be able to do it until the weekend).

[riverbanks]

Most times a 300 is a lean condition,those misbehaving cylinders are not providing the proper power,it's a process of elimination of fault,other could be intake gasket leak,low fuel pressure,low flow on cats,it sometimes is tough

[riverbanks]

And possible crankshaft relearn procedure

[carkhz316]

I will bet money on a fix being performing a crankshaft angle relearn. Do that first because it costs nothing but time (5 minutes) before firing up the parts cannon. Your symptoms are indicative of crank angle correlation issues. 

Edited September 25, 2019 by carkhz316

[dj_bosse]

This may be a stupid question, but how do you do the relearn without a high end scan tool? Everything I'm seeing requires a Tech II or equivalent tool.

[carkhz316]

Not a stupid question at all. It can be done with various aftermarket scan tools (not sure of which ones, but they're out there), and HP tuners (which is what I use) and EFI Live. However, I searched on my own out of curiosity and found that it CAN be done without a scan tool, which is news to me. 

 

http://www.totalcardiagnostics.com/support/Knowledgebase/Article/View/99/0/crank-relearn-instructions-with-and-without-an-obd-scannerreader

[riverbanks]

Found one also,gm-crankshaft-relearn-instructions.pdf, interesting,we basically do it every time we drive,so how the heck can they go bonkers?

[dj_bosse]

So it took me longer to get around to it than expected but the misfire did not follow the coils. I ordered a new crank sensor and got access to a high end scan tool, so if I have time before the weather turns cold I'll change out the sensor and do the relearn. Hopefully this is the fix. I'll try to post my results.