2005 Yukon XL Denali P0300 Code

[Jimmy4445]

My 2005 Yukon XL Denali, 6.0, 160k, throws a persistent and recurring P0300 code. It consistently stumbles and shifts hard in low gear, runs and shifts smoothly after the first shift, but code P0300 appears at 60-70 mph without fail after clearing it. There have not been any other codes.

I have changed plugs/wires and cleaned MAF. No improvement. Before I throw more parts any ideas based on the above? Ignition coil? Injectors? Vacuum/Gasket leak? Faulty MAF? I know the there are a lot of suspects, but I was hoping someone could help narrow it down based on my symptoms. Thank you very much in advance for any suggestions.

Freeze Data (two representative incidents):

throttle position 29.8
rpm 2851
load value 37.2
air flow rate (MAF) 50.38
map sensor 43 kpa
coolant temp 186
intake air temp 73
ignition timing advance 37.5 deg
short term fuel trim1 3.9%
long term fuel trim1 9.3
short term fuel trim2 2.3
long term fuel trim2 8.5
speed 68 mph
fuel system 1 closed
fuel system 2 closed




throttle position 26.6
rpm 2538
load value 32.1
air flow rate (MAF) 38.28
map sensor 39 kpa
coolant temp 186
intake air temp 73
ignition timing advance 39 deg
short term fuel trim1 1.5%
long term fuel trim1 8.5
short term fuel trim2 -2.2
long term fuel trim2 9.3
speed 60 mph
fuel system 1 closed
fuel system 2 closed

[gmtech1]

The 6.0L is very sensitive to build up in the throttle body, causing rough idle/misfire conditions. I'd check and clean the throttle body and see if that helps.

[Jimmy4445]

Originally Posted by gmtech1

The 6.0L is very sensitive to build up in the throttle body, causing rough idle/misfire conditions. I'd check and clean the throttle body and see if that helps.

Thanks for the suggestion.

I gave the throttle body a good cleaning today. Despite the fact it was very dirty it did not fix the stumbling trying to find gear.

I guess next I'll look into a vacuum leak somewhere, then dirty or bad injectors.

Still getting only a p0300, nothing else, so if anyone has any ideas I'd love to hear. Maybe the camshaft position sensor?

Thanks in advance.