P0300 at 75mph 03 yukon xl denali 6.0 4l65e

[erock198]

I know you are going to tell me not to drive that fast...duh... but I only do it on the highway I promise Anyway, I've replaced plugs and wires and battery and am still at a loss. 03 yukon xl denali 6.0 4l65e. oh, replaced transmission too.

 

[kwOH]

I cant remember the exact code but I know knock sensors sometimes only come on after 60mph.

 

[M1Gunner]

I know you are going to tell me not to drive that fast...duh... but I only do it on the highway I promise Anyway, I've replaced plugs and wires and battery and am still at a loss. 03 yukon xl denali 6.0 4l65e. oh, replaced transmission too.

I would start by doing a quick double check of the air flow to the throttle body.
Clean filter and the intake air duct isn't cracked anywhere. Also that the exhaust isn't partially restricted. Can do that throwing a vacuum gauge on the intake manifold.

(FUEL TRIM DATA is your best friend!)

Then I would hook a fuel pressure gauge and to verify your pressure is maintained at that speed. If fuel is eliminated from possible cause...

Then probably start with verifying the function of cam sensor.

***also when was the last time you changed your fuel filter?

 

[swathdiver]

P0300 is an engine misfire code. It should have triggered another code telling you which cylinder was misfiring. If it did not, try a crank relearn after looking to solve any vibration issues or a damaged reluctor wheel. The book says even rough road conditions could set the code. Also check your wires and coils, high resistance in any of them could set the code too.

 

[M1Gunner]

300 is random misfire. If a single cylinder was at fault then it would set 301, 302, 303... etc. The fact that it is random means that the cause is something that would effect all cylinders I.e. Lean condition, rich condition, unwanted spark advance or retard etc.

Also, if it was a faulty crank sensor or its reluctor wheel was damaged then it would effect performance at all speeds. Not to mention the injector pulse AND spark would be effected if it was crank related. With that would more than likely throw a Crank/Cam correlation code.

 

[swathdiver]

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