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Gary, ASE Certified Technician
Category: GMC
Satisfied Customers: 16873
Experience:  GCM master tech over 25 years
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Unable to diagnose p0300 Radom misfire mechanic says its in

Customer Question

Unable to diagnose p0300 Radom misfire mechanic says its in cylinder 5 at 17000 rpm or 30 percent. It's sometimes bucks going up steep hills but runs great otherwise . Driving back from sears ck engine light came on again.
Also says runs good for 290000 miles
My guess fuel filter/pressure or wires or spark plug
Submitted: 1 year ago.
Category: GMC
Expert:  Gary replied 1 year ago.

i am sorry your having this problem

the mechanic confirmed it was JUST % 5 cylinder doing this ?

Customer: replied 1 year ago.
only at 30% power Below and above 30% no misfire
Customer: replied 1 year ago.
Also said could be valve problem
Expert:  Gary replied 1 year ago.

if this is only a specific cylinder then the fuel pump should not be the issue

these issues come down to a bad cap and rotor bad # ***** plug ot plug wire OR sticking valves in the valve guides for # ***** cylinder which is somewhat of a common issue with these 5.7 R engines

Expert:  Gary replied 1 year ago.

how are you making out ?