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rpmdiag, Tech and Instructor
Category: Chevy
Satisfied Customers: 8130
Experience:  Since 1975
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I have a 2003 5.3 L Silverado with 51000 miles on it (that

Customer Question

I have a 2003 5.3 L Silverado with 51000 miles on it (that number is *****). I bought the truck new. I have a P0300 trouble code with the associated misfire detected at idle along with an off-idle stumble. At higher RPM's no misfire is detected. I would like someone to help walk me through trouble shooting who is patient. I have many years of amateur auto mechanics. I have started the process but have many possible issues to investigate.
Submitted: 1 year ago.
Category: Chevy
Expert:  rpmdiag replied 1 year ago.

Hi, I’m Ron and I can help you.
I have been in the auto repair business with my own shops since 1975 and also teach
automotive repair. I am confident we can resolve your problem together. Please
understand that I don't know your skill level. In most cases when you have a problem
with the way the vehicle performs, specialized tooling is required to acquire computer
data/information or engine testing results. We didn’t build these complicated vehicles; we
just diagnose and fix them. Also understand that I can't see, smell, hear, or touch the
vehicle either, so it may take a few replies to get to satisfactory results. I am also going to
direct you with the fastest, easiest and most common things to check first. Please be
patient and permit me to do what I do best.
I also try to give all of the information you may need, expecting a “positive rating” in
return. I will also continue assisting you after you have rated me, if necessary. If there
is any reason I cannot get a positive rating, please let me know why “in advance”, so I
can go another direction for you. I am sorry you’re having this problem; I’ll help you
through it.
The problem is either a vacuum leak or a CRANK sensor. It's also possible that the problem is a flash update for the computer.

You need to attach a scan tool and check the fuel trims at idle and load, then check MODE 6 for misfire data including. freeze frame data so we know the conditions that the code was set.

Thanks, Ron

Customer: replied 1 year ago.
I understand your message. I am currently checking plugs and plug wires. I suspect a vacuum leak because I believe I can hear it from the location of the right bank. I do not have a scan tool but I believe this idle misfire is in both open and closed loop. Can you help me eliminate vacuum leak sources after I replace the plugs? I know this is not the preferred way to proceed.
Expert:  rpmdiag replied 1 year ago.
A smoke machine.
Thanks again, and please don't forget to rate me and keep this link in case you need me again later on.
Customer: replied 1 year ago.
I realize that the P0300 code can be complex. I did resolve the problem on my own. There is a GM tech service bulletin that relates to my problem, which I found on through my own research. The problem was the OEM plugs which are not recommended. How poor is GM engineering? I removed the plugs and replaced them with the newly recommended plugs, l also changed plug wires and the issue is resolved.
Expert:  rpmdiag replied 1 year ago.

Great job, please rate me for my time.

Thanks, Ron