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Technician Chris
Technician Chris, Technician
Category: Car
Satisfied Customers: 367
Experience:  I am a certified ASE Master Technician with L1 Advanced Diagnostic Specialist and over 15 years experience.
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2003 sierra 2500 hd 6.0 gas Sle: Hi i have 2003 gmc sierra

Customer Question

Hi i have 2003 gmc sierra 2500 hd 6.0 gas
Random misfire po300and knocking noise
Truck runs strong After 60km then engine light flashes constint
Coils plugs fuel filter done cleaned maf code went away and most of
The knocking for 40 miles the came back.also the engine light flashing
Became more constint after i changed exhaust manafolds earlier but the knocking
Still was there.
Submitted: 1 year ago.
Category: Car
Expert:  Technician Chris replied 1 year ago.
What are the codes from the check engine light? This will help greatly. You can visit an auto parts store and have them pull the codes for free.
Customer: replied 1 year ago.
Po300
Expert:  Technician Chris replied 1 year ago.
That is the only code? Usually there will be more if there is a hard misfire. That code is just a generic misfire code, and doesn't really tell how to pinpoint the issue. A misfire can be caused by fuel, spark, or compression issues. Spark is the most common. I would start by checking the plugs and wires for any signs of wear. You say you have already replaced the plugs? How long ago? What about the wires? This engine has 1 coil per cylinder, so likely the coils are all ok if you don't have any codes that pin point 1 specific cylinder. If the problem is not spark related, I would then move on to the fuel system. Fuel filter is new, but when was the last time you cleaned out the injectors? A fuel system cleaning is recommended every 30K miles or so. If just one injector is slightly clogged, it can cause a random misfire issue.
Customer: replied 1 year ago.
How about the knocking in relation to the misfire
Expert:  Technician Chris replied 1 year ago.

A knock or "ping" is caused by detonation which is premature combustion. This will occur from advanced timing or running too lean (too much air / not enough fuel.) In your case, I would suspect the latter. Most likely both issues are going to be from the same source. Probably fuel related. I know Firestone will perform a fuel system cleaning for $99 so that would be a great place to start. Even if the issue is elsewhere, a fuel system cleaning is a great service to perform on any vehicle over 30K miles. What was the issue with the exhaust manifolds? Were they cracked and leaking?

Customer: replied 1 year ago.
Yes all the above
However earlier i pointed out i clean maf and all was good
For short time then resumed
some suspect internal top end work
But i disagree after i heard 95 perc knocking
Gone after maf cleaning
I snow plow with this truck its my bread and butter
I dont mind paying the bucks if it is motor work thats
Causing misfire .
So dont what to find out later it dudnt work


Expert:  Technician Chris replied 1 year ago.

Have you ever replaced the mass air flow sensor? If it is acting erratic, it could have an internal fault giving false readings to the computer which would cause a running lean issue and possible knocking and misfires. Typically that part alone is around $150, but replacing it is easy. I would advise taking the vehicle to a shop for a diag. before just throwing a new mass air flow sensor at it. However, it is the one part above all others that controls the amount of fuel the engine gets. The PCM controls the fuel of course, but it uses the mass air flow to tell it how much air the engine is getting, if the reading is wrong, the computer doesn't know how much air is actually getting to the engine.

Expert:  Technician Chris replied 1 year ago.
I don't understand. You left me a poor rating. Are you not satisfied with my service? Do you have follow up questions? I will be more than happy to assist further. You have to understand, that with out physically being with the vehicle, and able to run tests myself, I cannot know what is going on 100%. I only have the information that you give me.

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Technician Chris
Technician Chris
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I am a certified ASE Master Technician with L1 Advanced Diagnostic Specialist and over 15 years experience.