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Gm Tech (Cam)
Gm Tech (Cam), GMC Mechanic
Category: GMC
Satisfied Customers: 27667
Experience:  GM Grand Master Technician 2007. 11 year experience.
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GMC Sierra: service soon light is flashing. engine running

Customer Question

service soon light is flashing. engine running rough idle. just changed o2 sensor driver side. 2005 sierra 2500 hd with 6L gasoline.
Submitted: 3 years ago.
Category: GMC
Expert:  Pete replied 3 years ago.
Hello,I will be glad to try and assist you with this,Welcome to Just Answer.

Hi.

With a flashing service engine light this indicates the engine computer has detected an engine misfire that can damage the catalytic converter is left un treated/repaired for to long.

This can be caused by a failed ignition coil,plug wire or spark plug or it can be caused by a restricted fuel injector or even low compression on one or more cylinders.

What needs to be done here first is to have the fault codes checked in the engine computer with a scan tool that can communicate with this computer by a shop or dealer.

Then it needs to be located as to which cylinder is misfiring and a scan tool that has misfire data will pinpoint the misfiring cylinder.

Once that has been located then it needs tested to see if spark is at fault or if an injector or low compression is at fault and then once located then it can be repaired.

More common is a faulty ignition coil to a spark plug or a restricted or failed fuel injector.

Let me know if you need anything else,click accept if you feel satisfied with your answer as this is how I get credited for my time,Thanks Pete!
Customer: replied 3 years ago.
Fault code P0300
Expert:  Pete replied 3 years ago.
P0300 is a code that sets for random cylinder misfire,it still needs to be located to which cylinder is misfiring then it can be repaired.If this scan tool that read this code reads out engine data try and locate the engine data for engine misfires and see which cylinder is showing the misfire,this will aid in diagnosing which component is at fault.

You can also try unplugging each coil one at a time to find the cylinder at fault,if you unplug a coil and it doesn't change the way it runs then this is the cylinder at fault.

Let me know if you need anything else,click accept if you feel satisfied with your answer as this is how I get credited for my time,Thanks Pete!
Customer: replied 3 years ago.
Could it be the EGR valve?
Expert:  Pete replied 3 years ago.
Yes,a stuck open egr valve will cause engine misfires but usually in this case if it was then you would have an egr fault code set as well.

Let me know if you need anything else,click accept if you feel satisfied with your answer as this is how I get credited for my time,Thanks Pete!
Customer: replied 3 years ago.
One more bit of information. The code reader gives no specific cylinder miss fire. The engine seems to run fine at higher rpm. I would like to know your opinion as to if the problem may be caused by a faulty coil. When I changed the driver side o2 sensor, it seemed to run smoother and the service engine soon light did not light continuous as before for a while, however, it started to flash and has not reapperared as long as I kept higher rpm's while operating. I didn't want to go through the process of illimination as it gets expensive. Once again, the code reader showed P0300, random misfire.
Expert:  Pete replied 3 years ago.
O.k.

Your code reader doesn't give this info like some scan tools do.A local parts store might read the codes and show the data with their scan tool to let you know which one is misfiring and they usually will do this free of charge as well so you might check into this.

At higher rpm if the light either doesn't come on or flash then the misfire hasn't been detected and if it was a faulty coil or restricted fuel injector you would still have a misfire at higher rpms in most cases so this may be low compression as a faulty valve or broken valve spring can cause misfires at idle or lower speeds but in some cases may not misfire at higher speeds.

A full compression test will help in determining if it is compression related.

Let me know if you need anything else,click accept if you feel satisfied with your answer as this is how I get credited for my time,Thanks Pete!
Customer: replied 3 years ago.
Can you give any other symptons related to your last answer?
Expert:  Pete replied 3 years ago.
If it is compression related meaning low compression on one cylinder then most likely the cylinder head valve or valve springs are at fault for the cylinder that is misfiring.Is this what you were looking for?
Customer: replied 3 years ago.
I thought it was electrical, by way of faulty sensor or ignition coil. Low compression in one cylinder causing a misfire should be detectable with the updated code reader that I purchased 2 days ago. It is actually a diagnostic reader with real time information, i.e., many different engine functions. As it is updated and a single cylinder misfire should read as a P0301 through P0308, it reads as P0300 as a random misfire. I don't want to over do it, however, it doesn't make sense that a single cylinder is misfiring. Noticed some smoke from tailpipe for a short time last week. Just installed new radiator last weekend.
Expert:  Pete replied 3 years ago.
I will opt out and put this question back on the list for other experts to see if they can assist more on this.

Thanks Pete!
Customer: replied 3 years ago.
When will I be contacted?
Customer: replied 3 years ago.
Relist: Incomplete answer.
Expert:  Gm Tech (Cam) replied 3 years ago.
Hello, I'm Cam. Please include all the details about your issue so I can accurately advise you. I cannot see your vehicle so please be detailed.

Have you checked the fuel pressure? Spec is 55-62 psi.

Second, did you check for plugged catalytic converters? This is done with a gauge in place of the front oxygen sensors and more than 1psi at any time will cause all of this.
Customer: replied 3 years ago.
2005 GMC Sierra 2500HD, 6.0L, 364 ci VIN U. Will check fuel pressure within 10 minutes if diagnostic meter has that option. Have not checked for plugged catalytic converters. What is the procedure for that test?
Expert:  Gm Tech (Cam) replied 3 years ago.
Pressure test is done with a manual pressure gauge on the port of the passengers side fuel rail.

Take the front oxygen sensors and either drive with them out the pipe or put a back pressure gauge to thread into one of the oxygen sensor holes.
Customer: replied 3 years ago.
I will perform these tests asap. I will get back to you then. I will accept your answer for now and will contact when tests are complete. Will reply to email.
Expert:  Gm Tech (Cam) replied 3 years ago.
Perfect. Thanks. I will watch for your reply.
Gm Tech (Cam), GMC Mechanic
Category: GMC
Satisfied Customers: 27667
Experience: GM Grand Master Technician 2007. 11 year experience.
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Gm Tech (Cam)
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GMC Mechanic
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GM Grand Master Technician 2007. 11 year experience.