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2018 Grand Sport has P0300 CEL / code

 
Old 05-07-2019, 04:06 PM
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CADbrian
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Default 2018 Grand Sport has P0300 CEL / code

The reason for this post is because CEL / code P0300 is very very vague and troublesome to accurately determine its cause without dealership-grade diagnostic equipment. Here is my story and experience and I hope that it will help others who may also be experiencing the same issue. I've searched all the other C7 P0300 threads and found them to be different than mine.

My car is a 2018 Grand Sport and for the record my car is well cared for, never abused, frequently enjoyed, and has only 1,195 miles.

Problem(s):
1. a flashing yellow Check Engine Light (CEL) icon
2. an amber Service Notice / CEL in the DIC that read Service Rear Axle and Service Stabilitrac (see images below).
3. the carís engine was idling rough, not smooth like it normally does.

The morning of May 4, 2019, was the culmination of about a weekís worth of work to wash and detail my car (2018 Grand Sport) for Corvettes & Coffee at a local dealership. The morning of May 4, 2019, I started the car, pulled it out (no more than 30 feet), and noticed a flashing yellow Check Engine Light (CEL) icon and an amber Service Notice / CEL in the DIC that read Service Rear Axle and Service Stabilitrac (see images below). Not captured in the photos was the fact that the carís engine was idling rough, not smooth like it normally does. Figure 3 below shows the battery status.


Figure 1



Figure 2



Figure 3

On May 6th I connected a scan gauge to the car and was able to pull off a P0300 code, which of course is not very helpful to anyone in isolating the cause of the miscellaneous misfire code. Iíve recently learned that the source of the P0300 code could be a LOT of things.

On May 7th my dealerís service manager (50 miles away) called me and tells me the best thing for the car is to hook it up to his dealershipís GM diagnostics tools. I know that and totally understand that. After researching CEL code P0300 and some of its possible causes, I am not at all comfortable driving the vehicle as I wouldnít want to damage anything than is not already damaged. Subsequently, this will require that I trailer my car to the dealership, but because itís not super convenient for me to trailer the car to the dealership (or any dealership regardless of how close or far), he suggested that I check that the spark plug wires are connected/seated properly as this is about the only thing that I can do with my limited mechanical skills. 😊

With my service managerís tutelage I removed the upper and lower rail covers on the driverís side of the car. I detached and then reattached all the connections below (Figure 4) and above (Figure 5) each of the 4 coils of the driverís side. I didnít have the correct tool(s) to be able to reach, sufficiently grasp, and remove the spark plug wire at the spark plugs (driverís side). Therefore, I was not able to confirm or deny that these connections were not as they should be.


Figure 4



Figure 5

In the passenger sideís lower rail cover there is a tube that fits into a grommet that sits in the middle of the lower rail cover, and because I didnít know how to detach it, my service manager thought it would be best to leave it alone. This, of course, prevented me from being able to remove the lower rail cover on the passenger side to expose and test the connections between the spark plug wires, coils, etc. on the passenger side of the vehicle.

Once I buttoned everything up (replaced and secured the rail covers), the manager of the dealership called, and I was able to start the vehicle while he was on the phone with me, but unfortunately, nothing had changed. Still a rough idle. All the same messages returned: Service Stabilitrac, Service Rear Axle, flashing yellow CEL icon. Toward the end of the conversation with the manager, the carís idle (or maybe better described as ďRPMĒ) kept getting lower and lower, as if the car was going to die (stop running). Before it had a chance to die, the dealership manager instructed me to turn the car off. Thatís where it sits right now.

Iíll update this thread with my experiences and findings as things develop and unfold in hopes that it helps someone else in a similar circumstance. Please feel free to share your experience and / or advice. Thanks.


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Old 05-07-2019, 04:33 PM
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laserdude2215
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Stop working on it yourself! You are risking damage to perfectly good parts. Drive it/flatbed it to your local dealer.
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Old 05-07-2019, 04:58 PM
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Oh, I've done all that I can do. I don't plan on fixing it myself. I just tried to rule out the possibility that perhaps a spark plug wire came disconnected somehow. My plan is to put it in the trailer and take it to my dealership as soon as I can but thought I'd start this thread to share my experiences and findings with others who may find themselves in a similar circumstance.
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Old 05-08-2019, 08:51 PM
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Good luck man. Let us know how it goes. This seems like a really strange error.
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Old 05-08-2019, 11:01 PM
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I told my service manager that I wanted to be there (to gather information and then to share it with the Corvette Forum) as the technician diagnoses and resolves the source of the CEL code 0300. I totally expected him to deny my request stating reasons of liability but my service manager is actually pleased to have me there saying most people don't care about things like this any more. It may not be until next week until I can get the car there but I am looking forward to the journey and the educational process along the way.
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Old 05-08-2019, 11:10 PM
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Brace yourself for the moment when he starts pulling the valve covers off. Good luck.
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Old 05-08-2019, 11:58 PM
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I would charge battery over night. Low voltage Can cause those faults. Your scanner might not show low voltage. Has it been sitting for the week you were washing and keeping doors and hatch open? Do you use the trickle charger? Just a thought.

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Old 05-09-2019, 12:21 AM
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The oil pressure at idle is high. Why?
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Old 05-09-2019, 02:13 AM
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Can your local scan tool read the freeze frame files? If it can then you may be able to see where the misfire is coming from. When I had my high speed missfire issue the History file would show cylinder number 8 misfired 64 times when a P0300 code was set. I didn't get a P0308 code until there were a lot more misfires.

Bill
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Old 05-09-2019, 12:21 PM
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Originally Posted by Mehrlovin View Post
Good luck man. Let us know how it goes. This seems like a really strange error.
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Old 05-09-2019, 12:21 PM
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Originally Posted by laserdude2215 View Post
Brace yourself for the moment when he starts pulling the valve covers off. Good luck.
Have you been through something similar where the valve covers needed to be pulled off with your car?
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Old 05-09-2019, 12:38 PM
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Originally Posted by aliix View Post
I would charge battery over night. Low voltage Can cause those faults.
Since the day I got my car, any time my car isn't being driven it is on the battery charger. In fact, here is a photo of the charger that I use - perhaps the easiest-to-use charger ever.



Originally Posted by aliix View Post
Your scanner might not show low voltage.
Agreed. All I have is the ScanGauge 2.

Originally Posted by aliix View Post
Has it been sitting for the week you were washing and keeping doors and hatch open? Do you use the trickle charger? Just a thought.
During the week I was detailing it, I would have to move the car from here to there (around my shop) by starting it up and moving it around, but at the end of each day the car was always parked back inside the CarCapsule and on the charger.


Thanks for helping me to think of some things that may have caused the CEL.

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Old 05-09-2019, 12:40 PM
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Originally Posted by Jaylars View Post
The oil pressure at idle is high. Why?
Good question. I don't know. Furthermore, I don't know or don't remember what my car's normal oil pressure is just after start up. Can anyone comment or share theirs?
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Old 05-09-2019, 12:52 PM
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Originally Posted by Bill Dearborn View Post
Can your local scan tool read the freeze frame files? If it can then you may be able to see where the misfire is coming from. When I had my high speed missfire issue the History file would show cylinder number 8 misfired 64 times when a P0300 code was set. I didn't get a P0308 code until there were a lot more misfires.

Bill
I would not consider my scan tool to be professional grade by any means. It's just the only thing I had (purchased a few years ago for a different vehicle) that I thought might be able to provide me with a code to investigate. I was hoping for a more definitive code such as P030X but my scan tool is either not that great or there haven't been enough misfires to generate that specific 0P030X code. What scan tool do you use?

It looks like next week when I'll have the ability (time) to get it in my trailer and delivered to my dealer for some diagnostics.

Do you guys know if there are any scan tools that one can use to obtain more detailed information? I would be very embarrassed if I took it in and diagnostics revealed a bad spark plug. But without better information than I currently have, I really don't want to start changing things and potentially making things worse. Know what I mean?

I'm really looking forward to the educational journey and promise to share the info with the Corvette Forum.

If you guys have any other tips or suggestions please keep them coming.

Thanks.
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Old 05-10-2019, 12:00 PM
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Originally Posted by CADbrian View Post
Good question. I don't know. Furthermore, I don't know or don't remember what my car's normal oil pressure is just after start up. Can anyone comment or share theirs?
Typically around 27psi. You might want to get that checked out asap.
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Old 05-10-2019, 03:49 PM
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Originally Posted by Jaylars View Post
Typically around 27psi. You might want to get that checked out asap.
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Old 05-10-2019, 04:05 PM
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Yesterday a local GM dealership sent over a technician with a "midi" and, in addition to the P0300 code, pulled the following information:
P26C5 - Exhaust Flow Control Valve (I pulled fuse 42 a long time ago - while I'm not a GM technician I don't think my misfire / rough idle issue is stemming from fuse 42 being absent)
P050D - Rough Idle Cold Start

Once the technician got back to his dealership, the service manager sent me the following message:
Whatever dealer you take your vehicle to have them look at technical service bulletin PIP5498L, DTC,s P050D and P0300 with rough idle on cold start, possible injector issue.

It looks like next week is when I'll have the ability (time) to get it in my trailer and delivered to my dealer for some diagnostics.

Stay tuned...
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Old 05-10-2019, 04:33 PM
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I think the key phrase in the post above is "possible injector issue".

If it does indeed turn out to be a bad injector, is this repair easy and routine for a dealership? Or scary (like I should be concerned about my young engine)?

Thanks.
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Old 05-12-2019, 02:07 PM
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Originally Posted by Jaylars View Post
The oil pressure at idle is high. Why?
After reading the message from Jaylars, I sent the following email to my GM dealership's service manager:
"Someone on the Corvette Forum looked at the images below and inferred that the oil pressure was high. Stating that his is about 27, whereas mine is just above 50. Is my fellow Corvette Forum member correct? Is my oil pressure (upon startup) high? Something else to look at next week?"

This was his response:
"Nope I assure you cold start oil pressure you would expect to see 40 to 50 psi on a healthy new engine. As the engine warms up the oil will thin and the pressure at idle will drop. If you were seeing pressure in the 25 psi range on a cold start that may present a problem with low oil pressure after the engine heats up. In other words your oil pressure indicates a very healthy engine and no reason for concern."

I share this with the CF, not to start an argument about correct PSI (because I am not an expert on motors), but instead to share a GM service manager's opinion (and reduce one's subsequent concern level) that if your relatively young and all stock LT1 motor has an oil pressure PSI reading that looks like mine upon startup, your motor might be okay.

Jaylars, does your oil pressure go high (like mine) at startup and then come down as the motor warms up? Let us know your experience. Thanks.
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Old 05-12-2019, 08:28 PM
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OK, I got it. Initial cold start psi. I was thinking standard idle @ operating temp. I failed to look at your coolant temps and put 2+2 together.

Interestingly though, I never had over 50 psi at cold idle. Mine is typically under 40psi till rpm is stable at idle and then just under 30 psi when @ operating temp and idle. Makes me wonder now, lol.

Anyone else want to add to this?
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