Rough Idle, Misfire, Help Diagnosing Codes

FishinDVM · June 1, 2019

Hey guys, 2014 5.3 Sierra AT with 85k on it. Codes on it are as follows:

- p050d, p0451 - These have been on and off for about 2 or 3 weeks now. Not always there, when cleared they came back but only after a day or two, certainly not every drive. I didnt actually notice a "rough idle" until this past week though. Again, not all the time, but a few times.

- Stabilitrak service - Came on 2 weeks ago, never repeated after cleared, so far at least.

- p0300, p0308 - These just happened. Random misfire 2 days ago. Cylinder 8 today. When these happened, the CEL light flashed and I also got a U0401. I noticed that while these codes were active, the truck would downshift hard. Basically, I'd feel an extra 'thud' when applying the brake gently and slowing from like 20mph to 5mph, it felt the the transmission dropped into a low gear too soon. That's just how it FELT to me anyways. But while these codes were active, it would repeat every time I was slowing down at low speed I'd get that 'thud'... every time... I just cleared it now, drove around, not happening once the CEL light is off, driving fine at least for this current second.

- I assume I'm going through oil faster too... since the "engine oil low add oil" light came on for the first time ever about 3 weeks ago... and I'm at about 39% oil life according to my computer (which is accurate, I added 8 quarts myself in mid February).

Now, I do oil changes when my computer gets down to around 25%, 8 quarts full synthetic mobil 1. I have cleaned MAF, throttle body, new battery, new oil/filter, new air filter, all back in February. Here are my questions:

1) Part of me wants to replace all the spark plugs and coil packs and see what happens. The other part of me wants to take it to the dealership in case it is injector/AFM/lifter related. Thoughts on the possible causes?

2) I assume I am still on powertrain warranty? I bought my 2014 in SEPTEMBER of 2014 NEW and I currently have 85k. So I didnt break the 100k mark and I assume the warranty starts on the DATE OF PURCHASE so I have til September? They didnt drop the warranty to 60k until the year 2016 models correct?

3) If it is a lifter, would it make sense for a p0308? My thought was that if its AFM V4 related it would be one of the others. I dont remember the numbers but I want to say cylinder 8 isnt one of them, but the p0300 random certainly could fit this?

Now 2 things I have NOT been doing. And of course in retrospect I should have done my research and figured this out long ago for preventative maintenance. I didnt shut off my AFM all this time, and... I havent done anything to decarbonize the intakes. While having a manual cleaning and shutting off the AFM are now on my list, I figure I'll have to solve this first. Thanks for the input, Chris.

Jhinkle82 · October 23, 2019

Just wondering if anyone has posted anything related to this. I’m experiencing same issues, except; I’ve replaced all 8 injectors, spark plugs, coil packs, wires, MAF Sensor, Baro Sensor, Fuel Vapor Solenoid. I’ve confirmed compression on all cylinders @ 148-150 PSI and checked for vacuum leaks. Nothing I do has changed the P0308 & U0401 codes and I’m plum out of ideas. Faulty ECM or CAN-BUS? Please somebody! Lol

2015 Sierra 1500 4x4 L83 Gen V Flex

L86 All Terrain · October 29, 2019

On 10/23/2019 at 12:12 PM, Jhinkle82 said:

Just wondering if anyone has posted anything related to this. I’m experiencing same issues, except; I’ve replaced all 8 injectors, spark plugs, coil packs, wires, MAF Sensor, Baro Sensor, Fuel Vapor Solenoid. I’ve confirmed compression on all cylinders @ 148-150 PSI and checked for vacuum leaks. Nothing I do has changed the P0308 & U0401 codes and I’m plum out of ideas. Faulty ECM or CAN-BUS? Please somebody! Lol

2015 Sierra 1500 4x4 L83 Gen V Flex

P0308 is Cylinder 8 misfire. You could have got a bad plug right out of the box, bad coil or wire. I have had NFG new spark plugs on a few occasions. I would take the coil/wire from Cylinder 7 and install on Cylinder 8 and see if you get a P0307 code instead, if you do then your coil or wire is bad, if it is still P0308 then you probably have a bad plug on Cylinder 8. U0401 is a generic code related to whacky info in the ECM, so there could be a bad ECM, or just bad the P0308 code causing the 401 code to trip. Not familiar with the U0401 code, but here is some info I found on it.

Damaged, burnt, shorted, disconnected, or corroded wiring and/or connectors
Abnormal system voltages
On some applications, defective engine and driveability sensors can cause this code, or contribute to its setting. Note however that in these cases, defective sensors will be indicated by dedicated codes
One or more defective control modules. Note that control module failure is rare, and the fault must be sought elsewhere before any control module is replaced

The third bullet makes me believe it could be related to your misfire code. Check that #8 situation out.

Jhinkle82 · October 29, 2019

L86 All Terrain thanks for the input! Through all of this I have focused on the ignition portion of the code in replacing plugs (twice), wires (OEM) and Coil Packs (All Eight). To no avail, I then focused on the Fuel Delivery by replacing all 8 injectors and HP fuel piping. When nothing changed, I began looking at the compression factor of the number 8 cylinder. I first checked and verified compression at 148 psi (OEM New calls for 150 psi) I checked all cylinders for comparison @ 149-150 psi.

With all that said, I’m leaning now toward a broken lifter spring on #8 I’ve read a few posts where the spring can be broken and be silent, AND show god compression on cold turn but fail on running compression. Just a thought.....opinions?

Lsgun1 · October 30, 2019

You can always pop the valve cover on the cylinder 8 side and turn the engine over to see if the valve tries to open or not.

g2full · October 30, 2019

1, 4, 6, and 7 are the AFM cylinders.

EDIT - My truck is currently at the dealer for a cylinder 7 misfire. Maybe coincidence however with my flashing CEL and a misfire code my truck also would downshift hard. Hard to the point that I would put it neutral when coming to a stop or slowing for a hard corner on the way to the dealer.

Edited October 30, 2019 by g2full
adding text

Ozer · October 30, 2019

When i was chasing misfires i replaced all plugs and wires and eventually coils BUT, when i went to change the #7 plug the wire has corroded and actually fell off the plug so no spark was going to #7.

My hard shifting and jerking/shudder ended up being a bad Torque Converter = New Trans!

Edited October 30, 2019 by Ozer