Rough Idle, Misfire, Help diagnosing codes

[FishinDVM]

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.

[Paintor]

Fill it with oil. Drive for a couple days so all cleared codes come back.

 

Take to Dealer and request to have oil change and fix all of the above.

[jay webb]

ok the P050D is related to injectors so is the P0300 and other misfires. the P050D is cold start misfire. the dealer will need to do an injector balance test to see what injectors failed. and replace as needed. 

 

the p0451 is not related to the misfires, it has to do with the fuel tank pressure sensor, typically the sensor goes bad  its on top of the fuel pump , tank has to be dropped to test/replace

 

oil usage... you can ask for a oil consumption test. basically they will do an oil change, have you come back ever 500/1000 miles and check level and top off and record how much to top off. if it meets a certain limit then might need engine repair. 

 

Engines require a period of time to BREAK IN so that moving parts are properly seated. Therefore, oil economy should not be tested until the vehicle has accumulated at least 6400 km (4000 mi). An exception would be allowed only if an engine is reported to be using more than 0.946 liter (1 qt) in 1600 km (1000 mi).

Verify that the engine has no external leaks. Repair as necessary.

Verify that the engine is at normal operating temperature (see Owner's Manual or Service Manual).

Park the vehicle on a level surface.

Wait at least 15 minutes, after the engine is shut off, before checking the oil level to make sure that the oil has had time to drain back into the crankcase.

Verify that the oil level is at, but not above, the full mark on the dipstick, and that the proper viscosity and quality oil are being used as recommended in the Owner's Manual.

Record the vehicle mileage, date, and exact oil level on the form included in this bulletin.

Ask the customer to verify the oil level, each time the vehicle is fueled, following steps 1–6 and return the vehicle to the dealership if the oil level is found at or below the add mark, 0.946 liter (1 qt) low. If the oil level remains above the add mark, the customer should continue to operate the vehicle and verify the engine oil level until 3200 km (2000 mi) has accumulated before returning to the dealership for a final evaluation.

If the final evaluation shows that the engine uses more than 0.946 liter (1 qt) in 3200 km (2000 mi), follow the published symptom diagnostics as described in the appropriate Service Manual. If the oil consumption test shows that the engine uses less than 0.946 liter (1 qt) in 3200 km (2000 mi), explain to the customer that their engine meets the guidelines for oil consumption.

 

the U code is a lost of communication code. that can effect alot of things . that would have to be diag by the dealer.