p0300 2009 5.3 LY5 P0300 code

chapel · July 1, 2018

I started off with a search but most were for GMT800s.

Just yesterday my 2009 Silverado 5.3 started throwing P0300 codes. The only real change that's happened recently is it's gotten quite hot out compared to when I got it over the spring. It's always had a bit of a stumble at idle, but nothing serious.

I noticed today going up a steep incline under load on the highway, the MIL was flashing.

Seems like there's a lot of potential answers for this code as well:

Bad spark plugs

Bad ignition coils

Fuel delivery problems

collapsed lifters

Crank Variation Relearn.

I was wondering if there's a better way to narrow this down without simply throwing parts at it or bringing it to the shop.

Any help would be appreciated.

sefiroxx · July 1, 2018

Need a better scanner to tell you which cylinder. Then look at plug, wire or coil on they cylinder. Then injector.

Supreme Pizza · July 3, 2018

P0300 CHEVROLET - Cylinder Misfire Detected Random Cylinders Possible causes Faulty spark plug (s) Faulty ignition coil (s) Clogged or faulty fuel injector (s) Intake air leak Fuel injectors harness is open or shorted Fuel Injectors circuit poor electrical connection Ignition coils harness is open or shorted Ignition coils circuit poor electrical connection Insufficient cylinders compression Incorrect fuel pressure

Read more: https://www.autocodes.com/p0300_chevrolet.html

Tech notes The P0300 code means that a cylinder(s) is misfiring or is randomly misfiring. Start by checking for intake leaks, intake gaskets are a common caused of multi-cylinder misfiring. If no leak is found the next step is to replace the spark plugs. If the problem persists more tests needs to be done to diagnose the problem, see "Possible Causes".

When is the code detected? The control module monitors the crankshaft speed and has detected a misfire condition Possible symptoms Engine Light ON (or Service Engine Soon Warning Light) Lack/loss of power Hard start Engine hesitation P0300 CHEVROLET Description When a misfire occurs, engine speed will fluctuate. If the engine speed fluctuates enough to cause the Crankshaft Position (CKP) sensor signal to vary, the Engine Control Module (ECM) can determine that a misfire is occurring.

Read more: https://www.autocodes.com/p0300_chevrolet.html

txab · July 3, 2018

Start with intake leaks, including manifold gaskets

swathdiver · July 7, 2018

On 7/1/2018 at 3:55 PM, chapel said:

I was wondering if there's a better way to narrow this down without simply throwing parts at it or bringing it to the shop.

If you're going to keep this truck you should get a Tech2 and a subscription to AllDataDIY and or the shop manual in pdf form from SeriousCTBuyer on ebay. The Tech2 can pull up codes in history, it can test your coils, injectors, AFM system, etc. It can see just about everything electrical and electronic and interact with them too. Without it, you're just guessing.

If she threw the light while going up a grade, my guess is cats, but you need to get a good scan tool on it so there's no question as to what is wrong.

chapel · July 12, 2018

plugs 1 and 7 were oily

wires for 1 and 7 were frayed.

replaced plugs and wires.

also, noticed some ground wire and fusible link connector corrosion. Might look into getting that fixed before winter.

chapel · July 12, 2018

On 7/7/2018 at 10:57 AM, swathdiver said:

If you're going to keep this truck you should get a Tech2 and a subscription to AllDataDIY and or the shop manual in pdf form from SeriousCTBuyer on ebay. The Tech2 can pull up codes in history, it can test your coils, injectors, AFM system, etc. It can see just about everything electrical and electronic and interact with them too. Without it, you're just guessing.

What's a good price and place to get a Tech2 anyway?

swathdiver · July 12, 2018

21 minutes ago, chapel said:

What's a good price and place to get a Tech2 anyway?

Ebay sellers with good reviews, the latest software is 33.004 so make sure it has that. You can get them with and without the hard plastic case. Expect to pay anywhere from $300-350 or so delivered. It will pay for itself in short order with money saved by quickly and accurately diagnosing and even heading off problems.

chapel · July 12, 2018

thanks. are the clones OK? seems like it's difficult to determine if it's real or a clone on ebay... or amazon... or literally anyone selling one.

swathdiver · July 12, 2018

3 hours ago, chapel said:

thanks. are the clones OK? seems like it's difficult to determine if it's real or a clone on ebay... or amazon... or literally anyone selling one.

Various companies were licensed to make them over the years. The last is/was Vetronix. GM has moved on to another system. OTC, Bosch and Vetronix all have slightly different features. The Chinese factory started selling them, maybe they changed them just enough to make it legal like? Mine does not have training software nor the ability to copy PCMCIA cards but it can program and do everything else.

chapel · July 13, 2018

Ok, thanks! i'll take a look

I used to be a systems integrator for BMW, Audi, Porsche, Acura, Lamborghini and Mini focusing on diagnostics equipment installation and maintenance actually The last job I did in that field was replacing the BMW Group Tester systems with their new (at the time) ISIS system

Spencer Campbell · July 14, 2021

For those still seeing this issue, as others said, I recommend that you start with spark plugs.

As you can see in the images I provided, my 245,000 mile 2009 Suburban LT with the 5.3L VIN 3 V8 was sending a P0300 "Multiple Random Misfires" code caused by near total failure of several plugs. To the best of my knowledge, the previous owner had changed the plugs before I purchased the vehicle at 205,000 miles, meaning these plugs were only driven for ~40,000 miles.

Initially, the vehicle began to idle rough and, living in NW Iowa with dirt roads, I changed the air filter first (it was due anyhow). The rough idle was not improved. What surprised me was how fast the power loss onset was. The change wasn't gradual at all. I was driving on the highway the next day and decided to pick up some plugs. Before I could get to the auto store, the truck couldn't rev higher than about 2,000 RPM and was making very little power. I was able to take residential roads at 25 MPH to get to the auto store and bought 8 plugs.

What I saw when I got home surprised me. 1, 4, 6, and 7 were completely mucked and their electrodes eroded. Cylinder 6's plug was completely destroyed—the insulator nose had been cracked down the side and the center electrode had eroded significantly. Cylinder 1's center electrode had been eroded down to the insulator nose. Plugs in cylinders 2, 3, 5, and 8.

So why is this happening?

First, I think the plugs were already past their prime. However, there is an obvious pattern that plugs 1, 4, 6, and 7 are wrecked while 2, 3, 5, and 8 show more normal wear.

First, there is a TSB (10-06-01-008F) out due to excessive oil consumption, primarily on cylinders 1 and 7. The TSB is issued to replace the left valve cover on 07-11 5.3s to prevent excessive oil from passing through the PCV (positive crankcase ventilation) valve. I have a plan to replace the valve cover.

Second, as mentioned by others, the AFM system is often to blame. It is no coincidence that plugs 1, 4, 6, and 7 are the AFM plugs and those are the plugs most damaged in my case. Excessive oil consumption on top of normal wear and tear and extended highway driving likely had the greatest impact on these plugs. On cylinder 6, the broken insulator nose is likely due to the dramatic temperatures changes when coming up to and down from operating temperature up from and down the ambient during our -30F days we've had.

I hope this helps someone else.