P0300 - where to start

psa55

Well-Known Member
#1
I've read lots on here about the p0300 error. Since the error code, I have driven 50km. I have 50km more to drive home. Hopefully it will give me a more specific error code with cylinder.

I replaced the spark plugs at 160 000km as recommended with the proper AC delcos. Truck has 243 000km on it now. I have never changed a coil, or any sensors.

My question is where to start? Is there a diagram of the vacuum system somewhere I could use to help me trace those lines? From there, what order next?

Lastly, what would you consider routine maintenance here... should I consider changing the CPAS regardless. What about the Camshaft Position sensor(CPS), or the Crankshaft Position Sensor (CKP)?

I know I can't drive it like this for long, without risk of a CAT failure and would prefer to get the proper issue addressed and necessary maintenance done in due time.

Thanks!
 
#2
I'd get a Bluetooth OBD adapter with the Torque app to read live data to see things like misfires and fuel trims. There are many things that can cause misfires but need to get some data to point us in the right direction.
 
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psa55

psa55

Well-Known Member
#3
I've got a bluetooth adapter and used it to read the code. I'll try to monitor it on the way home to see if I can get more specific data.
 
#4
Check the following:
Misfires of all cylinders, Long term and short term fuel trims, engine temperature, vacuum (MAP).
 
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psa55

psa55

Well-Known Member
#5
I ran the truck a few times and couldn't get it to set a more specific code. Everything seemed to be running as normal, smooth idle, etc. Check engine light would not flash. I didn't mean to, but when looking for codes I cleared the P0300. I've run since then 4-5 days all over the city and the code has not returned. Most trips have been logged in Torque. I've emailed a few different logs to myself as .csv files that I can view in a Spreadsheet. I get most of the data: GPS, Bearing, Altitude, etc... but there is a G: x,y,z, calibrated. Not sure what that represents.

Should I be getting other data?

I can upload it here if that would help. I'm open to suggestions? If I've got an issue here, I'd like to get it solved. Thanks!
 

MRRSM

Lifetime VIP Supporter
#6
P0300 Code by nature implies random misfires that can have causes almost too numerous to list:

http://www.obd2-code.com/2017/08/p0300-multiple-cylinder-misfire.html

First... Do NOT Clear any Codes. If your Scanner is capable... try to view the Historical Misfire Counts and correlate those to Historical Long Term and Short Term Fuel Trims. Even though the P0300 is so general as to confuse the diagnosis... narrowing down other related Spark and Fuel/Air Ratio issues that point to a more specific place to look will refine your chances of narrowing down the problem. But don't get "Lost in The Numbers" by trying to commit all of this to memory.

Make a small Grid Chart and Number a Top Down List with All Six Cylinders... 1-6 and jot down the "Bean Counts" for any out of range counts for these issues. Check all prior codes and then search Google for any combinations of these issues to see what they indicate as clusters or individual codes, When you finish ...your penciled Graph will help you figure out the next moves... and when you find those other possible places to work on... search GMTN for "The Threads That Help". Good Automotive Diagnostics blend the OBD2 Side with the Observation and Wrench Turning side under the hood to get things done.
 
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psa55

psa55

Well-Known Member
#8
I've logged a lot of data the past couple weeks and I've watched a bunch of videos on how to best use and learn about the Torque App. Way more there then I realised. So, I snapped screenshots this afternoon of my issue.... Misfire History of Cylinder 6 and Cylinder 4 tell a pretty bleak story. Current misfires on both those cylinders as well... so when I discovered this, I was on my way to work... came out tonight, started the truck and it popped the P0300 again after I cleared it a couple weeks ago. Current misfires were on Cylinder 4 that popped the code.

I can swap Coil Packs around and see what happens, but I'm thinking there have been misfires on all cylinders and they are probably due at 245 000 kms on them.

Go with: ACDELCO D1935E for coil packs? If I swap em around to identify if it is just a couple coil packs, I could go with the Denso's but man, they are pricey.
Spark Plugs (ACDELCO 41-103)were done at 160 000km, so only half of the life is used... should I bite the bullet and replace them too?

I plan to remove and clean the Throttle Body and MAF at the same time, and the CPAS.
Thoughts?
 

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#9
It's wise to swap #4 coil to a different hole and see if the miss follows. Can you post the freeze frame data and fuel trims at idle and under a load?
 

MRRSM

Lifetime VIP Supporter
#10
With just over a Decade of Wear and Tear on your 2008 Vehicle and 245,000 Km behind it... your Misfire woes might have another Cause and Origin in the form of Clogged Fuel Injectors. You could search High and Low for a deal on separate EFIs... a separate EFI Harness and a separate Fuel Pressure Regulator and NEVER find a Better Deal than this one for around $100.00 for the entire set on Amazon.com:

https://www.amazon.com/dp/B072BZGRNW/?tag=gmtnation-20


DELPHIOEMEFIRAILS.jpg


Likewise ...you could also search just as far and wide and be hard pressed to find anyone that has had as much hands-on experience with working on these Multech II EFIs as yours truly...and that includes their disassembly, inspection and Complete Off The Vehicle Cleaning and Pressure and Balance Testing... right down to the finest of details. I have seen first hand what these identical EFI Fuel Filter Baskets look like up close and personal on High Mileage sets of EFIs that resemble unclean toilets and these are the images and videos I used to document this realty which can be seen at this link to my Photobucket:

http://s557.photobucket.com/user/60dgrzbelow0/library/HOMEMADEEFICLEANER?sort=3&page=1

DSC03142.jpg

DSC03143.jpg

DSC03144.jpg

....so keep them in mind if you simply cannot pin down any definite problem with the Electronic Circuitry, Spark Plugs or Connector Wiring during you continued investigation.
 
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psa55

psa55

Well-Known Member
#11
@MRRSM That sounds really plausible. Thanks for the detail and pics. I got some info tonight out running kids around. I set up displays for cylinder #4 and #6 for current misfires... When the code popped the first time a few weeks ago, and tonight, it set right at ignition. Tonight with the dials up, I got 2 misfires on #6 soon as I started the truck. Then soon as I pulled away they disappeared. Everything seemed normal, until I let me foot off the gas to coast up to a red light... couple more misfires shown on #6. Even got a couple while idling at another light.

Seems like my issues on #6 might be related to fuel delivery.. If that is the case, is there something else I can use to diagnose. I might have to start by swapping coil packs around to see what happens.

Might be different issues at play here with each cylinder.
 
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psa55

psa55

Well-Known Member
#12
@Tdub What display would give me fuel trim data in Torque, I'll find it and screenshot it at idle and under load. And I'll swap #4 coil around on the weekend.

Trying to look for Step-by-Step on fuel injector swap on here as well, but I haven't found it yet... just that it appears the intake manifold would have to come off.
 
#13
Fuel injectors are the last thing to look at since they are not that easy to replace, requiring the removal of the intake manifold. It can happen that an injector goes bad or gets clogged but this should be confirmed with an injector balance test, which is only available using a high end scanner or Tech 2. Coils are the most common, vacuum leaks are also a possibility, bad compression is also possible. These you should look at before the injectors.

For coils, stick with ACDelco or Delphi, which are the same. I've had a bad experience with Standard. I've even gone with used ones from junkers.
 
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psa55

psa55

Well-Known Member
#14
Not coils or Plugs... Swapped em around tonight, issues stayed on #4 and 6. Tried a new plug in #6, but no change. Torque keeps picking up the misfires, but I don't feel anything while driving or idling.

Gonna check around the intake manifold for vacuum leaks. Will have to get some carb cleaner as per Eric the Car Guy and will check the bolts on the manifold as per this video.

 
#15
Did you check the compression just to eliminate that? After that, all that's left would be the injectors. Did you check the fuel pressure?
 

MRRSM

Lifetime VIP Supporter
#16
+1-^-... and while the subject of compression testing is fresh in your mind... consider that in such High Mileage LL8 Engines.. over the long term, the Carbon Buildup has been forming thick, semi-permanent deposits of this crap around the Valve Seats, inside of the Upper Combustion Chambers and on the Piston Tops that will be more substantial than you can imagine.

The build up and presence of this stuff is an additional mass of material that tends to Glow Red-Hot at higher RPM, Raise Compression and Alter "Quench" levels enough to Cause Pre-Ignition problems in engines with compression ratios greater than 9:1. The misinformation about why the use of High Test Gasoline is better at preventing this problem in higher compression engines is NOT that it "explodes with more energy"... but rather that it burns more slowly at higher compression levels using Combustion Modifiers added to prevent 'Engine Knock' pre-ignition problems. Low Grade gasoline is MORE likely to cause these pre-ignition events.

There are some excellent contemporary threads that discuss using the Liquid and Foam versions of ACDelco Top Engine Cleaner (TEC) here at GMTN that will show you the best way to accomplish carbon Clean-Out using this stuff to dissolve and get rid of this Carbon Build-Up problem. This is the most recent one that will help with one of the very few effective methods available to get this job done. If you organize your Compression Test along with using the ACDelco TEC... you would be able to Kill Two Birds... with One Stone.

https://gmtnation.com/forums/threads/baked-on-oil-plugs-5-and-6.18750/
 
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psa55

psa55

Well-Known Member
#17
Hard to find time to keep at this, but I'm not giving up. My buddy dropped off a MAC tools engine compression tester this morning. I ran compression tests tonight, and I think it looks alright. Cylinder 5 seems a little weak, but not too bad. Results pic attached.

I did tighten up the intake manifold bolts and many spun easier than I think they should have. I tried everything, but couldn't get to the very back bolt. Couldn't even get my hand there to try and get box wrenches or any combination of sockets on it. After tightening, I tried carb cleaner around the intake manifold but couldn't find any spot that had a leak noted by change in engine sound. I am going to try this again.

First time doing most of these tests, and I'm learning a lot in the process. I did try to find fuel pressure info in torque, but all if could find was fuel tank pressure. The gauge I set up for fuel tank pressue never reads.

If my next vacuum test comes up with nothing, am I down to injectors? If so, I see some garages offer a Fuel Injector Cleaner service where they state they hook up some devices and do a full fuel system cleaning. Would this be worth my while?

I mean, if it's just Seafoam in my gas or another product, I am sure I can handle that myself. And I'm willing to give that a try as well.

Once I get through this, I still need to clean my Throttle Body, and CPAS, and I will complete the Top Engine Clean as recommended by @MRRSM. Wish I was dealing with this in July or August instead of all the family craziness of Sept. and Oct.
 

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#18
There are some hard core injector cleaners that you connect a pressurized can straight to the fuel rail and you disable the fuel pump until the can is empty. I've seen it work wonders under certain situations. Also cleans up some of the carbon
 

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