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Misfire P0300 *Possibly Resolved*

sennister

Full Member
Joined
Jan 26, 2005
Messages
3,082
Location
Grant, MN (aka Stillwater, MN)
Hello,

I was driving my truck down the highway today and after I punched it to accelerate around a car to take my exit I noticed that my check engine light was flashing. Knowing that the flashing check engine light means stop and since I was taking that exit anyhow I exited and turned off my radio so I could hear how the truck was running. Everything sounded fine and the light went out. I kept driving to the store that I was going to and on the way home I punched it again to see what happens. Again radio off from 55 to 70+ and the check engine light starts to flash. I got off the gas and it flashed for 5 sec and went out. The DIC was displaying no message. When I got home I connected my scan tool to see what what was stored. Under pending codes it reported P0300  Missfire Multiple Cylinders. I then started the truck and when though the computer to look at the data that has been logged. Here is what I found:

Cylinder          Misfire Count
1                    0
2                    98
3                    0
4                    0
5                    16
6                    350
7                    0
8                    2

I have done some searches and found several postings of P0300 but most seem related to an aftermarket alarm or happen at idle. My condition only happens at WOT at higher RPMs (about 5000 RPM).

History of vehicle that I know of.

I bought it used in Jan. It has 62xxx miles. 2003 Z71. No Aftermarket Alarm. Everything stock except the K&N Filter. I did just do my first oil change on the truck and used Mobile 1 5w-30. I don't know what it had before but I would think dino oil. This oil change was a little over 3 weeks ago but this oil change was done right before I left for Japan. So the truck has been sitting in my driveway for the most of that time. I did tell my wife to drive it every now and then. I think she drove it about 3 times when I was gone on short trips to the store and what not. Also she drove it to work the day I came back so she could pick me and all my luggage up in the truck that has more room than the Audi and she reported no issues. This didn't surprise me as it only seems to happen at WOT. It currently has 1/4 tank of fuel.

I was thinking that it might be fuel as it has been sitting. But I would think it would take longer than 3 weeks for gas to go bad. I also read that contaminated fuel can cause this code but the primary cylinder that will have a missfire is Cylinder 7 and then 2 and 8. It was explained that these cylinders suffer from this because of how the fuel rail works. I do show misfires on 2 and 8 but 6 is my biggest offender not 7. So I'm not sure this is my problem. Another mention was injectors. I'm not sure about that though. As the light goes out I might keep an eye on it and see what happens after I refuel. I'm not going to clear the codes just yet in the event that I'm off to the dealer. It's a good thing that I insisted that the dealer included a drivetrain warranty because of the miles on the truck.

Thanks for any help you can provide.
 
There can be a lot of different reasons for a misfiring cylinder. Plugs, wires, coil, valve, etc. Different things you can do to diagnose it if wanting to do it yourself. Rotate spark plugs, wires, coils, injectors, NOT all at the same time. This checking to see if the misfire "follows" any of these parts. I haven't personally seen it yet, but I've also heard that the intake gaskets on these trucks are prone to leaking. You can check this by spraying carb cleaner around the intake gaskets while "listening" to the RPMS. Lastly, which can be a LOT harder, as I don't think there's a vaccuum port on our intakes, is to hook up a vaccuum gauge. While driving it, get it to misfire, and watch the gauge. If the needle bounces while its misfiring, indicates a mechanical valvetrain issue. I recently had this happen on an old bodystyle 1999 5.7 Suburban, and ended up pulling the heads off for a valve job. Valves were sticking.

I'd bet that the intake gaskets are leaking on yours. (Guy that told me about those, is a new co-worker that came from a Chevy dealership).
 
I'm not sure what it is. I'll try some of these steps. I do have an extended warranty on the truck so while I would like to diagnose it I'm not sure I will be fixing it. I just don't have the time to do it as I'm out of town alot with work.

It's just strange that it only happens at WOT with RPM high. So it could be a vacuum leak or intake leak like you mentioned. I read through alot of TSBs and didn't see anything that really met all my symptoms.

I'll take it easy until I get into my next tank of gas and see if the condition can be rproduced
 
Well I'm about 100 miles into a fresh tank of gas and it didn't flash CEL on a low speed run up to 5500 RPM. I'll have to wait until I get out on a highway to see if I can get it to come on at the 70MPH+ run where I typically see it illuminate.
 
Well no luck so far. On my highway run the truck started misfireing again.  :E:

I picked up some carb cleaner and I'm going to try the trick with spraying around the intake gaskets and listen to the RPM. Heck my OBDII cable is long enough so I can bring it under the hood and it will display the real time RPM. So I will be able to monitor it. I need to get the software installed on my computer so I can log the data on a highway run. My scanner is capable of logging 90 sec worth of data and then graphs it out with the software. So that might help as well.
 
Take your pick...

Bulletins/Campaigns/Preliminary Information

  1. #PIP3232A 1999-2005 GEN III Cold Engine Misfire at Idle with Possible P0171 and/or P0174 - keywords 4.8 5.3 6.0 aire cold crack hestiate idle intermittent internal intake L59 LR4 LM4 LM7 LQ4 LQ9 P0300 - (Mar 8, 2005)

  2. #PIP3056 97-05 Vehicles with 4.8 5.3 5.7 and 6.0 Gen III V8 Engines and Misfires on One Bank - kw CEL diagnostics driveability DTC P0171 P0172 P0174 P0175 P0300 P0301 P0302 P0303 P0304 P0305 P0306 P0307 P0308 - (Jul 12, 2004)

  3. #PIP3055 99-05 Full Size Trucks, Utilities and Vans with 4.8 5.3 6.0 V8 Engines That Have a P0300 Due to Fuel Contamination Cylinder 7 Misfire - kw CEL code contaminated DTC fuel gas L59 LM7 LR4 LQ4 LQ9 miss - (Jul 8, 2004)

  4. #02-06-05-004A Info - Misfire DTCs P0300, P1380, P1381 and Catalytic Converter Damage Due to Installation of Alarm Systems - (Jan 9, 2004)

 
Sperry,

I don't think it's any of these. I have looked at all of them in the engine section except #2. I can't seem to find the full version of that one. 

1. No problems when engine is cold. Happens only at highway speeds when running at 5000+ RPM and engine at normal operating temp. I don't know if it does it when cold at these RPMs as I wouldn't push a cold engine that hard.

2. I happens in more than one bank. (Unless this fault is on both banks it may not be the issue. Do you have more info on this one?)

3. This one really seems to stress that most of the misfires would be in #7 this is explained by how the fuel rail is layed out and how the fuel is delivered. I have had no misfires in #7 and #6 and #2 are the biggest offenders.

4. I don't have an aftermarket alarm.

One that you didn't mention that may have a role but I'm not experiencing all the symptoms is:

Info - Automatic Transmission Shift, Engine Driveability Concerns or Service Engine Soon (SES) Light On as a Result of the Use of an Excessively/Over-Oiled Aftermarket, Reusable Air Filter #04-07-30-013A - (Jan 25, 2005)

It mentioned that there are drivability issues and a CEL but didn't say what the code is that it would throw. I do have a K&N Drop in filter. I installed it maybe 1000 miles ago and it was pre-oiled. How do you tell if there is too much oil on it. This is my first K&N filter. I don't really think this is it as I haven't noticed any performance issues. Infact it keeps pulling normally when the CEL comes on and the truck is reporting a missfire.

I'm almost to the point where I throw in the towel and take it in. I do have a drivetrain warranty so it should be covered.
 
They did say "Over-Oiled" but how does one tell? I would imagine the level K&N uses initially is OK. It's the sopping-wet dripping filters that is the obvious. So I would suppose that just enough oil and no more would be the safe amount..

I'll pull #2 for you and post it in the tech section, if it isn't there already! (I didn't check! :2:) And post a link..

 
sperry said:
They did say "Over-Oiled" but how does one tell? I would imagine the level K&N uses initially is OK. It's the sopping-wet dripping filters that is the obvious. So I would suppose that just enough oil and no more would be the safe amount..

I'll pull #2 for you and post it in the tech section, if it isn't there already! (I didn't check! :2:) And post a link..

This was my thought exacly. What is over oiled??? It's not very clear. Maybe I should post a question in the intake mods section. There are plenty of people running the K&N filters. I would also think that the initial oil applied by the factory would be correct. They would be wasting money by over oiling them. Again I don't really think this is the cause of my problem. It is just one of the known issues that I came across that could be the cause. I still have my OEM paper filter that I could drop in to see if there is any change.

Thanks for the post of #2. I didn't think I had seen that one.

Also in the latest TechLink (May 05) that you posted the 5th item states a problem with the Upper Manifold Gasket Leak it would cause a P0300 to set. This is where I really think the problem is. This would make the most sence as far as what I'm seeing. The vacuume created at high RPM could cause enough of a leak to cause my issue. It states that this applies to the 5.3L L59 (E85 capable), as well as the other Gen 3 and Gen 4 engines. I know I don't have the L59 but I would think we have either the Gen 3 or Gen 4 engines. I'm not sure what years were Gen 3 & Gen 4.

As a side note does the 03 AV use a fuel pressure regulator. If so is it for the entire system or does each bank have it's own? I know they are totally different powerplants but the 4.3l has a fuel pressure regulator that is known to fail and cause a random missfire. I know this as my Blazer had this problem. If I remember correctly this caused the engine to run too rich. I'm not seeing a too rich condition in any of my OBDII values so I'm thinking it's not this.
 
Perhaps it's the injectors just affected from some bad fuel? My old POS Ferd chugs rich once in a while, then after some injector cleaner it clears up.

I would align with the manifold gasket service document too, I do remember that I will need to edit it in a couple to 3 weeks as the link said "current month". (cnt_mo)

Look in the Engine exterior misc in the Technical Information Section, the R&R may point that part out.

And, swap around the coils (I never checked if you can, but seems likely). See if they are arcing over and the fault follows them, when you sub them to another spot.

Maybe even swap the injectors??
 
Here's some info, and yes, there is a regulator..

"The fuel pump is mounted in the fuel sender assembly reservoir. The fuel pump is an electric high pressure pump. Fuel is pumped to the fuel rail at a specified flow and pressure. Excess fuel from the fuel rail assembly returns to the fuel tank through the fuel return pipe. The fuel pump delivers a constant flow of fuel to the engine during low fuel conditions and aggressive vehicle maneuvers. The powertrain control module (PCM) controls the electric fuel pump operation through a fuel pump relay. The fuel pump flex pipe acts to dampen the fuel pulses and noise generated by the fuel pump.

The fuel feed pipe carries fuel from the fuel tank to the fuel rail assembly. The fuel return pipe carries fuel from the fuel rail assembly back to the fuel tank."

I'll see if I can find the entire document, of if I already posted it! :2:

---------------

P #7600
 
OK, Here it is..

Fuel Systems Info - 101

Down the document a short ways...

Maybe something in there will give you a better understanding and and perhaps an idea that may assist in the diagnosis.

HTH!
 
Well I had to fly to Louisville on a business trip so I dropped off the AV at the dealer and had them take a look at this as I am covered under a Drive Train Extended Warranty. I called and made an appt for today and had dropped the truck off over the weekend. I had to pull the Blazer back out and drive that. Wow I got spoiled in the AV. Anyhow, the dealer called and left me a voicemail to call them when I was on the flight to Louisville. Once I got settled here I gave them a ring and here is what they said...

1. Misfire has been caused by a blocked Cat. He stated that even though the truck is over 36,000 miles (62,xxx miles) this will be coved under the 8 year what ever mile warranty I think he said 80,000 miles. Anyhow what ever the year/mileage warranty on emissions is. I had printed out a laundry list of TSBs and detailed descriptions as to what it was doing to help the mechanic diag this quickly. I also listed some of the more common TSBs that refered to a P0300 code and why they didn't apply in this situation to help eliminate a few possibilities.

2. The Intermediate Steering Shaft needed to be replaced. I know I have read about this in the past and that there was a redesign that took place to resolve the issue in the future so I told them to replace it. That rund $150.

3. The two lic plate lights are out... Hmmm I never checked them but ok. They are going to replace the bulbs and see what that does.

4. They wanted me to agree to doing the flush of the diffs and transmission. I said no thanks I'll take care of that.

I guess we will see if this resolved my misfire condition. I can see how a cloged cat would cause this. I have read about several people reporting this problem with Cats. I wonder why there are so many problems with them?

 
Well I picked up my truck last night and like others have noted the truck has alot more power. I'm wondering if my MPG will increase? It's not like it's been bad it's just that with a plugged Cat it had to hurt.

As far as the intermediate steering shaft. It must have been bad when I bought the truck. I just figured it was normal as after all this is a truck. Now the steering is silky smoothe.

The Cat was covered under warranty. It doesn't list a price on the paperwork but the Service Writer mentioned that the $600 Cat replacement was covered under the emissions warranty. The intermediate steering shaft and the repair of license plate lights came to $158.75 including labor and taxes so that wasn't too bad. Although I don't know what the steering shaft runs. Either way I don't know when I would have time to take care of it so I just decided to let them do it.

Now time for Dueler Revos  >:D I might go get them today. If I don't get that done today then it will be next week before I get it done.
 
Hmmmm, the plugged cat's are usually caused by the misfire.. `Must be a loop where one causes the other - causes the one..

Here is some info. About 1/3'd way down where the oxygen sensor info starts.
 
Both MainOne and I had the same exact symptoms and it ended up to be clogged cats. On mine it was the passenger side and I don't remember on MainOne's....I htink others have had this issue also.....
By the way we also both had '03's coincidence or a known problem by GM  ???
 
sperry said:
Hmmmm, the plugged cat's are usually caused by the misfire.. `Must be a loop where one causes the other - causes the one..

Here is some info. About 1/3'd way down where the oxygen sensor info starts.

Yeah, that's what I thought. If the O2 sensors are not reading right then it could run too rich and kill the cat. I'm not sure. Maybe it wouldn't hurt to swap them.
 
wow, took a long time to read.. but damn good information. Sperry.. you never cease to amaze me.

Keep up the good work guys  (y)
 
TNAV said:
Both MainOne and I had the same exact symptoms and it ended up to be clogged cats. On mine it was the passenger side and I don't remember on MainOne's....I htink others have had this issue also.....
By the way we also both had '03's coincidence or a known problem by GM  ???

I plan on crawling under the truck to see which one is new. I haven't done so yet and the paperwork didn't say. All I know is that it smells funny. I'm sure it's just the cat getting broke in or maybe they have something on the pipe that is burning off. It kind of smelled like burning oil at first but the smell is going away.
 
How can you tell when you cats go bad?? I just started with the noise at hard acceleration.? Code was P0303, Cylinder 3 misfire.? Switched the wires, plugs, and coils one at a time with cylinders 3 and 5 and nothing changed.? Was going to dive into the injector tomorrow but wondered how to find out if the cat is bad....
I have an 02 with 62,000 miles.  I am the original owner.  New fuel filter.  No other problems since I bought the truck.
 
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