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2007 Chevy Avalanche 5.3 LC9 with code P0300

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21 Sep 2019 15:21 #33746 by D C
I have a 2007 Chevy Avalanche with the flex fuel, AFM 5.3, LC9, VIN3 that has been having issues for the past month or so and is throwing a P0300.
Most of the time, when it's running rough it's storing the code as pending, and not setting the MIL.
Frequently, the Traction/Stability warning comes on, which makes sense when the ECM loses it's ability to precisely control torque.

Symptoms:
Rough Idle - occasionally
Hesitation when accelerating from a stop, or when driving at speed and accelerating
Severe hesitating and stumbling when doing a WOT (i.e. accelerating onto the freeway)

Data:
With a "cold" start this morning, the data (Torque-Pro & a cheap bluetooth scan widget) shows misfires on multiple cylinders.
Cylinder 1 is the worst, followed by 6, then 4, 3, and 7 to a much lesser extent. Yes, it's MOSTLY on the AFM cylinders, but not entirely. Both banks, too.
After about 2 minutes, as the idle speed drops (and the engine load, and MAF, and throttle and everything else that goes with it) the misfires quiet down, but pop up randomly among several cylinders.


WOT
When doing a WOT from a standstill with the engine warm, the hesitation is severe (as is the misfiring)
Cylinder 1 is the worst, followed closely by 4. 6 starts misfiring toward the end of the event. The others are pretty quiet.
Doing a WOT from a standstill onto the freeway, I only see misfires in 1 & 4. Oddly enough, once I let off on the throttle, 3 misfires briefly.

Oil pressure seems ok throughout, around 50 psi.

What I have done so far:
Upon looking underhood, I noticed that the port into the intake from the right valve cover was disconnected. Cleaned & re-connected with no change
Swapped the MAF with my '08 Suburban, also with the same engine (the Suburban engine is out for a rebuild thanks to a wiped cam lobe on #5 AND a collapsed lifter on #6. Oh what fun these engines are)
Fuel cleaner (Techron)

The randomness of the misfire makes me think it's not a collapsed lifter, though I'll confirm with a compression test. Odds seem low that multiple lifters would collapse at the same time.
That it's primarily on the AFM cylinders makes me suspect the AFM crap in the valley cover.
That it's primarily under load and not at idle has me a bit stumped.
The randomness makes me think it's not a plug, plug wire, or coil.

Thoughts? I have loads of data to comb through and answer questions with, too.

Dave

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  • Chad
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21 Sep 2019 18:55 #33750 by Chad
What is you fuel/alcohol composition? How are your fuel trims?

"Knowledge is a weapon. Arm yourself, well, before going to do battle."
"Understanding a question is half an answer."

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21 Sep 2019 19:32 #33752 by D C
Ethanol: 12.9%
Here is the idling graph with short term fuel trims on it


And with long term



Over a longer drive with steady state and WOTs, I get these:
Short Term
Bank 1
Avg -0.620361781
Min -15.625
Max 13.28125

Bank2
Avg -0.54245552
Min -19.53125
Max 11.71875

Long Term
Bank 1
Avg -3.901260265
Min -15.625
Max 0

Bank 2
Avg -4.738110173
Min -18.75
Max 0

I forgot to mention that I checked fuel pressure, too. ~61 psi at idle. Blipping the throttle just causes a slight shake to the needle. Running sustained at ~3000 rpm is no change. All while parked.

Idling while in drive adds enough load that it misfires much more frequently than unloaded, as well.

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22 Sep 2019 09:35 #33755 by Tyler
The miss when cold, plus the high fuel trims, makes me think of leaking intake manifold gaskets. It's not as common on the LC9 as it was with the LM7 and L59, but still happens. The good ol' ScannerDanner water test will work perfectly if you test it at idle on a cold startup. ;)

However, that doesn't explain the hesitation and misfiring while driving. :unsure: Because you're seeing miss counts on #1 and #4, you may be looking at damaged spark plugs from oil consumption (AFM cylinders). #1 is right out in the open, so I'd suggest pulling it for evidence of oil or ceramic damage.

I really don't think you're looking at AFM problem just yet. A completely stuck lifter/wiped cam would likely be a dead miss, as you know. I've seen failing lifters stick and unstick, but you can always catch them with a relative compression test when they stick.

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22 Sep 2019 14:30 #33772 by D C
Oddly enough, with 180k on the clock, this AFM truck is not an oil burner. At least not at an unusual rate like my Suburban was. Weird, I know.

Pulled the #1 plug. The plug wire came apart in the process, leaving the clip on the plug. Unfortunately, that wasn't the problem. I did cannibalize my Suburban for a plug wire though.

Here's the old plug. Has about 20k miles on it.


Swapped the plug, truck hasn't been run since yesterday. Fired it up... NO CHANGE.

#1 is still the most problematic, followed by #4, and #6 but every cylinder is misfiring at some point.

Once the coolant temp came up, the misfiring settled down from constant to occasional.

Scratching my head on this one. Off to drive my son's '98 Corvette, which I'll be posting about in the near future....

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22 Sep 2019 17:11 #33778 by Tyler
Ah, I didn't know you'd changed the plugs recently. :blush: Swing and a miss!

I'd still advise trying the water test on the intake. Here's an SD video demonstrating the idea if you're unfamiliar:



Technically it's a different engine, but the intake design is nearly identical to yours.

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22 Sep 2019 19:16 #33785 by D C
The plugs were worth a shot. It was my son's truck, and is now my future DIL's truck, so I haven't exactly been the one doing the maintenance on it. I thought the plugs had closer to 80k on them until he said something.

I looked up the water test earlier. I'll give it a shot. I was surprised to see multiple leaks in that intake in the video. I would have expected maybe one, but not several. Been through intake manifold gasket changes on the Suburban in my effort to hope away a wiped lobe. Definitely not difficult, and the water test is easy.

With so many cylinders misfiring, I was figuring it has to be a global issue. either something affecting the entire intake, or fuel.

That it had problems and the tube from the right valve cover was off did make sense... until it didn't seem to matter.

I'll do the water test next. Hopefully tomorrow after work.

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23 Sep 2019 18:55 #33818 by D C
Water test completed on a warm engine tonight. No luck. Flooded the heck out of everything, too.

Going to have my son repeat it in the morning on a cold engine.

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24 Sep 2019 11:18 #33836 by D C
Well nuts. No luck with the water test. I'll try it myself in the morning just to be sure though.

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25 Sep 2019 06:07 #33851 by D C
The truck passed the water test with flying colors. Ran quite a bit of water on it without a single hiccup.

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29 Sep 2019 09:31 #33913 by D C
Any suggestions on a next step for diagnostics?

I have a spare known-good injector that I could swap for the worst one (#1) and see if it makes any difference. Relatively low-effort and free check.

Beyond that, I'm pretty well stuck for diagnostics that seem to be within reason at all.

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29 Sep 2019 13:07 #33917 by D C
Rainy and about 52 out this morning, so I figured it'd be a good time to get another look at some cold start data.

Cylinder 1 is still the most prominent misfire followed by #8, which has been barely making a showing in the past. Then #3 is next, followed by the others.

These plots are the startup. Sitting in the driveway for a few minutes, letting it idle, then pulling away.

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30 Sep 2019 08:53 #33932 by Tyler
If you have an injector on hand, that may not be a bad idea? Alternately, if you have a fuel pressure gauge and a bidirectional scanner, you can run though the injector drop test. It'll cycle the pump and run the injectors for you, so all you have to do is watch the gauge. :cheer:

While we're on the subject, how often does this truck get refueled? Thinking about a possible water or bad gas issue. It usually happens that the water or contaminants end up in the ends of the fuel rails, like #1 and #8. Not saying you should drain your tank or anything, just thinking out loud.

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30 Sep 2019 20:34 #33952 by D C
Unfortunately, I don't have a bi-directional scanner, though I could possibly get my hands on a Tech2.

As for fueling, it's actually been rather frequent recently. Not knowing if it has a fuel sender issue, it's been kept around 1/4 tank (nobody wants to try to drop a full tank if it comes to that).

I can always swap the entire fuel rail with my Suburban, if it comes to that. But draining the rail and swapping an injector can happen this weekend pretty easily. I'll give that a go.

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05 Oct 2019 10:50 #34033 by D C
Well, I have an ever-growing list of things that it ISN'T.

I ended up just swapping the entire fuel rail and all of the injectors with my Suburban (which doesn't have the intake on yet, so it's easy).

No change. #1 is still the worst cylinder for misfires, but the next-worst seems to vary with no real pattern. It's MUCH worse under load, i.e sitting in gear instead of idling in park.

Didn't get a chance to get access to a Tech2 yet., either.


I keep thinking back to when I first popped the hood on this thing when it was running rough & misfiring, and found the hose from the right valve cover completely disconnected. But nothing changed when I put it back on. That seems strange.

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