Help us help you. By posting the year, make, model and engine near the beginning of your help request, followed by the symptoms (no start, high idle, misfire etc.) Along with any prevalent Diagnostic Trouble Codes, aka DTCs, other forum members will be able to help you get to a solution more quickly and easily!

2007 Chevy Avalanche 5.3 LC9 with code P0300

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4 years 6 months ago #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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4 years 6 months ago #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."

I have learned more by being wrong, than I have by being right. :-)

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4 years 6 months ago #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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4 years 6 months ago #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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4 years 6 months ago #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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4 years 6 months ago #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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4 years 6 months ago #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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4 years 6 months ago #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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4 years 6 months ago #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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4 years 6 months ago #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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4 years 5 months ago #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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4 years 5 months ago #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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4 years 5 months ago #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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4 years 5 months ago #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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4 years 5 months ago #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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4 years 5 months ago #34291 by Toddf
Any new updates? Having similar issue with my truck 06 lq4.

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4 years 5 months ago #34313 by VegasJAK
flex fuel engines are known for the system not resetting the alcohol % back when using gas... check the alcohol content of your gas. If you have a high alcohol content you will have to reset the %. If you have been refueling small amounts of gas and flex fuels this causes the problem. This will cause a random misfire...

"an open mind let's knowledge flow in and wisdom flow out for a man who has neither never listens to those who have both".
Being wrong doesn't bother me, it's being right and not understanding why that does

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4 years 4 months ago #34699 by D C
How does the alcohol % get reset? I presume I can do it with a Tech2 or a "real" scan tool?

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4 years 4 months ago #34700 by D C
No updates Todd
I've been travelling for 3 weeks, and haven't taken a look at anything.

BUT...My son's girlfriend drives it. We'd been keeping it around 1/4 tank, thinking that's better to do if the tank needed to be dropped.

We put a couple bottles of HEET in it to help with any water that might be in the fuel. And then it was filled. She said it seemed to run noticeably better when the tank is full. Interesting.

So she's been keeping the tank full, but then reported that it's been not as good recently, so she's been taking data this past week. Haven't had a chance to look at any of it though.

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4 years 4 months ago #35130 by D C
Well, weirdness continues. My son's fiance said that leaving work last week, she could hardly get the truck to go 35mph, it was misfiring so bad (naturally, my scanner was in a different vehicle at the time). BUT... once she got about a mile from work, it was like someone flipped a switch and it started running normally.

Previously, the misfire was most dominant on #1. I swapped the coil and plug wire between #1 and #3. #1 got better. #3 did not get markedly worse.

The other day, it was idling fine (parked in the warm garage), then when I went out for a drive it was misfiring BADLY on #4 and #6 (engine was warmed up). Worse than it ever has. Only under acceleration though. Driving on the freeway at 65mph, doing enough of a throttle tip-in to get a downshift, and then accelerated to 75 with the engine around 3k rpm or so.

So I swapped coils & wires. Between #3 & #4, and between #5 & #7. Drove it yesterday afternoon. Same drive on the same freeway. Wasn't misfiring much at all. Without the scanner, I wouldn't have thought anything was wrong. Still a handful of random misfires, but not enough to even trigger a pending P0300

The truck then sat outside in the ~30 degree weather for a couple hours and when I started it back up, #5 was misfiring like crazy at idle. Once it warmed up, it was just like how I drove it earlier. Hardly anything to speak of. Did throttle tip-ins for a few miles on the way home and it was pretty boring.

So there is definitely a temperature effect or cold-engine effect here. Strange that it kind-of moved with the coil swap.

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