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2007 Chevy Avalanche 5.3 LC9 with code P0300

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02 Nov 2019 15:00 #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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02 Nov 2019 15:04 #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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19 Nov 2019 05:53 #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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27 Dec 2019 20:19 #36178 by D C
With all of the odd behaviour with the coil and wire swaps, I decided to throw both at it. Swapped all of the coils (and the remaining wires) from my 2008 Suburban onto the 2007 Avalanche, and it's been running perfectly fine since.

Ordered some used coils off ebay for the Suburban, and new wires as well, and all is fine.

I can see ONE coil or wire being bad, but not multiple. Very strange.

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