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Old November 29th, 2008, 03:26 PM   #1
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Default chevy 5.3 & random misfire problem?

hey all got a problem with my Avalanche I cannot seem to track down.

specs:
2003 avalanche z71 4x4
5.3 V8
65,620 miles
cats were warranty swapped out a year ago.

so two weeks ago I went out and start my truck before work and it's warming up and the check engine light comes on. i plug in the scanner and it says P0300 random/multiple misfire. I clear the light and drive to work. it sits outside all day and I figure I was going to get the CEL come back on. nope. next morning I get the same thing.

not knowing what the problem is and the scanner is not isolating it to any single cylinder last weekend I changed the plugs, fuel filter, pcv, cleaned out the throttle body. truck ran great. no light in the mornings so I assumed it was all good.

yesterday I went out in the morning and started it and the CEL comes back and the scanner again says P0300 random/multiple misfire and it is idling a bit rough. clear the light and when the temp gets to around 160 (read by the scanner not the guage) it idles fine.

since this morning was cold I went out and started it up and sprayed carb cleaner all around the outside and inside of the intake figuring it was an intake gasket problem but rough idle didn't fluctuate according to the scanner it was rough but the carb cleaner at the intake didn't make a difference? I got a spray bottle with water and sprayed the wires thinking that maybe the cold start was causing a possible moisture problem and once the wires got warm and dry the problem would go away but again I could not duplicate the problem once the truck was warmed up.

my mpg hasn't changed and it runs great except for a few minutes in the morning and it's got me stumped. any ideas?

Last edited by ovrlnd; November 29th, 2008 at 03:45 PM.
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Old November 30th, 2008, 08:33 AM   #2
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I tend to wonder if you are having a condinsation issue with your ignition system? the idea that it does it only in the mornig leeds me to thing theres a cracked coil or water getting into something that has to cook it off.
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Old November 30th, 2008, 11:06 AM   #3
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I tend to wonder if you are having a condinsation issue with your ignition system? the idea that it does it only in the mornig leeds me to thing theres a cracked coil or water getting into something that has to cook it off.
I cleaned the MAF and the readings on the scanner from before and after were too close to indicate an issue there (although that would be a rich situation) but doesn't hurt to check. same issues this morning and the idle got worse then once warmed it was fine.

the scanner says that the heated O2 and O2 are working correctly. since the CEL light will sometimes flash I thought it may be an O2 sensor issue but the tool says otherwise. the MAP sensor reads 10.2 at idle and the MAF reads .79. all other systems come back fine.

My neighbor (a ford dealer mechanic) was thinking the same thing you mentioned about the condensation issue. he was the one who recommended that I spray water on the wires and coil to see if I could duplicate the issue.

again this morning I removed the intake cover and tried to see if I could get the carb cleaner on the intake. I have used 1/2 can on it over the last two days and no dice.
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Old November 30th, 2008, 11:32 AM   #4
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bad knock sensor......replaced a few of them on the 5.3 and 6.0L's
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Old November 30th, 2008, 11:35 AM   #5
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bad knock sensor......replaced a few of them on the 5.3 and 6.0L's
really? it will cause this issue? I have read that moisture can seep around the sensors and cause problems. to change those I have to pull the intake?
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Old November 30th, 2008, 11:36 AM   #6
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yes knock sensors under the intake......i've ran into it with similar complaints as to what you describe.
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Old November 30th, 2008, 01:12 PM   #7
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Check the fuel pressure regulator too. I have replaced alot of those on those models. Remove the vacuum hose going to it and see if it is leaking fuel. If it is the diaphram is bad and it needs to be replaced. That will give you a hard start warm also..
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Old November 30th, 2008, 02:38 PM   #8
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Watch your o2's or short term fuel trims while spraying the lower intake. I use propane or brake cleaner. If your o2's go high and/or you trims go neg. it is probably the o-rings. Sometimes it won't affect idle to much but you will see it with the scanner.
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Old November 30th, 2008, 05:54 PM   #9
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Check the fuel pressure regulator too. I have replaced alot of those on those models. Remove the vacuum hose going to it and see if it is leaking fuel. If it is the diaphram is bad and it needs to be replaced. That will give you a hard start warm also..
thanks. went out and checked it but no fuel out of the vacuum hose

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Watch your o2's or short term fuel trims while spraying the lower intake. I use propane or brake cleaner. If your o2's go high and/or you trims go neg. it is probably the o-rings. Sometimes it won't affect idle to much but you will see it with the scanner.
O-rings on what? injectors?
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Old November 30th, 2008, 06:02 PM   #10
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thanks. went out and checked it but no fuel out of the vacuum hose



O-rings on what? injectors?
Lower intake o-rings/seals


I do quite of these, we also have a smoke machine to help find the vac. leaks but I still prefer propane/brake cleaner.

Make sure you test it cold, they seal up when they get warm
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Old December 1st, 2008, 09:30 AM   #11
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update. so it sits from 7 last night until 7:50 this morning and no issues at all. wtf? i am starting to wonder if it just hasn't been occasional bad gas. ran fuel system cleaner through a tank a week ago. this morning on my fill up i added some drygas so i'll see what happens.
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Old December 3rd, 2008, 02:36 PM   #12
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well it died out yesterday and had to take it to the dealer. crack in the back of the upper "quality" plastic manifold.
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Old August 4th, 2010, 05:57 PM   #13
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Default P0300 Random Misfire

I have a 2003 Avalanche with 5.3 4x4, and I had a similar issue. I noticed when I was driving everything seemed to be fine but when I came to a stop my truck was idling a bit rough. My engine light started flashing and then just stayed on. I hooked up my scanner and the only code was P0300 random misfire. I took it in and had a fuel injection clean done. Same issue. The mechanic told me that it was the intake gasket. Hoping that it wasn't I changed the plugs and wires but the problem persisted... however, all of the plugs that run down the passenger side were fouled (covered in black soot). I realized that all of the cylinders were getting way too much fuel. All of these cylinders are regulated by one of four O2 sensors. I changed the sensor and Everything has been running great ever sense. Hope this helps and Good luck.
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Old August 9th, 2010, 08:18 AM   #14
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does anyone know if this is this common on newer engines as well? my 06 2500 6.0 has a rough idle in the morning when cold but doesn't set a light. i'm going to check the manifold for cracks now i know where to look. changed plugs once and it never made a difference.
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Old June 23rd, 2013, 02:01 PM   #15
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Greetings All,
I want to say thanks for the Great Forum. I've had a few issues over the last 10 years on my 2003 z71 4x4 but none of which ever frustrated me as much as the dreaded P0300 code. Mine started with a P0306 I believe, nevertheless my scanner was telling me cylinder 6 had misfired. No problem I thought, at least it isolated the Cylinder. My truck has 255,000 on it right now so I swapped the coil of my LS1 and on the next drive P0300 came up. Prior to this other than using about a quart of oil between changes it has always had power to spare. I put back the coil and plug wire and still had P0300 I have never had a coil issue or misfire ever until now. After reading the numerous threads here and all over the net as well as every dang service bulletin I had convinced myself I had an intake manifold leak. It took me forever to find it, and it was minimal at best. I changed the gasket to the Modern style and it was good for about a day then P0300 returned. Next I was sure the Cats were clogged because I removed the O2,s (still plugged in) and my idle smoothed out and I had Power again but not as good as normal. I live in Indiana where we have no vehicle inspection so Straight Pipes were installed in Place of Clogged Cats(I never got a CAT code).
Next I had my buddy who tunes come by to delete the rear O2's from the ECM tune, as well as delete CAT codes. When he arrived we went for a road test and P0300 pops up. He was able to communicate while we're driving and clear codes then check it out but the code kept coming back. My O2's all 4 yet still on the tune were working perfectly within specs but the Fuel Trims were turning Bank 2 off and telling Bank 1 to dump more fuel. He couldn't tune my problem away but did recommend I pull the intake back off because I may have pinched the camshaft sensors wire when putting the intake back together. He said it happens a lot. I tore it apart and it was all good, put it back together and it started fine till the ECM took over then P0300 back again.
Today I went over everything and whil unplugging the passenger side 02 sensor I noticed a spot of water got on my fingers. Upon inspection I could see a mild corrosion on one terminal in the engine harness side. More importantly I blew into the connector and a lot more water was in there. I grabbed an airline and blew it out and reconnected it, cleared the codes and went for the Best Ride ever in my 255,000 mile truck, no codes at all after 60 mile road trip. Truck runs great with no Cats, and rear O2's still plugged in taking readings. Early this week my Tuners coming by to Tweak it for the increased air flow, man it's nice to be able to turn the tires over again if I want to. Gotta Love them Chevy's!

Thanks for the Forum Again, very informative!
Oh btw the moisture build up was getting into the harness running inside the cover over the wires leafing to the O2 sensor, it was coming off my AC lines.
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Old June 25th, 2013, 10:54 AM   #16
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Bad injectors caused this code on my buggy...
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