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LS misfire

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skyhigh4by

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Have an 03 1500HD with an LQ4 and its throwing P0300 random misfire. Really don't want to just start throwing parts at in hopes that it fixes it.

This problem just came out of nowhere after being parked for a week. It misfires below 2000 rpm but once above that it clears right up and the CEL even goes out. Have an OTC Encore scanner and its showing most misfires happening on #8 followed by #7 then #2. Over the past few days it keeps getting worse.

Replaced a dead o2 sensor bank 2 sensor 2 but that didn't help anything. Plug wires are new. Have tried to do crank learn but for some reason the scanner says pre conditions not met or unable to perform function. Fuel pressure is good but the needle on the gauge is vibrating. Cant find any kind of vacuum leaks. Misted ether all around the top of the engine.

Any ideas? The part thats most confusing to me is that it totally clears up when you speed the engine up. It drives fine too.
 
spark plugs yet ?

and do ONLY factory ac delco plugs . newer design engines are that sensitive . . . .even more so I have seen on gm .

and if you wana make sure its plugs swap the worst one to a good cylinder and see if the miss fire moves to that cylinder .

if it dosnt try swapping a coil from miss firing cylinder to another good one and see if it moves to that one.
 
@sweetk30 Any particular reason for only using factory ac delco plugs? Pretty much everyone in the performance ls world runs ngk plugs, I've never had an issue with them.

Does it seem to miss alittle more when its cold/cooler outside? The older trucks are known to do this when the intake gasket starts getting brittle.
 
Tweekls1 said:
@sweetk30 Any particular reason for only using factory ac delco plugs? Pretty much everyone in the performance ls world runs ngk plugs, I've never had an issue with them.
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years in the shop as a tech . . . keep it simple stupid and least amount of come backs.

now if you know what your doing and if need be tune for it then go for aftermarket .
 
If you want it to skip,put BOSCH plugs in it--I have seen trucks being tuned up that ran much worse with bosch plugs than the old oem AC's that came out of them...dont know why,but chevy's just do not like BOSCH plugs,especially platinum's...
 
My shop truck is a v-10 dodge,ran great and it just started a random miss,replaced the plugs and runs great again
 
We've had this truck for only a few weeks. Original owner said knock sensors were recently changed but it was still getting the odd code for knock sensor low voltage because they never changed the wire harness. It ran great for the first week and then this started happening after sitting over a week. It was at first only missing when it was cold but now its all the time and keeps getting worse and worse to the point that its actually backfiring at idle but above 2000rpm it totally clears up.

Tried to find a vacuum leak but no luck spraying ether all around the intake. The intake was off not that long ago to replace the knock sensors so they shouldn't be brittle.

Im not sure what brand the plugs are or how old they are but the wires were done not long before we bought the truck.
 
Could be leaking on the bottom side of the intake gasket, the gaskets also get flat where the rubber no longer sticks out past the plastic frame.
 
I remember reading somewhere about a common issue with them and the rear injectors or something about them, gets dirty real easy, always the rear ones and usually #7.
 
My older brother had a truck at his work that drove him nuts with random misfire codes--the code indicated one cylinder was faulty,he replaced the injector on that cylinder,after trying new spark plugs and a coil,and it "fixed" it--for about 2 days,then another cylinder started misfiring..

After replacing all 8 injectors,the truck did the same thing--ran OK for a few days,then acted up again..it seemed like all the cylinders were taking turns misfiring...

He was baffled,and when he asked a local dealers service manager if there was any complaints or bulletins on that particular truck,he looked on their computer and there was none for that VIN #,but several others a year or so older had some injector rail issues and GM was replacing them under a silent recall--he bought a new fuel rail and installed it,and ta-da,no more codes thrown,or misfires...

He decided to cut open the original fuel rail and found it was full of red silty rust inside,and there were fine mesh screens built into the rails to trap any gunk--they were almost plugged completely on a few cylinders...the outside of the rail looked nice shiny chrome too,you'd never guess the inside was that bad by looking at it ..
 
I would do a standard tune up and report back.

Random misfire is almost never an injector, coil, or individual cylinder related component but it cant hurt to throw new plugs in it.

What type of fuel system does that motor have? Return or returnless? My first thoughts would be low fuel pressure from a bad filter or pump on its way out thats possibly be remedied by increase voltage when the alternator starts charging better at a higher RPM. With a decent scanner you could also check MAP reading and look for a TPS dead spot you're breaking past. Is it any better if you drive it harder? With a vacuum leak I would expect a knock retard but its not necessary and you said your knock sensors dont work anyway.
 
We have an '06 Tahoe that did the same thing. It would only throw the misfire code randomly. Sometimes it was fine and others the code would come up. I did all of the tune up stuff and had the same problem. It eventually began to run like crap over a period of 3-4 months. Problem ended up being a failing/failed lifter that was wearing out the cam. I ended up having to put another engine in the truck because it was cheaper to get a takeout motor than fix the original.
 
If the truck runs ok(and all the numbers look good)but just keeps setting a P0300, do a crank variation relearn(can be done with a "high end"scanner). 98% of the time that will fix it.
 
After plugs, plug wires, and coil packs, I would check the grounds to the heads. Often missed on LS swaps is retaining good ground on the aluminum heads. Lastly how is the MAF sensor? Is it dirty? Strange issues happen when that is out or dirty.
 
Meangreen1 said:
After plugs, plug wires, and coil packs, I would check the grounds to the heads. Often missed on LS swaps is retaining good ground on the aluminum heads. Lastly how is the MAF sensor? Is it dirty? Strange issues happen when that is out or dirty.
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When I was in the LS game the only time I had a misfire code was a dirty MAF from over oiling a KN air filter.
 
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