5.3 vortec envoy random misfire/ rough idle

2003 GMC envoy 200,xxx miles
Bought this truck with over 6 codes lit up
only issue was rough idle and misfire. P0449 p0300 p0134 p0135 p0154 p0442
What I fixed so far:
Injector bank 2 had a broken/ damaged wire (now I can hear all injector open and close)
Broken spark plug (#1) changed all driver side plugs still plan on charging the ones on the other side soon.
New plugs and seems like I’m getting spark from all wires.
Changed intake gasket and valve cover gaskets
New temperature sensor plug
New knock sensors and plugs.
Cleaned all injectors.
Now all the codes are gone. Except the random misfire code P0300.
New O2 sensors (previous owner had wired the O2 wrong)
O2 sensors bank 1 stays around .2V (recently found out it goes down to 0v at 2000 rpm
Bank 2 runs around .800/.900 V (still have to pull spark plugs to figure out what’s going on on that side.
I’m new to diagnostics and stilll trying to wrap my head around all this so all the help is well needed thanks guys.
Seems like it misfire got worse and now it pops. This is not my daily but would keep trying as soon as I get back home from being on the road.


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Welcome to the forum! Not a ton of 5.3 owners, relative to the 4.2; always nice to have another one on board. Less than 3% of all consumer-owned vehicles get to 200,000 miles, so the previous owner(s) had to have taken somewhat good care of it.

When you go to replace the bank 2 (pass side) plugs; you may find it easier to go from underneath. Pull the wheel; you'll find a flap on the inner well housing toward the rear, by the frame rail. You can also pull the entire housing back a bit, and if you remove it outright (acknowledging that it's a bit of work), you'll have all kinds of room on that side.

I didn't have to fully remove mine, but I did pull it back a bit, and I had my spring out on that side as I was replacing shocks - that gives a lot of room to get to the front two.

Have a bunch of extensions ready. A bit of anti-seize on the plug threads will help for their next replacement; those heads are aluminum. Oh - and make sure you use the AC Delco iridiums (41-103; IIRC); I bought mine with Bosch plugs, and replacing them made all the difference in idle quality.

While you're ostensibly done with it now... on the driver's side, I did the plugs on that side from up top, and #1 was the worst of them, for me (wound up moving all that wiring and the bracket, and it was still difficult after that) I may have even moved the alternator, but can't remember, TBH.

P0300s can be tough to track down. You may find the below helpful...

Since you were going to replace all the plugs anyway, continue with that. Put in new wires, as well (didn't see you mention these). Next item in that area would be to measure voltage at the coil packs. Unfortunately, since it's a 0300, swapping the packs with one another won't tell you much. If you can get one new one, you can then swap that out with likely candidates to eliminate a bad pack.

If you find a bad coil via process of elimination, getting a 2nd new one to have as a spare can be a good idea, in case you lose another one down the line. Oh, and since you have an '03, like me, you'll likely have the 'square' versions (square heatsinks). Make sure someone didn't swap in a round style - not good to mix / match the versions, IMO.

The O2 sensors may play a part, especially since you said they were wired wrong, and you see voltage issues on both sides. Ignore the rear sensors for now; while they may be marginal and even throw a code, the *front* ones are the critical ones to make sure they're operating optimally. If not, they can toast your cat(s) (which can contribute to the P0300s)
Do a backpressure test on them if needed, to check.

You've cleaned the fuel injectors; if you're getting good pressure with all of them, that eliminates another cause of the misfires. If one is weak, check to make sure the fuel rail pressure is good at that outlet. If more than one is weak, and especially if on opposite sides, make sure your fuel filter has good throughput (you have the external type, on the D/S frame rail just behind the front door.) If it's weak there, the fuel pump may be marginal. Unfortunately, it's an in-tank design, and there's no access cover up top (unless you cut one out) - you'll need to drop the tank to replace it.

IIRC, there's a fuel pressure regulator to check out, too, but I forget where it is as I write this. Pretty sure it's nearer the fuel rail than the pump end.

At that point, the 'major' things have been pretty much eliminated / ruled out for the misfires, but there's a few left. Start with those, and let us know how you come out.

Also, there are others here besides myself who are more knowledgeable, so stay tuned to see if any of them proffer any potential solutions I haven't mentioned.


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What kind (brand) of O2 sensors did you use? I hope they are the ACDelco's, if not, you are going to have problems.

I would first finish up on doing a straight up tune up, dont use the cheapo spark plugs, they will cause nothing but problems. I personally recommend the ACDelco double platinum's, but have been told the AutoLite Double Plats work just as well. Using anything else is asking for headaches.
sorry guys between work and school I kinda gave up on trying to figure out what’s going on with this vehicle lol.... so far
I have replaced intake gaskets dealership said they think it’s the cause “NOT”
I currently have all brand new plugs , spark plug wires, and all coils seem to give enough spark.
After I put the intake and everything back together cleared the codes and started messin with the harness a little bit. Now I’m getting P0174 a bank 2 lean code with the p0300 multiple misfire also car pops and runs like crap, also could the Evaporate selinoid po449 somewhat related to what’s going on here ? . Pulled spark plugs “the new ones after letting it run for 40 minutes” and they have a ton of carbon deposit on them already. How can I test the harness for proper voltage ?
P0449 is not likely to cause a lean condition. It's not giving an evap leak code.

Either one of two things might be causing a lean code. Either it has a vacuum leak or it's not getting enough fuel. @Reprise mentioned fuel pressure. Was this checked? For vacuum leaks, check with carb cleaner. Search for YouTube videos on how to do this.

Another thing to look for is water in the fuel. My son had bad misfires caused by this. We pumped out about 2 liters of water from the tank. We just disconnected the fuel line from the rail and ran the pump. Right away, we saw the water at the bottom.


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With 200,000 Miles on the Engine and with Carbon Clogged Spark Plugs you mentioned as another indicator... it might help to perform an ACDelco Top Engine Cleaner Treatment. While doing this task on the GM LL8 In-Line Six Cylinder Engine is very straightforward ...the V8 Engines may be a bit difficult .

However... after recently examining the 5.3L in my truck... I was thinking that by getting one of the Compression Kits from either Amazon (Skemidex Brand) or from Harbor Freight ... it would allow you the chance to Use just the Hose Threaded for the Spark Plugs from this Kit as the means to Guide the TEC Nozzle Spray into each Cylinder... and fill them up on both banks to the brim... without spilling a single drop.



After leaving that stuff performing a "Cylinder Soak" for say... half a day when the vehicle can sit idle... to avoid Hydro-Locking the Engine... first pull out the Fuel Pump Relay ( on a Fully Charged Battery) ...and then crank the Motor over long enough to push out the gooey excess TEC and Black Carbon Mung from inside of the (8) Cylinders and catch that stuff with some packed in Scott Blue Shop Towels. Doing this TEC Treatment will definitely improve Ring Compression and re-establish the proper TDC "Quench" that the Carbon Build Up and Gas Gummy Lacquer in combo ruins the C/R, causing pre-ignition and misfires when the stuff gets accumulated in the tops of every cylinder and packs into the Ring Grooves trapping them tightly therein.

AFAIK... There are no contemporary Threads that cover this specific procedure for the GM Vortec V8's... but Carbon Buildup ...is Carbon Buildup, regardless of the the engine design ...and this TEC stuff might surprise you with Good Results. The longer you leave it inside of the cylinders... the better it works. If the Hose is used with this TEC Powerful Solvent... flush the Rubber Line clean with some Spray Brake Cleaner afterwards so the hose is not ruined... and then it can do double duty as an intended Compression Tester, too if needs be.


@MRRSM , have you tested TEC and how it affects rubber hoses like in compression testers? This stuff is powerful and may just turn the hose to goo.


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It just might... but for the minimal amount time the ACDelco TEC would be getting sprayed down through the Neck of the Air-Fitting Side of the Compression Hose and past the Spark Plug Holes... it probably will not have that much of an effect on the Rubber Lining ... especially as suggested earlier that the Compression Hose should get purged with Brake Cleaner immediately after topping off all (8) Cylinders.

But ... What The Hell... Let's Give it 'Real Test Try Out' to find out for sure... Yes?

I have assembled everything needed to do this scientifically as shown in the attached Images. As you can observe... it does become necessary to remove the OEM installed Schrader Valve... so the Tool shown ...or something like it ...will be needed open up the pathway for the TEC Spray.

I'll be using use the ACDelco TEC Foaming Spray as displayed and completely spray down and submerge the entire Hose inside of a Plastic Zip-Lok Baggie and leave it in there for 24 Hours. Of course, ordinarily... This is something I would NEVER do on purpose. But the Question you raise here is a valid one... and it does deserve a definitive Answer. I'll follow up on this within a Day or so and post up the Results with Images:

Uhhh.... The 'CAN ' of TEC will NOT be inside of the Baggie during the Test... Just the Foaming TEC of course... I just wanted to show it in there now... to convey the idea of what is about to happen... :>)





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I have never had to do a top end clean up... will look more into it will def help clean all that gunk out. Just found out that the cats been removed and installed straight pipes (could the downstream o2 cause fouled spark plugs) this creating all these issues ? Not related but I have a 1998 trans am with an ls3 swapped rear O2’s removed and never had any issues with it


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OP - the Envoy s/b the same as the T/A, w/ regard to those downstream O2 sensors, I would imagine. However - if the converters have been removed (you have 2), that may (?) complicate the remedy somewhat, if emissions turn out to be the underlying cause.

Just a thought on the top engine cleaner, and putting it into the cylinders - I would think that having the associated cylinder at BDC would be beneficial when adding the cleaner - so doing this in at least *two* steps would give the best chance of freeing up more of the gunk?

If true, then I'd use one of those USB borescope cams (Amazon, eBay, et al) to determine which of the cylinders are at / near BDC... insert the cleaner in those cylinders only & wait the prescribed interval...then after extracting the cleaner from those, turn the crank so that the other half of the cylinders are at / near BDC, and then do those. I think you need 2 x 360 degree revolutions of the crank to get from TDC to BDC - the camera will help you determine that, though.
All the downstream O2 sensors do is report if the cats are working properly. To keep the CEL off, either the PCM was tuned or spacers (commonly called spark plug defoulers) were added to the sensors to pull them out of the exhaust stream and fake the cat's presence.

Sounds like you inherited a basket case with all these problems. Have you checked the basics? Vacuum leaks, fuel pressure, compression test?

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