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Discussion Starter · #1 ·
I know I'll probably get bounced out of here for posting this, but:

I no longer have my 2002 black LTZ, I've got a 2002 Suburban 1500 4wd with the 5.3 Vortec. Anyone have any ideas on the following issue?

Check engine light came on one afternoon. Started the vehicle, it seemed to be missing, very rough idle, but didn't stall.

Scanned the codes, P0300 Random/Multiple cylinder misfire. According to the scan tool, cylinder 4 is misfiring the most, with cylinder 3 having some as well (about 5:1 ratio cylinder 4 to 3). That's at idle. Under WOT, the misfires seem to stop on cylinders 3 & 4 (for the most part) and cylinder 5 starts missing, with cylinder 7 picking up a couple, intermittently. Soon as you let off the throttle and the RPM's drop, cylinders 4 and 3 start back up. I've read up on how the PCM detects misfires, and I am unsure if cylinder 3 is really having misfires, or if it's by chance just a result of number 4 having so many. Apparently, over 2000 RPM, the PCM gets data from an opposing cylinder to detect misfires, so, is that the reason why it "appears" to switch to cylinder 5 (with a few on 7) under WOT, or, is the misfire really moving?

I've:
changed plugs
swapped wires with cylinders that aren't misfiring (miss didn't follow the wires)
swapped coil packs with cylinders that aren't misfiring (miss didn't follow the coils)
changed fuel filter
Run some injector cleaner through it
vacuum tested the engine, got about 19" in.Hg at idle, dropped to 0 when opening the throttle, and increased back to about 25 in.Hg, then settled back in to 19in.Hg (seemed normal to me)

I suspected it could be a sticking EGR valve, but alas, the 2002's (at least some of them) don't have the EGR valves.

I don't *think* it's a catalytic converter, as I don't notice a rotten egg smell from the exhaust, and the vehicle doesn't drive like it has a restriction (if anything, I'd say it alternates between lean and rich judging by the exhaust smell). In fact, it still seems to produce plenty of power while accelerating, you almost wouldn't even think it had a misfire when the throttle is open. You really only feel it at idle. I strongly suspected a vacuum leak, but that doesn't seem to be the case (at least not from what I can tell). I'm running out of things to "cheaply" replace, with cat's and fuel pump in the $250~$500 range, coil packs at $50 a piece (x8). I'm not sure about the O2 sensors, the scan tool I used read them, and the levels appeared to be similar between bank 1 sensor 1 and bank 2 sensor 1, as well as b1s2 and b2s2 and there's no other code showing in the OBDII scan. I've also yet to perform a compression test on cylinder 4, to look for a stuck valve or swap fuel injectors from a working cylinder to cylinder 4.

And hopefully I'll be back in a Trailblazer EXT soon enough. :woot:
 

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2003 chevy trailblazer_lt_ext
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It looks like your having quite a few misfires on one bank of cylnders if you have a scan tool that you can monitor the before cat o2 sensors with i would see if the are working properly. If you have a bad 02 sensor it will cause it to most likely run lean and have a missfire. I had almost the exact same problem shortly after i bought my tb but i had ran into it enought times before to know what to check.
 

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Discussion Starter · #4 ·
No other electrical issues exhibited. This weekend I'm getting the scan tool again and swapping out the injector from cylinder 4 with one that's showing no misfires.

I'm wondering if one cylinder is totally (or nearly) dead, if it would cause a couple of others to show a few misfires at various RPMs.
 

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Discussion Starter · #5 ·
I'll post an update here, in case anyone runs into a similar problem. I swapped fuel injectors to no avail, did compression tests on all cylinders, another vacuum test (and sprayed throttle body cleaner around the intake/vacuum hoses as well). Previously, I'd ruled out a coil pack since I thought I had misfires on cylinders 3 and 4 regularly, and intermittently on the others (I think 2, 6, and 7 had one or two misfires here and there, nothing regular). After running some injector cleaner through and everything else I'd done, I got it down to only misfiring on cylinders 3 and 4, the rest were all running perfect. However, under WOT, cylinders 5 and 4 would misfire, and 3 would run normally. I did more research, and apparently the PCM uses a different alternate cylinder (via the crank position sensor) to detect misfires above 2000 RPM than it does below 2000 RPM. I began thinking cylinder 4 might be the only culprit, as originally it was the only plug that looked different than the rest when I pulled all 8.

One $50 coil pack later, and there's no misfires at all. So, when viewing real-time scan data, I guess it's good to keep in mind that a "random/multiple cylinder misfire" might not be on more than one cylinder. I suspect with the coil pack gone, the cylinder was pretty much totally dead. That might have caused the others (3 and 5 depending on RPM) to throw a code as well.
 

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2006 chevy
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I'll post an update here, in case anyone runs into a similar problem. I'd ruled out a coil pack since I thought I had misfires on cylinders 3 and 4 regularly, and intermittently on the others. After running some injector cleaner through, I got it down to only misfiring on cylinders 3 and 4, the rest were all running perfect. However, under WOT, cylinders 5 and 4 would misfire, and 3 would stop. I did more research, and apparently the PCM uses a different cylinder to detect misfires above 2000 RPM than it does below 2000 RPM. I began thinking cylinder 4 might be the only culprit, as originally it was the only plug that looked different than the rest when I pulled all 8.

One $50 coil pack later, and there's no misfires at all. So, when viewing real-time scan data, I guess it's good to keep in mind that a "random/multiple cylinder misfire" might not be on more than one cylinder. I suspect with the coil pack gone, the cylinder was pretty much totally dead. That might have caused the others (3 and 5 depending on RPM) to throw a code as well.
I wish I saw this post earlier, I could have helped you on this.. We had the same type issue with my Buddies Sierra and it was the coil pack..
 

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2005 gmc envoy_slt_xl
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Hey Guys,

I experienced the same thing this past weekend (while on vacation of all places) with #7. (2005 Envoy XL 5.3L V8)

I had a mechanic friend change the coil pack on #7, yet it still misfires at idle (we had no time to look at it farther -part store closed- and as we all had to go - including me back to Canada - was in NYC at the time).

We did run a scan and it did show #7 misifiring with that P0300 code.
I was about to change the injector (of #7) this Monday morning but feel, after reading up here, that that might not be the solution.

Any more ideas what might it be ?
(ps, we also ran a fuel injector cleaner as well).
 

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2005 gmc envoy_slt_xl
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GRRRRRRRRRRRRR,


I'm so pissed. Changed both coil and injector, but miss continued to no avail. Pulled out spark plug and it had carbon build up right to the tip (solid) connecting both ends (and platinum spark plugs too).

Anyways, if any of you ever need a used injector PM me (I live in Toronto, Canada) so take that into account re: postage.
 

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2002 chevy trailblazer_lt
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Similar problem on '02 TB 4.2L I6

How do you get the misfire counts for the different cylinders? Is that a function of the scan tool? I am seeing the same symptoms on my sister's 02 TB with the 4.2L I6. It runs rough and uses a lot of gas, and my scan tool only shows a PO300 code, nothing for individual cylinders. I have done some research and found many possibilities, including a bad coil, spark plug, injector, fuel pump, dirty throttle body, excess carbon buildup in the valves, and a few others. The spark plugs are AC Delco iridiums, less than a year old. I haven't pulled them yet to check for fouling or oil or water buildup in the wells. I was also going to do an upper intake cleaning with Sea Foam, and put half of the can of Sea Foam in the gas tank with about 1/2 a tank of gas to see if that helps. She had a similar problem about 8 months ago, which went away by replacing the plugs, cleaning the wells (had some oil in them), and replacing one coil because we had an intermittent miss code for cylinder 1 at that time. Now the same symptoms came back a few days ago, shortly after she filled the gas tank. Maybe it's just some bad gas, possibly contaminated with water? Any other ideas or thoughts? If I can find out how to get the misfire counts for each cylinder, then I could probably tell if it is an electrical problem (weak coil, high voltage arc path, contamination) or fuel problem (injector, low fuel pressure, bad gas?).
 

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2003 gmc envoy_slt
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A P0300 = multiple or random miss-fires. The last zero would be 1-6 if only one cylinder had a miss.

So you're getting a miss in more than one cylinder.

Any or all the symptoms you have listed could be the problem.
 

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I tried the upper induction cleaning with 1/2 a can of Sea Foam and put the other 1/2 can in the gas tank, which is about 1/2 full. I also cleaned the throttle body. I am part way through removing the coils and plugs to check and clean them. I am finding oil in each of the wells, about an inch deep. I am guessing this may be interfering with the electrical connection between the coil and plug, either shorting it to ground if the oil is electrically conductive enough, or blocking it. I think it will probably run fine after I finish this, but it will likely come back. Where is the oil coming from? I don't see anything obvious. Is there a valve cover gasket that could be leaking and draining oil into these wells, or some other gasket? How hard is it to replace a valve cover gasket? Or is it easier to just remove the coils and plugs once a year and clean them, as long as the oil leak doesn't get worse?
 

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2003 gmc envoy_slt
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It's most likely the valve cover gasket.

Have you tried tightening the bolts?

You don't need to remove the plugs. Just use a turkey baster to suck out the excess oil.
 
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