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`03 Trailblazer LTZ
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Discussion Starter · #1 ·
Hey guys,
I recently bought a `03 Trailblazer 4.2, it has roughly 82.000 miles on it and is in very good condition.
Except a week ago the SES light came on and my OBD scanner told me it´s a P0305. I quickly noticed the somewhat rough idling.
So I began the troubleshooting process by installing new spark plugs (was due anyway). Didn´t help.
I figured maybe the coil was bad, changed that with another cylinder, but it still gave me the misfire on #5.
I noticed some drops of oil on some of the coils, so I ordered a new valve cover gasket which I´m going to install this week. However the misfiring cylinder had no oil at the coil.
I´m thinking it may be the fuel injector, so I ordered a set of injectors to replace all of them and hopefully the problem will go away.

What else do I have to think about? Something I really need to check or test?

thanks guys
 

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I just had this happen to my '04 Envoy. Had a p0306 and p0300 pending code. Took it to a mechanic because I replaced the coil and spark plug and I was still getting the same codes. He did a compression test and I was only getting 85 lb. Turns out I had burnt valves in that cylinder. Big job and expensive. I lucked out and pressed the guy who sold me the vehicle to fix it or I would take him to small claims court if I didn't receive a refund. Good luck. Hopefully you have a less expensive fix on your hands.
 

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2004 Trailblazer LS 4.20 L
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did you reset the computer when you changed plugs and coils? maybe get you a new coil or two and make sure they are seated properly before swaping all the injectors. drive it for a bit in case the cylinder has gunk in it. otherwise you got it figured out. spark, fuel, air equals combustion. also you'll want a fuel filter and air filter probably. at least check them. you'll be good to go!
 

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Discussion Starter · #4 ·
Yes I did reset the computer and it didn’t show a code right away, but the misfire was still there, you could feel the car shaking a bit, you could hear it and feel it at the exhaust. Cleared the code every time I changed something and it usually takes a while for it to come back.
installing new injectors today and already replaced the air filter. New fuel filter maybe today or tomorrow. Thanks so far!
 

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Discussion Starter · #6 ·
So I changed the injector on #5 and it’s still idling rough, misfire didn’t go away. Doing a compression test next
 

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you've got to pull the pcm fuse or disconnect the battery to reset the computer. ( in case you may not have caught that part.) clearing codes is another bit.
 

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Discussion Starter · #8 ·
Cleared codes via an OBD Code reader, didn’t come back since a few days, but the misfire is still there. I didn’t drive though, just started it a few times in my driveway. I’m way too anxious about damaging the engine
 

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Cleared codes via an OBD Code reader, didn’t come back since a few days, but the misfire is still there. I didn’t drive though, just started it a few times in my driveway. I’m way too anxious about damaging the engine
its the blinking SES light that is bad news. the damage occurs to the catalytic converter because unburned fuel clogs it up. the misfire code is thrown when there is about one misfire in 100 cyles detected. the blinky light happens at ten misfires or more per 100. when this occurs the PCM greatly reduces performance to prevent damage to the converter. have you diverted the rain water from pouring on top of the block? have you checked the SAIS lines for condensation? sometimes I've heard that pump will blow water into the crank case if the check valve fails. that usually causes a number 4 misfire but probably worth looking at. one problem you will have in high humidity with short running times is condensation in the coil. without the longer run cycles water can get into the coil boots and wont get burned out before the motor gets shut down. could be as simple as that.
 

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Discussion Starter · #10 ·
Okay guys I did the compression test today.
Following results:
#1 165
#2 165
#3 160
#4 175
#5 160 the one with the problem
#6 160

on one hand I’m glad the misfiring cylinder didn’t show much lower compression than the others and is within the 10% range of the highest compression. On the other hand I’m starting to get lost with that misfire.
To recap:
New Plugs - misfire still there
New coil - misfire still there
New injector - misfire still there
Compression test - acceptable result

I‘m willing to throw a bunch of money and new parts on this thing because everything else is in premier condition and relatively low mileage, but I just don’t know what to do or swap out anymore
 

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It seems you've ALREADY thrown a bunch of money at this job. No problem for me - it's your money!

But that 160# isn't the answer on your engine - suppose the #5 cylinder isn't opening one or both exhaust valves? You'd still get decent (read: good) compression - but the DYNAMIC COMPRESSION would be bad.

What you've failed to do is DIAGNOSE this problem before shovelling money at it for ZERO GOOD RESULTS so far.
 

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Discussion Starter · #12 ·
All in all I’ve spent about 200$ so far. At first I was thinking it would be a quick fix, but it’s starting to seem like it might take a while.

So you’re saying it might be an exhaust valve problem? Guess the only way to find out is to do a leak down test, right?
 

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............. a leak-down test is to ascertain the condition of the rings (not the valves unless they are slightly leaking) - and you can eliminate them with a wet-test.
►It's a low pressure test that eliminates the necessity for there to be high pressures that can skew the compression test results.​
 

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Discussion Starter · #15 ·
Thanks! I think you’re supposed to cut off ignition and fuel to the cylinder being tested. Ignition is easy. But how do I cut off the fuel for one cylinder only? (Except maybe unplugging the injector, which is a lot of work to get to, there has to be an easier/quicker way?)
 

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White 2002 Trailblazer EXT; Dark Charcoal/Green 2005 Trailblazer EXT
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Just humor me by pulling the spark plug from that cylinder and checking to make sure the new coil is getting proper voltage and signal from the PCM (I'll tell you a simple way later in this post)!

My 02 EXT 4.2 started misfiring a few weeks ago, even though I replaced ALL the plugs with delco's AND ALL of the coils. After scratching my head for a week, I found the culprit! Something (squirrel or mouse, I guess) nibbled on the wiring and clipped a wire that worked coil #4!

To easily check to see if the coil/plug is firing is as follows. Take the coil and plug from the questionable cylinder and put an OLD plug in the cylinder, plug the coil back into the harness, put the boot and coil spring back on the coil, put the NEW plug into the boot, and crank the engine and look at the new plug to see if you can see the spark. No spark means no power to the coil OR no signal to the coil. Then all you have to do is to chase the wiring and check the PCM.

just to let you know, my "02 EXT has just over 200,000 miles on her original powertrain.

Also, most of the time you see oil in the spark plug holes is not due to a bad valve cover gasket on the 2002-2005 models with the plastic valve cover. Most of the time it's because the PCV hose on the driver's side of the engine between the PCM and the brake booster starts stopping up in the block with sludge. It's easy to clean out by pouring liquid (not spray) Berryman B12 down that hose. The PCV has a very little hole in the head and it easily gets stopped up, and when that happens, the engine builds pressure in the crankcase and pushes up on the valve cover, and that's what causes the leaks. I know that from experience. I changed that PITA cover gasket BEFORE I knew about the PCV issue, and the new gasket did NOT stop the leak until AFTER I cleaned the PCV! I now believe I didn't actually have to change the gasket if I cleaned the PCV first! It would have saved me a lot of headache!
 

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Discussion Starter · #17 ·
I tried to check for spark a few days ago, although I used another method (maybe not a good one?). I unclipped the wiring to the coil on the bad cylinder while idling and the rpm dropped, indicating that without the wire it doesn’t fire at all, therefore with the wire attached it’s producing a spark. I don’t know how reliable or correct that method was, but I will try your method just to be sure.

also thanks for the help with the oil!
 

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I tried to check for spark a few days ago, although I used another method (maybe not a good one?). I unclipped the wiring to the coil on the bad cylinder while idling and the rpm dropped, indicating that without the wire it doesn’t fire at all, therefore with the wire attached it’s producing a spark. I don’t know how reliable or correct that method was, but I will try your method just to be sure.

also thanks for the help with the oil!
I've done it your way also, but the way I mentioned will allow you to definately SEE the spark and confirm it's all ok there. Did you use DELCO spark plugs? The 4.2 is VERY finicky and doesn't play well with other spark plugs!

Also, there's a youtube video of how to "bench test" a trailblazer/envoy/colorado coil. The only thing I do different when I do the "bench test" is that I use power from the TB's car battery rather than the drill battery and voltage transformer that is used in the video. It's a lot easier.

Here's a link to the "bench test" I mentioned.
 

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Sorry about the repost.
 

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Discussion Starter · #20 ·
Will try that out tomorrow just to be sure.

I’m going to do the dynamic/running compression test too, but I still need to figure out if I have to unplug the the fuel injector for the cylinder that is being tested
 
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