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Discussion Starter · #1 ·
I put in brand new plugs and brand new coils drove around for about 30 minutes then my check engine light came on and it never came on before I change the plugs and coils now it said P0305 cylinder 5 misfire what did I do wrong please help it a 2005 Chevy trailblazer it ran fine for about 30minutes I even cut the AC on and it didn't even shake I thought I fixed it please any help would be grateful

2004 gmc envoy_slt
2,589 Posts
First , what plugs did you get? You need to use AC Delco 41-103 Iridiums, these engines like them. If you did use the AC plugs, recheck your coils to make sure they are seated properly on the plugs.

1,512 Posts
Like EB said --- these engine are really persnickety about the right brand of parts - especially in the ignition circuits.

I am coming to the belief that the GM ECMs cannot actually tell where a very minor miss is and it just either reports a P0300 or a P0305.

It seems to use #5 cylinder as a scapegoat .............. for whatever reason I cannot deduce.

| <---------- HOWEVER ---------> |

I've got a 2000-4.3 Astro Van that reports a P0305 intermittently and sets the CEL and then starts flashing it to tell me the cat is being ruined.

I have tested everything - coil, distributor, for the hell of it I also took the new distributor electronic parts for the 4.3 off my shelf and installed them as I don't like to pull that stupid, insipid engine doghouse out any more than once every 30 years or so.

  • I've scoped.
  • I've regular probed
  • I've logic-probed
  • I've scanned.
  • I've test driven
  • I changed the oil (there is no VVT in this engine) 3 times.
  • I poured into the crankcase some Berrymans B-12 and some Seafoam - in the hope that IF it's a lifter pumping up, the relief valve might just be gummed up and those very aromatic solvents might do some good.
  • I've replaced perfectly good parts with brand new ones - yeah - don't quote me about that stupid move - I know!

New plugs weren't necessary as these NGK Racer Iridium plugs have only 15K miles old and looked fabulous!​
The plug/coil wires all meg'd out at 18K - to 22K Ohms - all WNL!​
I installed a new spider injector 2 years ago with the electronic valves at the end of the nozzles - supposedly a very good upgrade.​
The cap and rotor were new although I did replace the rotor 'cause I don't want to go in there any more!​
I got feedback that the distributor could have a bad gear (it didn't) or the cam gear was wearing out (it was also good) and besides, how could a bad distributor pick on #5 cylinder, individually? It can't.​
I performed a hot engine vapor cleaning in the crankcase because it MIGHT be a lifter that was pumping up and holding the exhaust valve open at certain times. (it wasn't)​
So --- tomorrow I put the dumb thing all back together and if it acts up I'll shoot it.

For the record - it will run forever from Montana to Key West, FLA and back without a single hiccup.
► BUT - if you shut the engine off and let it sit for 15-20 minutes, THEN it starts missing on #5 and the CEL comes on, a' blinking and if you scan it, it reports P0305 and/or P0300.​
► Let it cool for a few hours and it runs like there's nothing wrong at all.​
I just put new tires on it and filled the gas tank - so I can't give up on it.

.. the cracked front bumper is from a bulldog trying to commit suicide by getting hit by my wife's Astro. He ran off - yelping and the bumper got cracked. ... but there's more to that story if you wanna know .........

My wife was pounding me on the head and right shoulder as I was playing automotive dodgeball driving, braking, emergency steering and trying to stop in traffic that was following me at 70 MPH and they were scattering all over the road trying to miss me and they were even in the other lanes - it was a mess!

Just in case you don't know it ----> It's counterintuitive to pound on the driver when he's trying his very best to mitigate a bad situation that's happening all around him like a slow motion train wreck and it certainly pushes the driver's brain into a defensive mood when it should really be in a high speed crash - survival mode.

All the screaming she was doing also was attacking my mental train of thought, making it hard to hear the tires of the cars behind me, locking up their brakes and exercising their ABS systems.

Gee - it was a fun night! My shoulder has never been the same - nor my ears.
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