I'm out of ideas you guys. This plug change has turned into a stupid multi-day event. Ridiculous. I've gotten zero replies from the Trailvoy.com forum. Don't know why I didn't think to start here.
2003 Envoy XL, 119k miles on original plugs. Mileage finally dropped to an average of 14 mpg and I decided to change the plugs. Truck was running just fine, no problems whatsoever besides to mileage.
I installed NGK (7397) TR5IX Iridium IX plugs. Gap on these measured .040, gap on old plugs was ridiculously big at .068, electrode was a dismal nub(yes, I know, it's my wife's car, I only mess with it when it breaks) The reach on these is slightly shorter than the stock plugs, I didn't think it was a big deal. I also had to install new wires as I broke several during removal. Advance only had Autolite brand wires which I have since taken back as two of them were running so poorly the truck wouldn't run. I've replaced them with Bosch wires and the truck is drivable. I troubleshot the wires with a timing light and replaced them one at time with the factory wires.
I've gotten to where the idle has an occasional stumble and the truck regularly keeps a pending P0300 code. Every now and then under steady throttle at highway speeds it will stumble but rarely.
Spoke with the GM parts guy today and he says he's had several people come in with rough idle/running issues on the NGK plugs. AC Delco plugs are also iridium and gapped at .040. Not sure I'm buying it but my wife needs to take the truck on a road trip and I can't take the chance that something happens to her on the road. I HATE throwing parts a problem but I have nothing else to go on since I've only change plugs/wires on a perfectly fine running vehicle.
At least mileage has improved 
Later,
Bruce
I think that truck has the 'ion sensing module' which fires the spark plug on the exhaust stroke, it measures the resistance and determines from that what to do next. The Isuzus which used that module had to have the correct spark plug or something similar would happen. I'd get the OE plugs and wires from the stealership. Or at least get the exact same spark plug (no cross references to different brands, etc) as what came in it.
On the fuel mileage: the most common cause is a thermostat that won't close fully. The engine never gets to operating temp, meaning the PCM operates things in the 'cold engine' (read: richer mixture) mode. On many vehicles the transmission won't go into overdrive until the coolant temp exceeds 149 deg F. The transmission also stays in low gear much longer because the PCM is heating the catalytic converters up to operating temperature. It won't trigger a MIL unless the 'thermostat rationality' (excessive warmup time, code P0128) is out of specs and that generally doesn't happen unless the ambient temperature is way down there.
Next is the formulation of fuel; E10 (10% ethanol, very common) will cause a fuel mileage drop since ethanol has less energy density than gasoline.
I buy my AC plugs from the local Napa. No reason to spend extra money at the dealership for the same plugs.
I'll somewhat agree on the t-stat, but also I would be thinking an EVAP/EGR problem too. I know in the trucks, the EVAP canister will get full of liquid instead of fumes, and cause a hard fillup and rough running.
isnt a plug change on a v8 trailblazer just so much fun? lovely access
Plug change was a blast! I cant wait to do again.
Thanks for the replies you guys.
Running shops for a few years has kinda opened my eyes on the whole spark plug thing. I don't know why, but some cars just don't like some plugs. My wife's car came with Denso platinums from the factory. At the time I replaced them, the local O'reilly was having a crazy sale on the Bosch platinums (2.99 each instead of 5.99 each for the Densos or 4.99 each for the NGK) so I did it. It ran like crap
My Chevy came with Delco platinums from the factory. I replaced them with Bosch platinums and its great.
Go figure. As an engineer, I've looked at resistances, induction values, insulator composition.... I can't figure it out, but its just a known anomaly.
In reply to curtis73:
yep. Jeep 4.0L's love the $1 champions they came with and really dont like any fancy plug ive tried. i used the cheapest variant of a one step colder plug i could find in my stroker and she loved it.
Replaced NGK plugs with some AC Delco plugs from Napa, $6.29 on sale. All problems are gone. Truck idles and runs like it should. Unbelievable. I guess you never stop learning...
Thanks to all who posted. My wife is very pleased.
