2004 envoy 4.2 6cyl. runs rough. had a new

With this info, I'm betting motor mounts. If they're bad, make sure and use AC Delco brand. Other brands will tend to crush easier, now you're back with the same problem.

sounds like cat converter just about completely clogged and the back pressure from it is choking the vehicle causing the cylinder failure --the exhaust cant escape and will kill [stall] the engine--- with it running feel the exhaust coming out if there is barely any flow from tailpipe cat conv has failed [clogged ] and will need replacement -----when check engine light flashes it is to advise that u have a malfunction that needs immediate attention or further damage may occur and or vehicle shut down may occure

First of all, did you use AC Delco Iridium plugs part number 41-103? These are the best plugs for this engine.
Second, did you firmly seat the coil pack when reinstalling them on each plug?
If it is running bad, are you getting the Check Engine light coming on? If so, what is the code?
When you changed the plugs, did you disconnect the battery? If you did, the rough running could mean that the throttle body should be cleaned.

Attached is a diagram of where the plugs and coilpacks are located, you need to remove the air intake resonator box to access this area. Plugs should be changed around 100K miles. Use only AC Delco 41-103 Iridium plugs. #1 is the coilpack and #5 is the spark plug

If you have a 4.2 L engine, Remove the 10mm bolts on the coils, on top of the engine, remove the electrical connector then the coil, your truck has individual coils, spark plugs are in the hole, 5/8 spark plug socket, don't buy cheap spark pluggs for this truck, it will miss fire, I prefer autolites Autolite Xp Iridium/Spark Plug

1st things first, clean that throttle body with carb cleaner with the battery disconnected.


Check your motor mounts and transmission mounts for the vibration at start up and when your foot is on the brake. I just went through this with my TB. The mounts may look ok visually, but they may be going flat. Changing the gear oil won't solve anything you have described, however, it's time to change, then change it.

Did you recently change your spark plugs to a brand other than AC Delco 41-103's? I've seen platinum plugs make our TB's run rough also the worst is Bosch. Stick with the iridium Delco's, will have a smoother running vehicle. Hope something in here helps.

Make: CHEVROLET
Model: SILVERADO 1500
Year: 2004
Engine: V8 - 4.8L - FI - - GAS

GAP = .040"

I suggets PLATINUM sparkplug (Morotcraft, Bosch, Champion)

Hope helps (remember to rate and comment this).

Location of spark plugsThey are located along the top of the motor under coil packs that fire the plugs. You have to remove the resonator box from the top of the motor to gain access.
Each coil pack is secured with two long skinny bolts and must be removed carefully. Then simply lift it off and remove the plug directly underneath it.
Use only AC Delco iridium plugs. 100k mile intervals.

make sure you have ngk or ac delco iridium plugs installed. Also common problem on the 2.2L ecotec is the cat plugging. Easy check is to remove the front o2 sensor (very easy 22mm wrench) and start the car and rev it. If it revs freely now and does not bog the cat is plugged. They are covered fed. emissions warranty 8yr 80k, all gm dealers stock the cats for the 2.2 ecotec.