[Huckleberry]

Only 40% of my rigs are Astro/Safaris.
20% is a 2005 Grand Prix GT, 3800 Series III V6, 109,000 miles.
Last week it was a quart low on oil.
It calls for 5W30, but I've always run full
synthetic 10W30 in it without any problems.
Well, I didn't have any of that on hand,
so I topped it off with 10W40 synthetic blend.
Now a week later it's running super rough and
blowing burnt-smelling smoke out the exhaust.
Check engine light came of briefly and went back off.
Did I kill it?

 

[Rustbelt_mVan]

The 3800 engines are known for being very tolerant in terms of oil viscosity. I highly doubt a single quart of 10w-40 would have any discernible impact on your engine. If anything, the heavier oil would be more likely to cause bearing or valve train failures.
Is the smoke oily (check your tailpipe)?. If you're burning enough oil to create visible smoke at all times, it's going to be going through it pretty quick. If it's a fuel or converter issue, you'll need to hook up to the ODB II and go from there.
109k is not much for a well-maintained 3800

 

[Huckleberry]

Bought a $20 scanner and it pulled up
"P0300 - Randon/multiple cylinder misfire".
I'm hoping maybe it's just a bad coil pack.
Went ahead and bought a set on Amazon, only $30.
When they get here I'll swap them out one
at a time and see if that was the problem.

 

[Huckleberry]

Thanks to YouTube, I learned how to
diagnose which coil pack is bad.
You take off a wire, start the car, and if the
terminal sparks back onto the coil pack, it's good.
Using that process, I narrowed it down to the middle one.
I bought a set of cheap ones off Amazon,
and so far two different ones didn't fix it.
The chances of getting two duds in a row seems highly
improbable, so the coil pack is probably not be the problem
Those coil packs are mounted on/plugged into the ICM,
so now I'm thinking that could be the problem.
I can't be the only one here who has a car with a 3800.
Insights welcome.

 

[Huckleberry]

Switched out the ICM, and now
she's running like a Swiss watch.
Took almost a week to get here from
Amazon, but it was less than half the
price of what the auto parts stores wanted.
I'm patient like that.