Yep, engine light came on while wife had Riv on trip...

Bad engine miss.
I have not been able to get the code yet, but the coolant level has not changed, no smoke or coolant smell at the exhaust at least. No change in oil level, no mix of fluids.
In thinking back, it may have been around in a small way the last week at 1200 to 1600 rpm under load.
Where does the fault reader plug in?

Riv in Reno

Had a similar problem in my 03 Silverado w/63k miles on it just last month. Check engine light came on and started flashing indicating an engine misfire. Changed the plugs, no help. Changed the plug wires, no help. Changed the coil pack, no help. Went to AutoZone to use their ODBII code reader (which should be located under the dash on the driver's side), and it indicated a misfire in cylinder #2. Tried the SeaFoam/vacuum line cleaning procedure. No help. Could be exhaust valve leaking or a bad injector so I swapped out the injector on cyl #2 with #4. Had the code read again and the misfire went to cyl #4. Aha! Bought some new injectors off eBay for $20 each since the parts store wanted $120 for EACH! and replaced all four onthat side. Check engine light went out and it runs smooth as silk with no error codes.

Can't recall if the 3.8l SC engine is TBI or individual injectors but that might be a source to check after you check the obvious like spark plugs, wires and coil pack. Then do the SeaFoam treatment.

get the code read before you start throwing money at it, auto zone does it for free and it can save you lots of time and frustration if you have a code to start with. the sea foam top clean will not help with a missfire since it doent go through the injectors(unless you put it in the gas tank). it may actually make the miss worse if your plugs have gone bad. if you have a broken or clogged injector, then normaly no amount of gas treatment will clear it, you just need to replace it. btw we have indiviual injectors for each cyl. all 6 of them are the same and if your into performance make sure they have been flow tested and that they all flow close to the same, in other words make sure the 6 injectors are matched.
if you find out you need them zzp has a good price if ebay doesnt help you-
http://www.zzperformance.com/grand_prix/products1.php?id=323&catid=113

What year is your Riv? AZ can't read '95s, only '96 and newer. Usually it's a misfire cured by plugs and wires (get GOOD WIRES) but have the code read before you spend any money on it.

SA102, That's the first I've heard of an injector going bad (although that was obviously your problem). Good troubleshooting technique.

-Derek

Good news...
The code came up as a misfire in (can you guess?) cyl #4. Ever notice it's never the easy to get at plug...
The coil tested out good, so I bought what they had to get back on the road quick. On set of crappy "lifetime guaranty" wires, and Bosch platinum +2 plugs. I did the easy front bank with wires and plugs first, then it started to snow on me. So I did just the wire to #4 and...
it works and the light went off. :-)
I'll change the back plugs and the last two wires this week.
Yes, I'll upgrade the wires later. It's what the store had.
Is it a good idea to use the dielectric (sp) goo on the pins to the coils?

Riv in Reno

ok now sell the bosch plugs back and get some autolite 605's
dielectric is not nessisary but it wont hurt either, just aids in removing them next time, and from corrosion iirc