My "family" car is a Buick Century.  It has the 3.1 ltr V6 in it.  The 3.1 is a direct descendent of the 2.8 engine in our V6 Fieros.

Last year, the engine started running rough and misfiring.  No engine codes came up so the dealer did a diagnostic and "found nothing wrong" and charged me for it.  Then in February of this year, the misfiring got bad enough that the car would stall.  Still no engine codes.  Finally in March an engine code came up.  I brought it to a trusted mechanic who informed me that the code only states that the engine was misfiring.  He changed the plugs and the wiring and it seemed to fix the problem, though I sometime thought it was still happening.  Well, last week, the misfiring happened again and no engine codes.  I brought it in and they discovered the the ignition coil was fried so replaced the coil pack.  The car went pretty well trouble free for 2 days and it's misfiring again...still no engine codes. 

Any suggestions?....let's be constructive.  It's already been suggest I get rid of the car.    I can't get rid of the car until the issues are fixed.  It's time to trade it in for something else.  So much for the car that was the #1 JD Powers survey for quality in 1999 and most reliable used car in 2004.

Thanks.

Dr.Fiero wrote: Look for something non ECM controlled. A sticky EGR valve comes to mind.  You could have a chunk of carbon kicking it open every now and then - this causes the cylinder nearest to it to go lean (and mis-fire).

Thanks...this is most useful insight as it seemed to me it was the #1 cyl that had a bad spark plug last in March.

Nope....that wasn't the problem

Another observation...the car seems to run fine when the tank is full.  As it gets below 1/2 it starts to get worse and is especially bad as I get to the last 1/4 tank of fuel.   It doesn't sound like a fuel issue as it seems only one or two cylinders are missing.  However, the fuel tank level seems to have a corellation.  My first thought is maybe the fuel filler cap as those things are supposed to be airtight and they do wear out.  However, no code regarding a lose cap was thrown. 

Well...the next thing happened.  Water pump blew.  So the Buick is on its 3rd water pump now.  The original went about 3 months after the warranty expired so the last one lasted longer.  A whole 5 years. 

Hopefully, we also solved the coughing and sputtering.  It appears the V6 engine in the Buick (same block as our Fieros) does not like Bosch spark plugs.   We put in NGK's.  Not sure why my mechanic couldn't get Delco plugs.   But if the NGK's start acting up in the next while, I'll just go and get the Delco plugs myself.   The Buick is now running fine again.  ... this issue might serve as a heads-up for us Fiero owners too.

Given the reliability record of this car, the next problem should crop up by...oh....January.