Hello everyone, first off, of coarse my bonneville is running like a top its the 3.1 sister engine in my 1998 Century that is giving me a headache. It starts hard and idles rough, upon mashing the pedal down it misfires and black smoke pours out of the exhaust until it gets to a higher RPM then seems to run fine. The service engine soon light is on. I took it to a trusted mechanic shop to have it scanned, no codes came up, they checked the egr and said that was good. They said it it may be the plugs,wires and or coil. So I installed new plugs and wires, still no improvement. Swapped each coil out to the Bonnevile and they all work great in the Bonnevile. With no codes being thrown and the service engine soon light still on, I do not know what is going on. Any info would be greatly appreciated.

On a side note, my sig says 99 century, that is a misprint, will change that as soon as I push submit

Sounds like a Oxygen sensor. Get it replaced ASAP if it is.

Oh and BTW, the 3.1 is not a sister engine to the 3800. The 3.1 (3100) is a 60 degree V6 and the 3800 is a 90 degree.

Holy crap, I love this forum, I pushed the submit buttom, went to change my sig and low an behold you answered my post already, I will replace the O2 sensor tomorrow or sometime this weekend. Thanks again!!!!

Maybe I should have said the evil step-sister to the 3800

I just rembered that I have a BUICK GN -T TYPE CASPER HEATED O2 SENSOR that I picked up off of ebay that I was going to install in my bike for an A/F gauge, will that by any chance work, its a 4 wire heated sensor, I dont know how "standardized" these are.

New O2 sensor didnt change a thing, but it wasnt a total waste, the old one was cruddy and dirty. I tested the coils with a test meter and they all check out good. Problem still persits, anyone know of anything else I can try before I take it in and get raked on labor?

Hmph.. this is odd. No check engine light at all?

Not alot to go wrong with those engines. had one in an olds. They like to go through back side plug wires, I changed mine yearly. Once it acted all quirky and I replaced the module under the coil packs and it was healed. Ohm your coil packs also.

Yesterday I put in a set of Autolite Plantinum2 with new plug wires(Xact), Today I put in a brand new Bosch O2 sensor, I checked the ohms on the coils today also(and swapped them to the bonneville) They were all in spec with the TECHINFO how-to. Also last night after it was dark I, I pulled the plug wires of each coil seperataly and seemed to me to have a nice arc between the posts. Sevice Engine Soon light is still on and although it runs a little smoother then before it still runs like crap. I pulled the front three plugs today and #6 was sooted up again.
Before Thrusday the car ran fine, but would hesitate slightly at about 60mph sometimes. Then thrusday morning I had to push the gas to get it started then it ran fine, after work same thing, but when the wife went to take it later that night is when all this started.
I think I have a MAF sensor, at least that is what it looks like to me, and according to Advance Auto'* website they have some listed for my car.
When the car was scanned he said the only thing it said was 2 cylinders were not firing(before I changed all this).
Car is close to its mileage for an oil change, would this make a difference if I changed it, also the rest of the fluids where changed early spring.

check engine light is on, but no codes came up on scan

Yesterday I put in a set of Autolite Plantinum2 with new plug wires(Xact), Today I put in a brand new Bosch O2 sensor, I checked the ohms on the coils today also(and swapped them to the bonneville) They were all in spec with the TECHINFO how-to. Also last night after it was dark I, I pulled the plug wires of each coil seperataly and seemed to me to have a nice arc between the posts. Sevice Engine Soon light is still on and although it runs a little smoother then before it still runs like crap. I pulled the front three plugs today and #6 was sooted up again.
Before Thrusday the car ran fine, but would hesitate slightly at about 60mph sometimes. Then thrusday morning I had to push the gas to get it started then it ran fine, after work same thing, but when the wife went to take it later that night is when all this started.
I think I have a MAF sensor, at least that is what it looks like to me, and according to Advance Auto'* website they have some listed for my car.
When the car was scanned he said the only thing it said was 2 cylinders were not firing(before I changed all this).
Car is close to its mileage for an oil change, would this make a difference if I changed it, also the rest of the fluids where changed early spring.