My car is misfiring when I am accelerating, it doesn't do it all the time. But it also happens ramdomly around 59 mph. I recently had the tranny replaced and also the MAP sensor, air filter and fuel filter. Has anyone heard of this happening, I am not a mechanic and I don't want to put much more into it. Thanks for the help..

When was the last time you did a tuneup?

Spark plugs and wires are the most likely culprits. Is the CEL (check engine light) flashing, or lit? If so, take the car to Autozone for a free diagnostic trouble code scan. That will help to narrow things down.

Otherwise, you should start by changing plugs and wires if they have not been changed in the last year or so.

Is it a random misfire, or isolated to one specific cylinder?
Get Autozone to scan it for DTC'* as suggested, and let us know what codes were found.
While you are there pick up a can of MAF cleaner, the MAF should be cleaned every time you change the oil, at least that is when I do it, if the MAF readings are off the car can misfire, usually that happens during idle though.

If they have not been replaced in a good while you can get some wires and plugs, I recommend sticking with AC Delco wires, and AC Delco Iridium plugs, they may cost a little more, but you'll get more mileage out of them, do not get Platinum plugs whatever you do, our engines do not seem to like them much.




How many miles does the car have?

Has the nylon 6,6 framed lower intake manifold gasket ever been replaced with the aluminum framed ones yet?
If not I know it may not relate to this misfire, it is something I highly suggest doing if it has never been done, if you plan on having the car for a while anyway, if the motor ever hydra-locks it could cost you an engine.

I had the plugs changed and wires checked. Repaced a coil pac, checked the vacuum lines(1 was not connected on the canister by the air filter. And it is still misfiring and now when its sitting at a light it chugs like a train(as my daughter describes it), that didn't happen before. The only thing is that the MAF sensor, I think not MAP sensor. The one on the drivers side with 3 wires, my brother tried to get it out and it cracked, he siliconed it, but could that make a difference? I don't know what else. I had the codes P0102 and P0108 before we started, no codes now. ????

I was told when I bought the car the plenum was changed, is that the same thing?

If the P0108 went away after replacing the MAP sensor then I think that should not be the problem, unless it starts throwing the code again.
The MAF sensor can be bad enough to cause a misfire, and not throw a DTC, I found that out with my 96' Park Ave, I also split my MAF cover, I was tapping on it to see if the idle fluctuated, which is something you can do to occasionally tell if it is going bad or not.
It being cracked, and silicone'd could be a problem, however I do not think that is the issue, since it having the MAF code before, and it is now cracked, I'd get that MAF Sensor replaced as soon as you can, did you ever get the MAF Sensor all the way out and clean the thermistors with MAF Cleaner?

The plenum is the upper intake manifold, it sits on the lower intake manifold, if you can see where the lower intake manifold meets the block, and see if the little bit of gasket you should be able to see is plastic, or aluminum.
There is a chance they may have changed it if they did the UIM, since it is a known issue with 3800 Series II'*

I was gone for a few days, I have a 2003
pontiac with the same engine and this week-end my brothers switched out the idler something sensor on the 2 cars, cleaned them first. Pontiac still runs fine, now Buick idles really rough at stop lights, also about every other time I start the car it is rough, the RPM'* go nuts jump to 3000 and then almost dies and then jumps again, but only to 2000 RPM'*. After that it runs fine till I get to a light. It has only hiccuped a couple of times going down the highway, any ideas? A guy I work with said he thinks there is an vacuum leak from the catalytic converter????? Anyone heard of the same problem? I am not sure the check engine light is working, so my brother is bringing his code checker over, is there a fuse for the check engine light?

Definitely give the MAF sensor a good cleaning and even try swapping it out with a known good one to try. The 3.8'* are bad for MAF sensors.

You said you had a transmission replaced? Was it a new or used tranny and was the misfire there before you changed it? Have seen torque converters fish bite and feel just like a misfire when they're under a fair amount of load. Typically under 60mph.

check the vacuum boot and the two lines going to the fuel pressure regulator and purge solenoid. any cracking or dry rot looking on those will cause your engine to have a vacuum leak