I have an '89 Pontiac Grand Prix with a 3.1 Liter V-6 MPFI Engine, usually I don't have my car worked on unless it won't go, stop, or the noise is so loud you can't hear the radio, but this problem is really starting to bug me.
When I am accelerating, the engine misses slightly and later at 45-55 MPH it chugs a couple of times. It chugs a little while in gear at a stop light also.
Let's go down the list of what it might possibly be, and what it probably is. Number one: we can pretty much eliminate a mechanical problem with the engine, assuming the "miss" is coming and going. A hard internal failure would result in a solid miss that won't stop until you fix it. Number two: Multi Port Fuel Injection ( MPFI ). There could be an intermittent cause of a misfire in this system, maybe a fuel injector on its way out, a problem with the computer that electronically controls the injectors, or perhaps a short or bad connection in the wiring between the two. Number three: the ignition system. This is the first place to start looking, and the most common cause of a miss in any engine. To get even more specific we go right to the secondary ignition system, in your case that includes the ignition coils, spark plug wires and spark plugs. Notice I left out a distributor cap. That's because you don't have a distributer. Your engine uses what they call DIS (Direct Ignition System) which utilizes a combination of sensors instead of a distributor.
Bottom line: the most likely problem is a breakdown in the plug wires and/or ignition coils. When a plug wire deteriorates, it increases in resistance, meaning it is that much harder for the spark to get from the coil (producing the spark) to the plug (receiving the spark). So if that 60,000 volt spark no longer has a good path to the spark plug, it will either break through the insulation of the bad wire to the nearest path to ground (the engine block) or not get out of the coil at all and start eating it away from the inside out.
That's why driving for a long period of time with bad plug wires, before replacing them, may bring you back in the shop a week later with a faulty coil. In other words, if you have been chugging along for quite a while you may have added to the expense of the repair.