I recently developed a disturbing problem with our car. We have a 2002 Buick LeSabre Limited 3.8-liter with an automatic. Recently we completed a round trip from Dallas to Des Moines and back, and after the last fill-up in Oklahoma the car developed what feels like a misfire condition.
It seems to occur only at an rpm range between 1500 and 2000, at 45 to 50 mph and still in 3rd gear. This occurs only when there is a load on the engine. I can rev the engine in park from 1000 rpm up to 2300 rpm and it runs perfectly smooth. I have run an injector cleaner (Seafoam) in two tanks and there has been no improvement.
Once it's in 4th gear (OD) it smoothes out with no indication that there is anything wrong. At first I thought it was a wheel balance problem, but when you let off the gas the stumble goes away, and coasting at 45-50 mph is smooth. New tires and balancing were done prior to the trip.
The first thing that comes to mind, and what I believe to be the most frequent cause of a misfire, is a failure in the secondary portion of the ignition system. That's the half of the system which actually produces and distributes the spark, including the ignition coils, ignition wires and sparkplugs.
The Buick 3.8-liter V6 uses a distributorless ignition with three two-post ignition coils. A technician may be able to narrow down which cylinder is acting up during a road test with a scan tool, or perhaps even replicate the condition in the shop. Knowing which is the problem cylinder makes diagnosis that much easier.
The coils, wires and plugs should also be visually inspected and resistance tested. Another possible cause of a misfire is the fuel injectors. Aftermarket products added to the fuel tank usually have no effect on injectors that are bad enough to cause symptoms. General Motors recommends a fuel injector test procedure using a special tool which compares fuel pressure drops through each injector. Then you know which ones to replace, and which ones may be saved with a special injector cleaning tool and solvent.
The 3.8-liter has also had a few cases of a hard-to-detect intermittent chuggle on light acceleration, caused by a defective throttle position sensor. It could even be a problem with the lockup torque converter engagement of the transmission. And this is just the top of the list of potential causes, so don't replace anything prior to a definitive diagnosis.