Recurring misfire

I have a 1997 Buick Century with and automatic transmission. It has 127,000 miles on it, although recently it got the engine replaced so the engine has roughly 91,000 miles on it. It has a recurring misfire problem when i get down on the throttle, such as when i go to pass somebody. It has gotten worse in the last week. I have taken it to the shop several times and they only got it to misfire once and they checked all the wiring and they replaced the ignition modulator. The problem went away for a few weeks and now it is back and getting worse. Examples of how i can make the car misfire:

1. If i go to pas somebody, I can floor the pedal. The car downshifts twice and the RPMs increase, but the vehicle does not increase speed

2. On the highway, if I am using the cruise control and i slow down, then use the “resume” button to get back up to speed. The car will downshift and then it will buck and bounce while slowly increasing speed. Also, the speedometer will wobble in this case, before indicating acceleration.

3. Merging onto a highway, at approximately 50-55 mph the car will buck and bounce while accelerating and shifting gears, but once it is solidly in gear it will work fine.

Notes: I know it misfires because the Check Engine light is flashing at me, and according to the manual that indicates a misfire condition. The problem usually clears up after 15-30 seconds. It does not exist during normal driving conditions as long as i use less than 30% of the throttle.

A mechanic needs to use normal diagnostic skills to find the cause of your driveability concern. I don’t know exactly what part was replaced (ignition module?) but usually when these type of parts start getting replaced the usual suspects have been given a clean bill of health.

Are you looking for ideas to bring to your mechanic (I never feel good about this) or are you going to read up and tool up and try it yourself?

The replaced something that controls the ignition. I was guessing at what it was called because I forgot .I am mostly just curious as to what could be causing the trouble. I trust the mechanic’s judgement, but I am also curios if there is anything I can do at home to try and alleviate it until I can get back to the shop. I have to use the highway to get to work everyday, so I’m also wondering if this will damage the engine.

Did they pull the codes to isolate the cylinder(s) that is/are misfiring? The most common causes of misfires on these cars are the coil packs or fuel injectors, as long as it’s not a random misfire. I don’t think I have ever seen an ignition module cause a misfire on a 3.1 (the only engine available in this car. The module normally just causes it to not start altogether, and seldom just goes bad), but I have seen plenty of coils go bad and plenty of clogged fuel injectors. If it’s a misfire on one cylinder, try replacing the coil pack for the cylinder that is misfiring. It’s cheap, probably 1/3 the price or less of the module you didn’t need. If it’s not that, find a shop that does BG fuel injector cleaning services. Make sure they have the can that cleans the fuel rail and have them perform the service to your car. It will probably cost around $100, but is a lot cheaper than replacing the fuel injector. I recommend the BG service (and only the BG service) because I have used it on numerous 3.1’s and 3.4’s and 99 times out of 100 the injector will work perfectly afterward. The results are long-lived too based on experiences with my own cars.

Is BG a brand name or what? I have neither seen nor heard of it before.

You can learn more about BG products here:

You can also find a mechanic near your location who uses them.
So long as the check engine light is going off, then you have some time to resolve the issue. However, if the CEL flashes and stays on flashing, you need to get the car towed.

I would go back to my mechanic and have him do more diagnosis at this point. It appears the first fix was either not correct or incomplete.