My 1989 Bonneville with 3.8 V6 is the one car in my life I just can't part with. It has developed a strange misfire (?). It will only happen when the transmission is in OD and lock-up, and you carefully lug it down to about 1600 RPM. There are isolated jolts that feel like ignition misfire, except that they are very strong. It is not a string of misfires; it is one isolated "thump". Unless you press on the accelerator, the misfire does not cause any change in transmission state--if you continue to carefully lug at low RPM, it will "thump" again in a few seconds. It can not be made to misfire under any conditions other than this. Hard accleration--no misfire. Engine speeds above about 1800--no misfire. Does not seem weather related.

The misfires are so jolting, that I even suspected some trans problem where lockup went out-in quickly. However, my best clue so far is that when I replaced plug wires, the problem became noticeably worse.

ALSO, I need to replace the harmonic balancer, but I can't get the bolt loose on the crank end. The balancer appears smooth on its face, so I don't think a puller is required, just get the bolt out and the balancer will slide out.

just chirping in on your HB question...

i KNOW for a fact that you will need a puller... looks like this

Your misfire isn't only under light load in OD. That'* when it'* MOST APPARENT. Change your plugs and wires. Typical symptom.

For the HB, you need to take the dustcover off your flywheel on the other end of the engine and jam something in there to prevent the motor from turning while you break the bolt loose.

Then use the puller.

You must jam something in there to prevent it from turning the other direction during re-install.

Willsren--thanks for reply. I have changed plugs and new wires. Plugs made no difference. Wires made problem noticeably worse.

Are there no HB'* that don't have to be removed with a puller? I'm familiar with that, and even have a puller stored somewhere, but under marginal lighting conditions I looked for threaded puller holes on the HB, but did not see any. I'll look again. Thanks.

The threaded holes are on the back surface, accessed through the 3 slots in the front face.

You can rent a puller from Autozone if you have one nearby. It won't have the right bolts for the 3 holes, so you'll have to buy those seperately.

If your plugs and wires were changed, and your wires made it WORSE, I suggest you check each wire'* resistance and post the numbers here.

The only other possibility is your coil pack. You may want to consider a Type2 AC Delco upgrade from a 92 1/2 or newer GM V6 car.

Willwren--I'll look at the HB closer, but it appeared to have a smooth, convex front surface with no holes or slots. I've known the car since it rolled off showroom floor, so I'm sure it has never been replaced. It just started making a noise almost exactly like a rod knocking. Discovered that the elastomer joining the two-piece assembly together had turned loose, and I could rotate the outer (pulley) part while the center stayed stationary on the crank.

The "Type 2, AC Delco" upgrade--are you talking about just the assembly with the three squarish ignition coils?

You want the ICM and 3 seperate coils off the newest 3800 you can find on the lot. jr's3800 can give you details on the upgrade and what bracket you may need. Well worth it. A hotter spark from the newer/better system will smooth out your idle and give you a stronger mid-range response in your power band.

It is very possible that you do need a puller for the HB, I was lucky enough that the engine out of the 88 trofeo I picked up came off without a puller, but that bolt IS a pita to get off.