What causes the engine to "miss" in O/D? updated
 

Hi, I'm brand new here and have a mechanical question already. :oops:

I just bought a 94 Bonneville a few weeks ago. I bought it used so i know little about what has been done to it. The car has 173,000 miles on it but the person I bought it from said the engine was replaced (I assume from possible a wrecked Bonneville) and supposedly has just around 60,000 miles. I love the car. The body is in great shape. I love the ride, the room and the overall quality. Great car.

Here's my problem. The engine misses a bit while accelerating but not very bad. Just here and there. But when I reach about 50 mph and it shifts into overdrive and the rpms drop the engine misses badly and shakes the car quite a bit. I can get it up to about 60 mph and it isn't nearly as bad but still happens. The previous owner said he put fuel injector cleaner in it to stop the miss. I have done that and it helps a bit but doesn't stop it from happening. The engine looks like the original so I assume it is very close to or the same year as the car. It's a 3.8 of course. I haven't done anything to it yet but I was planning on starting with new plugs and wires and then move on to replacing the coil packs after that. Is there any other advice or possibly a problem that causes this that a good tune up won't resolve? I love the car and really want to get it running good. It's great other then the miss. Thanks for any advice in advance.

First thing, Welcome to Bonneville Club :P

Don't worry about already having a problem, that is how many people find us 8)

So, as far as the miss, I would suggest with a full tune up, or at least like you said, plugs and wires. Don't go and replace the coil packs right away if that doesn't fix it. They can be tested using this Techinfo article Ignition coil troubleshooting You don't want to waste money on coils if they are already good.

Do the tune up and see what you have after that ;)

also, what trim level is your car? Please enter your year and trim level into your signature so that we can help you better ;)

I tried to update my sig with a pic and my car info but it isn't showing up. I'll keep working with it. But for now I'll just tell you it is a basic 1994 SE. I have read through these forums a little before I joined and you guys are brilliant and very helpful. I hope to have a great time here on this forum!

Now the sig is working! :D

Also, I want to add that I have access to coil packs that are in great shape. 2002 Grand Am with a 3.4 in it and runs great so i know the coils are all good. Rear ended so I was going to strip the coil packs from that car and put them on mine. Any other parts I could strip that any of you know of would be helpful too. I found out that the coils packs are the same part number and interchangeable but any advice on other stuff to swipe befor it goes to the junk yard would be helpful as well.

Well, probably nothing but the coils will be interchangeable.

Your sig pic, though nice 8) is too big. Please update the pic and make it conform to this New Signature Requirements (effective a year ago)

I hope the sig is within the rules. If not let me know, thanks.

How hard is it to change the coil packs? Do they just bolt off and unplug then bolt on and plug back in? This is my first time working on an engine with coil packs. I've always worked on cars with distributors.

You need a 5.5mm socket, and they come right off. It's usually not the coils though. You need to be looking at plugs and wires first.

Also consider cleaning your throttle body.