I was thinking about your description of symptoms and wondered if it could be water or some other contaminant in your fuel tank. If the car runs OK until the fuel gets sloshed around and mixed up with the contaminant. Water should fall to the lowest point of the fuel tank. I don't know if the fuel pick up is set slightly above that point or not. If the pickup is slightly above the water, and the car will idle fine until it starts bouncing around.??? You would know from experience of when the car runs poorly if this is a likely possibility. Anyway, a couple of bottles of dry-gas (Heet or equivalent) should make short work of the water if that is the problem.

Before you do a lot of other stuff - check and double check to make sure the spark plug wires are routed correctly.

TRIPLE check the spark plug wires. You would have had the back 3 disconnected to remove the UIM.

As per your advise I will check the wiring, as per the order below.

246
135

In terms of the fuel, last week (mid week) i filled up with 91 Octane fuel and added injector cleaner. No difference during the end of last week. Saturday night, I added filled the fuel again with 91 Octane and added another injector cleaner. Since the UIM replacement 155106kms, I have added about 670 kms.

Thought this may help.

Yep, standing in front of the car, that is how the cylinders are numbered:
.2..4..6
1..3..5
You might also want to check the label on the injector cleaner. If water is the problem, you want to add an alcohol like methanol or ethanol to absorb the water and make it miscible with the gasoline.

Update:

Got a code reader and it looks like cylinder 3 was misfiring under load. During idle no issues but as soon as i get it up to 3000-4000rpms, i got over 100 misfire within 3 minutes. Total driving time was 10 minutes and registered 788 misfire. I got 2 or 3 missfires on cylinder 6 but not enough to register a code.

These are new plugs but will replace them all. Any suggestion of type of plugs. Have Delco 7mm silicon plugs on now.

Also, the car is still shaking and the mechn thinks it wheels or steering. Let me know if there are any links or forum topics to check the steering.

I would like to take this time to thank all members of this forum. Your in depth knowledge and patience is outstanding. Since purchasing the bonneville in 2001 I have been having multiple issues and reluntantly paying the price for it. Since discovering the Bonneville Club I have increased my understanding of the past and current issues with a positive attitude in knowning on how to diagnose and hopefully fix these issues.

Again THANK YOU and enjoy your rides.

Did you pull plug #3 and see what it looks like? It'* possible that it could have become fouled in the whole process. Check the resistance of the plug wire also and then check the condition of the coil, in particular the cap where the wire attaches.

The vibration could have a lot of causes, including a poorly running engine. Tire balance, and bad tires would be the most common.

OK, In just saw this thread and notice the guys here have been doing a good job trying to work you through your issues. As you have already noticed, when you have multiple issues at once, it is more difficult to diagnose and eliminate them one by one.

A few things I noticed after reading the whole thing:

1) You should still look into replacing the old O2 sensor, as I don't see that you've done that yet. In answer to your earlier question, you should have two O2 sensors, but you would do fine just replacing the sensor on bank 1 (in your rear exhaust manifold).

2) You should still check out your CAT. Get under the car when it'* cold and bang on the cat. If it rattles, then you should definitely replace it. Or take it to a shop and have them check the back pressure (a more definitive way to know).

Oh yeah, I just realized that you said you bought a code reader. Is it just a code reader or a scanner that will give you live data? Reason being, a scanner can easily indicate whether your O2 sensor(*) is/are bad. It will also gve you an indication of your Cat'* performance.

3) I didn't see anywhere that you have checked/replaced your fuel filter. It'* an inexpensive maintenance item that should be done anyway every 50,000 or so miles.

4) With the misfires you are experiencing, yes, you should check/change those plugs and wires. Use AC Delco or NGK TR55 plugs and AC Delco or Belden wires. Frankly from your last post, the wires sound like a likely culprit.

5) Double check all battery cable connections, both at the battery and at the terminal ends. Check for corrosion or looseness, even under the cable end boots.

Let us know about each of these items and we can go from there.

As far as the shaking problem, I suggest you start a seperate thread for that, for better focus. But maybe wait until you have the engine issues resolved first to keep your focus.

Update:

Changed all the spark wires and still the car misfiring on 3 and 6 (only on load). Spark plugs look good.

Moved, ignition Coils between 3-6 and 2-5 and now registering misfires on 5. Checked the resistance on coil packs:

Coil 3-6 Secondary 6.60K Primary 0.9 omh
Coil 2-5 secondary 6.49K Primary 0.5
Coil 1-4 secondary 6.46K Primary 0.5

With the above reading should i look at replacing the Coil Pack 3-6??

Considering that the miss moved with the coil, I would think that good enough reason to replace that coil.