Oldsmobile/Buick Repair: Engine stuttering, spark plug wires, spark plug wire

Question
We have a '93 Buick LeSabre with a 3.8 engine.

When going at a steady speed (below 30 mph to 60+) the engine runs roughly and very unevenly. However, when accelerating, the engine smooths out and feels like a new car. I've recently changed spark plugs and wires, used injector cleaner in the past couple of fill-ups, and had the local shop do a tune-up. The shop also suspected a loose harmonic balancer and did some welding there.

It still bucks when going slow and steady and smooths out upon acceleration. The problem occured suddenly with no gradule degradation of performance.

The shop tried fiddeling with something for an hour or so and then concluded that maybe a valve chipped off and I should have a valve job on the engine. It seems to me that they're guessing.

I have personally disconnected each spark plug wire from the coil pack and discovered that when I disconnect the #4 wire, there is no reduction in idle speed yet the spark looks healthy.

Do you have any ideas?

Thank you.  

Answer
Hello Thurman,
I have a couple ideas.
One is,,,I don't know about the welding, but can think of no good reason to weld around a harmonic balancer.
Yes to the guessing, so find a different mechanic. That place may end up costing you a bunch.
It is entirely possible that a valve is not seating quite as well as the rest. A compression test should be performed to see.
It is possible that a valve is slightly burned. It also could be a spark plug, although if it was the same miss before and after the tune up, I doubt that.
Try swapping a couple of the spark plug wires. Not just one end, but completely switch them. Then see if the symptom stayed with #4, or went with the plug wire to the new location. If it stayed at #4, then swap some plugs.
All this is free, just takes work.
If the problem stays at 4, and/or the compression on 4 is lower, there is still another possibility . I have had valve springs break, and cause conditions similar to yours.
There are probably two springs on each valve, one inside the other. They can actually be changed without removing the head. Mention that possibility.
Good luck,
Van