misfire cylinder 6 2001 buick lesabre

Ok after repairing the upper intake manifold I now get the P0306 code popping up. I have tried clearing the code but it keeps coming back. I have a slight roughness to the engine (nothing super crazy but it is there)

here is what I tried (on cylinder 6)

1. I thought maybe one of the new sparkplugs that I put in the car may have been faulty so I replaced it with the older one that I just took off the car. This did nothing.

2. I then figured maybe my ignition wire was the cause. I had a spare ignition wire to try and again this didn't work.

3. I then thought maybe the coil was the cause. So I switched the number 2 and 5 coil with the 6 and ??(can't remember what number) I figured if I got a misfire in the number 2 cylinder that would be my problem. But number 6 is still the problem child.

4. I then figured maybe I should check the compression. It is dark out and when I checked it I was getting a bit frustrated with the whole thing. But I think I had the compression tester screwed in tight enough. Anyway I got 108 psi. Cylinder 1 in comparison was 160 psi.

(could this be my problem???)

5. My other thought is that the injector could be flaky. I used a garden hose and I'm quite sure I heard the ticking of the injector but I don't know if the injector was plugged up or something.


Anyhow any ideas about what I did or what I should check next.

The car runs pretty good but as I said it does have a slight roughness (shake) to it. If I didn't know the car I might even say it was a normal vibration.

Was the 108 compression from a dry test, if so try squirting a little oil in the cylinder then test it. IF increases you know you have weak rings. The difference of 10% or less between cylinder compressions is recommemded.

That was a dry test I did. I didn't have anything to squirt the oil in with so I didn't do the wet test.

IIRC, your car sat for an extended period of time after the UIM failure, is this correct? If so, it is possible the valves on #6 were closed during this time, which allowed coolant to sit behind the valves allowing rust & corrosion to deteriorate the valve seats. There is nothing you can do about this without pulling the head, & doing a valve job. If I were you, I would continue driving the car, with the hopes that the valves will eventually seat themselves. Have you checked for vacuum leaks around the injector O-rings?

sounds good I'll keep driving and hope for the best