I pretty well ruled out the coil by switching the coil end of the spark plug wires from cyl 3 to 4 and vice versa. Since it fires both of those cylinders at once, I would have expected to see my misfire code move from 3 to 4. It did not. It stayed on 3. Putting a vacuum pump on the EGR valve at idle had little effect on the way the engine ran, so I was pretty sure it was the EGR ports.
Fast forward to last Sunday. I pulled the upper manifold and found the EGR ports half plugged on 1, 2, 4, & 5. 6 was totally plugged. 3 was almost completely open. I cleaned them out with a piece of wire and compressed air. I buttoned it back up and haven't stored a code since. I feel much better about it now.
Good news and thanks for sharing. Very glad you got your problem resolved.
Have you any ideas about why the problem might occur? I put 175,000 miles on my 2000 and have (I think) 158,000 on my '05 and I have never seen any signs of problems brewing in the area you just addressed.
I wish I had an idea about why this a problem on some engines, but not others.
There was very little hard crusty carbon. I suppose what was clogging the ports was hard, but just outside the port and throughout the entire manifold and plenum was this sticky gooey tarlike substance. Reminded me of varnish in an old carburetor. That kinda baffles me because there should be no fuel up in that area, and all the air flow should be downward. I have no idea how all that crap gets up into the plenum. PCV valve maybe? It doesn't use oil, though. Anyway, I wonder if the tar/varnish stuff gives the carbon something to start sticking to and it just builds after that?
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