I have a 2001 ford ranger. I just recently purchased it, and I thought I'd do a tune up on it so I did. I put new wires&plugs and now when I drive in town my check engine light comes on, it shows code p306, to which I know to be a #6 cylinder mis-fire, I've chanded the plug&wire 2times now& I still get the same code. If I clear the code& drive it on the high-way the light never comes on, but in town it will. The size of my motor is a 3.0L, do you have any idea what it might be?
Two things to try are switching the number 6 injector with one from a different cylinder to see if the misfire moves to that other cylinder, and check the routing of the plug wires. If any are too close together, the current through one can induce a voltage spike into another one causing that plug to fire at the wrong time.
Misfires are detected by the rotational speed of the crankshaft slowing down momentarily. That is what allows us to feel it. At highway speeds that may not last long enough to be detected, but the misfire event will last longer at slower speeds and that may be the only time it is detected.
October, 26, 2012 AT 9:47 PM
Forgot to mention that many engines do not like fancy spark plugs or those from some manufacturers. As a test, try reinstalling the old plug in cylinder 6. If that stops the code from setting, check the new one's gap and brand. Check the new plug wire too or switch it with a different one. If the end wasn't fully seated, (we all do that sooner or later), the terminal can burn away leading to a misfire. That will take some time to show up.