Second post on this one - has me stumped. Wife’s car - 2005 Taurus, 6 cyl, 3L, OHV, 83K miles. CEL came on and code came back as “P305 - cylinder 5 misfire detected”. The car had been running rough. I changed the plugs, wires and coil. After changing these (and clearing the code) the car seemed to run better for about a week. Today it suddenly started running rough again and the CEL came on with the same code. I’m guessing now this is probably fuel related as opposed to spark related??? As a side comment, the tachometer has been fluctuating as of late as well (again, intermittently).
The next thing you really need to do is check the compression.
This could be a purely electrical problem. I know little about any Ford, but if the car has individual coils, try swapping the #5 with the #2 and see if the problem moves. Failing that, change the wires, #5 with #2. hey should be similar enough in length to swap them.
You may want to have a thorough check of the fuses, etc, too. You didn’t say where you live, but Northern cars suffer more from failing connections than Southern cars. Regardless of where you live, check those out, too.
Let us know.
I plan on doing a compression test. By luck, the #5 cylinder happens to be the middle one facing the front of the engine (easy to test). As for the coil, the 05 Taurus has one coil pack for the 6 cylinders and not individual ones. I was curious if the tachometer going crazy might be related to a component that could be causing the misfire (or the misfire causing the tach to go crazy) ??
If you suspect fuel injector, try switching the #5 injector with a neighboring injector to see if the misfire follows the injector. If it does, you know the injector is at fault.
The 3.0 is a great engine and should not have a compression issue with only a measly 83k miles on the clock but I’m in agreement that a compression check is always a must when an engine performance problem exists.
A fluctuating tachometer often points to a problem in the secondary ignition but since you’ve already changed that stuff maybe a problem in the ignition controls, ECM, wire harness, etc.?
(Wild guess there.)