Mis-Fire revisited

[SlkMerc]

Hello Gentlemen,

After taking some great direction from you MM Gurus regarding a #8 cylinder misfire: Swapping the coils from one cylinder to another. I got another check engine light on showing the same code on the same #8 hole.

Could a bad plug be the culprit? I have 60k on the SlkMerc and have no clue if she's had plugs changed before as I bought her 2nd hand. It idles rougher than it previously did and I know that some of you mentioned that plugs could be the problem. Advice is anticipated.


Thanks

Don

[JACook]

Could a bad plug be the culprit? I have 60k on the SlkMerc and have no clue if she's had plugs changed before as I bought her 2nd hand. It idles rougher than it previously did and I know that some of you mentioned that plugs could be the problem. Advice is anticipated.

Spark plugs are certainly an easy place to start. If you've never changed plugs on a Ford Modular
before, be aware that they require a bit more care and feeding than the plugs in yer old Small-Block
Chebby. There's been plenty written in the forums already on this subject, so I won't rehash it all again,
unless I'm asked.

The other thing I'd do is dump a bottle of Techron in the gas tank.

Misfire detection is done using the crank angle sensor. When a misfire occurs, the crankshaft very
briefly slows down, then speeds back up again. This change in motion can be felt inside the car, and
it can also be measured by the crank angle sensor. The ECU sees the variance in crankshaft speed
pulses, and it knows which cylinder it just told to fire. If a cylinder does this enough times over a
prescribed period, the ECU sets a code, and turns on the light.

A single cylinder misfire code can be caused by literally anything that could cause that cylinder to
misfire. Ignition, fuel injector, a vacuum leak, burned valve... whatever. The most likely cause is a
spark plug, the second most likely would be a dirty injector.

Whenever I'm dealing with a single cylinder misfire, I like to scatter multiple parts around to other
cylinders, so the next time I get a misfire hit, I pretty much know what I need to replace. I'll move
the spark plug, coil pack, plug wire/COP boot, and injector all to different cylinders (taking notes of
what I put where, of course, since my memory isn't what it once was). :o

Then I wait.

If the misfire moves to another cylinder, I've nailed it. If it doesn't move, I've eliminated a whole bunch
of stuff, and I move on to looking at things like vacuum leaks and such.

Just what I do...