Misfiring problem

Truck Info: 2004 Ford Ranger XLT 2wd 3.0L (obviously im in the 3.0 forum). Stock motor with about 106k.

Okay, time to give the low down scoop. About two months ago I was getting a really rough idle when the engine was warm. It was giving the whole truck a shake and was throwing codes for cylinders #4 and #6 misfiring. So I pulled the plugs and a few of them were no good. No biggie right?

I replaced the spark plugs with Iridium plugs and a brand new set of spark plug wires. I prefer to replace them together if everything is getting changed. After replacing those, the truck has ran great until the past couple weeks.

I am now getting a misfire code for cylinder #5. All the plugs and wires look good and everything is on secure. When I start the truck up in the morning, it runs flawlessly. Once it gets pretty warm, if I come to an idle, after about 45 seconds it will start to get that misfire shake. Check engine light will start flashing. If I press the gas, it goes away. Go back to idle, it will start flashing again. If I ignore it, it goes solid. Later in the day when I go to start my truck, I will get a code occasionally for a misfire upon start up.

Any help would be appreciated! My thought is possibly the coil pack starting to go, but aren't those like either they work or they are dead?

I know that you said that you checked the wires, but the similar thing happened to me, it was a bad coil and I did the coil, plugs, and wires. Everything worked fine for awhile then I got the misfire situation like you and after a little looking I found that one of my plugs was grounding out every once in awhile (I guess when I hit a bump it started to ground on a bolt behind the engine). I am just throwing out a suggestion for something to look for. Start with cheap easy fixes first.

Coils can be effected by heat, so can spark plug wiring.

You can test the coil cold and then warm with an OHM meter.
There are 3 coils in the coil pack
The coil will have a 4 pin connector.

12volts
GND
GND
GND

The 12volts is on when key is on
Each of the 3 grounds control 2 spark plugs, these are controlled by the computer.
Your coil spark plug wires should be
3 -- 4
2 -- 6
1 -- 5

So 3 and 4 are on the same coil and will fire at the same time
2 and 6
1 and 5

Now with a bad coil shouldn't both 1 and 5 miss, not just 5?
Well yes if coil is dead that would happen.
But if coil is just out of spec only 5 might miss.

With 2 spark plugs on 1 coil they are "series" wired, so +.....-.....+....-
This means one spark plug will fire from center electrode to ground electrode(normal), the other spark plug will fire in reverse, from ground electrode to center electrode.
Now normally this doesn't matter, but the reverse spark is not quite as hot as the normal spark, so if a coil is getting out of spec. you might get a single cylinder miss.

To test a coil pack set OHM meter at 200 ohms
Unplug coil wires, connector and spark plug wires
Pins could be this way
GND
GND
GND
12v
Touch the 12v pin on the coil with 1 probe then touch one of the GND pins with the other probe
.5 to 1.4 ohms is what you should see
Leave 12v probe in place and move to next GND pin, test all 3 pins.
They should all be about the same.

Now touch one probe to a GND pin and the other probe to one of it's spark plug holes
Should get 7.0-11.5 ohms, test 2nd spark plug hole, should be the same.
Pins are in order
GND 3 -- 4
GND 2 -- 6
GND 1 -- 5
12v

Test all 6 spark plug holes this way.

This doesn't mean coil is good, but it will show if coil is bad.

Hook everything back up, start engine, get it nice and hot and then test it again.
Ohms will go up a bit, that's normal.

Now all ohm meters are not created equal, lol, so what you are really looking for is 1 coil not being the same as the other 2 in the pack.
So if you are getting 1.8 ohms don't freak out.
If you get 1.8, 1.6 and 2.5 I would suspect the 2.5 as a failing coil

There is a good video on testing coil packs
Google: How to Check an Ignition Coil Pack the easy way

Its a Ford Mustang V6 but exactly the same as your coil, spec-wise

Is it possible that clogged fuel filter could be causing this also?

A couple people have told me that they had similar problems and changing their fuel filters solved the misfiring.

Possible, and fuel filter is only $10 and worth changing as regular maintenance.

But from your description it isn't a top of the list cause, limited flow in the fuel filter or fuel pump is usually noticed at higher speeds first, a lack of power above 50mph for example, but idle would be fine, and cold or warm wouldn't matter, a clog is a clog, and the engine uses more fuel when cold, so you would get the opposite effect you describe now.

Okay, so I got around to doing the voltage test. On the high voltage test (checking the towers)

3-4 was 12.01 kohms
2-6 was 11.88
1-5 was 11.87

On the low voltage (checking the pins)

3-4 was .7
2-6 was 1.3
1-5 was 1.6


Only thing is that it doesn't make since that the cylinder misfiring and the coil pack doesn't relate. But then realized I was being dumb and PO304 means the #4 cylinder is misfiring. Should I just replace the pack or do further diagnostics to see if it goes farther than the coil pack to see if its something else?

I would head to the parts store with the OHM meter in hand, and get a new coil pack and test it at the counter.
Buy it if it has better numbers.

Problem is still persisting. Coil pack is all getting good sparks and reading fine. Going to clean the IAC and throw in a new gasket. Then hope to change the fuel filter and pull the injectors to give them a clean. From what I read, next step would be valves being mushroomed? Still debating on replacing coil pack regardless.

Have you tried checking spark when it's misfiring? You may have but its always good to double check in case you missed something.

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Olsin6 said:

Problem is still persisting.

Click to expand...

Some things you can do;

Get a can of CO2 and spray it on each coil one at a time when you get hot engine misfire. Cooling the coil may eliminate miss and narrow down coil.

When the engine is cold you get rich fuel which may mask injector problem until engine heats and fuel ratio is back to running mix. You could switch places on two injectors to see if problem moves.