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I have a 2003 2.0 Zetec that's my daily driver. Yesterday it started misfiring and sputtering at idle while driving home on the highway in stop and go traffic during a moderate snow storm. I hooked up my laptop and obd2 scanner and got codes 316 and 304 on cylinder 4. I thought at first the wire or plug was bad. I pulled the plug and it looked good and made sure the wire was in snug. Still no change. I then got my spark plug tester and was able to get the boot on the plug and wire on it and it lit up indicating to me continuity. Now that I think about it should have grounded the plug while out and see if it got spark. I then thought maybe the coil was bad and checked all the other plugs and wires and put the tester on all of the other 3 and they all seem to light up the tester. I didn't check them with my multimeter because I'm not sure what the value range is for resistance on them. I just ordered a Chilton Manual for it. I changed the coil just to check it, got a new one from Napa, and no change in the idle or going down the road.

I've searched this forum and others for and have found a broad range of things that can cause the codes and symptoms mentioned. From bad fuel, clogged injectors, fuel filters falling apart and getting clog in the injectors, to bad ecm and pcm's, sensors, and even a bad alternator.

I'm seeing if anyone has had this issue or a similar one and what your remedy was or any suggestions on where to look. Also, how do you test these coils ? I can not seem to find anything these, all I could find was the COP versions.

Oh, and my laptop does live data logging and it seems that its running rich, which if it was a fuel blockage somewhere wouldn't it run lean ?
 

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Update - After posting this yesterday I ran out to get some fuel injector cleaner with my other car. I got some Lucas Power Clean and used about half the bottle, it says 1 bottle treats 30 gallons. I then went filled the tank up because it was at about 1/4 and thought maybe I had some bad fuel or water in the tank. I drove the car to work today, approx 19 miles. It seems to run the opposite, at idle it seem nice and smooth and then sputters on load or going less then 35 mph. Where as on Saturday it was the opposite, rough idle and running ok but rich at 15 to 25 mph in the snow. After 40 or so today it smooths out again but anything higher than 50 it starts to miss quite a bit and the CEL flashes.
 

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Usually when a coil is shot, the resin on the bottom shows hairline cracks. I'd check the plug for the coil--I've replaced two and they've both given the same symptoms as you describe. They seem to fail right where the wire meets the rubber plug. Quick test--wiggle the harness while the car is running and see if you get a misfire. If yes, you've found your problem.

To check if the wire/boot is your problem, just swap the wire with another and see if the misfire transfers to that cylinder. If it does, then it's new-wires time.

The misfire is causing it to run rich...not enough spark for the fuel available, so it blows out instead of combusting properly.
 

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Thanks for the reply. I got it going just before you posted but was't around a computer or ipad to reply back. Apparently both the old and new coils were both bad. I found a place not far off my daily commute that can test coils. The old one was just barely producing enough voltage to ignite the fuel gas mix according to the specs the computer has for the tester. The new one must be off or not compatible for the car because it was off too. The tech's guess was the that the no 4 spark plug wire had just enough resistance to make the spark plug not spark or not enough to fire the cylinder and the coils performance was slowing going down. I exchanged the coil and installed it and it seems fine at the moment. Now thinking about it the fuel mileage was slowing going down, thought it was just because of the winter blended fuel, but if the coil was going that could cause it.

Also, you're right about the fuel mix being rich if the cylinder wasn't firing. I was just worried about something clogging an injector.

I hooked the laptop up and it says it's running normal, not lean or rich. I didn't clear the codes but they're in as stored. I'll see if the check engine light goes out on its own after a couple starts.

Thanks for the input. Now I just need to find the slight exhaust leak and the power steering fluid leak.
 
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