ym42

I have been dealing with this for a while. I have a serious misfire when the engine is cold. I have been trying to trace it for a while but no luck so far. I thought I traced it to one bad spark-plug but after replacing it its the same story ( I am using Champions now, just replaced them about 10K ago ). When I press the accelerator down to certain degree the engine immediately develops a bad and SUDDEN misfire, which only happens after I cross a certain invisible threshold with my foot. There is no problem when the engine is warm. And the transition from smooth operation to a steady misfire is very sudden, and I feel it might be more than one cylinder but I am not sure - feels like I am losing 20-25% of the power. I think something electronic... Where should I start?

autos221

My traction control sometimes makes my car misfire, good thing mines a manual so i can restart it while coasting

BigDinTexas

How did your plugs look when you swapped them? Did they look rich/lean?

When was the last time you replaced the fuel filter? You have over 100K miles, so you probably did it already, but it's worth asking.

What about the COPs? Have you replaced them?

Don

tbar

Have you pulled and looked at all the spark plugs.

ym42

Yes, I looked at the plugs when I changed them, all looked the same but I am no expert in this... How are they supposed to look? I have one in front of me now... The filter should not matter in this, but yes it was changed something like 30k ago...

lenko

I had almost the identical problems with my 06 GT and Champion 7989.
Just make sure you check them closely. Here's what I posted on another site.

Bought mine from RockAuto about a year ago, one plug the little cone slipped over the spark area (brand new in box). Rock Auto wouldn't warranty their sale and had to go to Champion Canada who arranged to replace locally. The past few weeks, Oct 2011, my 06 Mustang GT has been driving me crazy, running rough and misfiring after cold start. Finally, finally after trying everything with Maf, throttle body, different tunes and almost checking phasers, timing and worrying about broken comp cams valve spring, I decided to put the original motorcraft spark plugs in and voila, car is as smooth as silk again. Examined the Champions 7989 and found on one that the metal neck just above the threads and the socket head was cracked and broken on both sides. Absolutely no reason for this to be broken as car had ran well for over a year and the plugs had not been touched. My experience with Champions. Looking for a good replacement as neither spark plug Motorcraft or Champion seems reliable for these cars.

ym42

Yes, I thought I have traced the misfire to one spark-plug, but after changing it for another Champion its still there ( do not know if its the same code or not )... How I traced it at first - moved the plug and the code moved with it... I am pretty tired with this problem chase already, took me something like 5 hours already... Could I have messed something up while adjusting the gap? Used some special tools? The electrodes area is very clean on pulled plugs, the metal cylinder area that extends from the threads has some soot on it but I guess its normal... Also I have to mention that even before, when the car was new, it would still run rough when cold, sometimes completely stall if I started driving immediately after starting it with the engine cold. It did not happen all the time, only when cold ( even in the summer ) and driven immediately after start. If it stalled it would be hard to start, and after start there would be lot of black smoke from the pypes... I thought its kind of normal, and I read some other people complaining about the same problem, someone mentioned could be the o2 sensor/heaters not reaching normal temperature, and advised to warm up the car for a little bit... By the way, I do usually warm up the car for a minute or so before driving now, still would misfire... I do not know if it would stall if I drove it immediately like it used to...

Simon1

You either have:

A vaccum leak (pull and tug on every vacuum line and inspect them thuroughly)
A bad coil and/or plug
A loose air intake (which is what I am leaning towards)
Possibly a dirty throttle body.
Double check the coils are plugged in properly.

lenko

The broken spark plug I had was not easy to see. It was just a hairline crack just above the socket area on the metal. I simply removed the Champions and installed the motorcraft plugs and it ran better. I didn't replace that Champion to see if it was the problem. I still want to run Champions for the one piece factor and have a friend who went SC and is giving me his Champions at which time I will reinstall and test them, but this will be later. It sounds like you have had rich running right from the start, but for your current misfire, I would check your plugs carefully. If you have the old plugs, reinstall them and see if that helps. Good luck.

p.s. I used my XCal2 and also my laptop diagnostic program and they did not show and codes on my misfire, so my cracked plug was undetected.

ym42

I already checked my TB, actually cleaned it, it was clean because I had seen it dirty before and felt the effects of that. Intake is tight but I can retest it. My idle is normal and mpg on highway are good as usual. The only other abnormality that I have is that when I drive rather slowly on a level road, around 45-50 mph I can feel the car surging a bit on and off which kind of puzzles me because I think it's caused by one cylinder misfiring at times. It kind of feels like it gets some different throttle input when I'm holding my foot super steady. It's almost imperceptible... Took me a long time to believe in what I feel. Of course I should add that I also have a cat code, p0430... Cat efficiency below treshold. Replaced the front o2 sensors, should I replace the rears too? Is there any function that rear sensors have that affects how rich the car runs? That's what I'm feeling...