I have a mustang with 175k miles and had the problem you all are having-misfiring cylinder, fixed by changing old plug with carbon (yes, CARBON) buildup only to replace again in a month or 2.
One of 2 problems, probably both:
-The valve cover on 99-00 mustangs on the drivers side has a design flaw that allows oil to be ejected through the PCV valve into the upper intake manifold. Here is a good explanation, and includes possible solutions:
http://www.miracerros.com/mustang/pcv_filter.htm
This was not my problem, but it can't hurt to fix.
-leaky fuel valve seal- the seal leaks oil into the cylinder, which causes the car to burn oil and produces carbon buildup in the cylinder and on the plug, eventually fouling it out completely. I bet your car smokes when you start it but not so much when it runs, right? This is because when you turn off the car, your seal cools, then shrinks, exposing the leak. The oil in the head leaks through the seal after it cools and lays on top of the piston. When you start the car, all that oil burns off-thus the blue smoke, and the fouling plug. Once the car warms up, the seal expands, closing the leak, and the smoke goes away, or reduces significantly. The oil that has to be burnt during startup is what causes the plug to foul so quickly, and if the problem is not corrected, you will have more problems requiring more money most likely in the head-such as lifters or valve guides wearing oblong.
Solution 1-replace the heads. They run about $200-$250(depending on the year, since the 99 had duel intake ports and the 00-04 had single intake ports--don't believe me-call any Ford parts center and have them look it up, since even they normally don't know this) or you can get performance heads from crt racing performance, supersix motorsports, morana racing or rpm something or other-$550-$1300, but lots of fun!
Solution 2-replace all valve seals-they cost about $50 at any chain auto parts store, and about half a day's time. Requires you to take off valve covers (so you will need gaskets, or use the old ones, I wouldn't-cost about $30) and an air compressor that will blow at least 90 psi (a 2 horse power one should do) and an attachment for it that will screw into the spark plug hole-$15-$30. Consult your haynes or chiltons auto manual for instructions.
Solution 3-(what I did)-change your oil every 1000 miles RELIGIOUSLY and with every change add RESTORE-comes in a silver can, runs about $7 at walmart or any auto parts store. RESTORE contains additives that help restore compression, gaskets and the like. There are several other products that do the same thing, but this is what I used, its been around for a while. The problem should be fixed withing 5 cycles. If it isn't fixed after 10, replace the seals. I have driven the car about 10,000 miles since I did this with no RESTORE and the problem has not come back.
Also, you may notice right now that your oil gets very dirty very quickly (about 500 miles after the change). Check your oil now and smell it-you will most likely smell fuel. This problem also comes from the valve, and you will be able to tell if it is working because this will slowly go away. I don't know where the dirt comes from, most likely carbon buildup, but this goes away too. My oil looks new after 3000 miles since I did this.
In the mean time, a few things you can do to fix the rough idle, which is a completely different problem is:
1)clean the IAC with brake cleaner (no it doesn't hurt the electrical components, you can clean most sensors with it because it evaporates quickly). It is right behind the throttle body on the upper intake manifold on the backside, is about 4 inches long, cylindrical, and will have a wire coming off it. You can remove it with a stubby phillips head screwdriver, only 2 screws. Spray and wipe the inside clean as best you can. Unplug the negative battery cable before you do this and leave unplugged for a few hours. This completely wipes the computer and makes it write a new MAF (mass air flow) table among other things, allowing it to adjust to the greater amount of idle air flowing into the engine.
2) While the battery is unplugged, take the worthless POS air silencer off the side of your airbox (the rubber thing going from the air filter housing into the fenderwell) and replace your airfilter with a performance air filter. You don't need a K&N, they are all made from the same thing-cotton. Or better yet, buy a cold air intake kit from ebay. Even cheaper-make your own! There is plenty of material online about how to do this, mine is made of PVC.
3) Also while the battery is unplugged, spray the MAF sensor with some brake cleaner, especially if you don't replace your air filter correctly/on time. When you remove the airbox, lift up the MAF housing (big black plastic housing still attached to the car), remove the front (4 clips, flat screwdriver works fine), Now, see the little hole at the top? Yeah, spray in there. DO NOT TOUCH THE SENSOR (inside the hole) WITH ANYTHING IT WILL ABSOLUTELY BRAKE!!!! A new one is about $300.
4)Replace the fuel filter. This is a pain because Ford decided to put clips INSIDE the fuel line (because for some reason they thought the ones on the outside weren't good enough, which they are). You will most likely end up ripping them out-don't panic, they are pointless, the clips on the outside are fine and do not allow the filter to shift at all.
5)Run fuel system cleaner in your next 2 tanks of gas.
6)If you are feeling particularly daring, remove the upper intake manifold (very easy, can be done in half an hour). Notice all the black stuff in the runners-get it out. Brake cleaner works fine to loosen it, but you will need something to scrape it out with. A plastic spoon works pretty good. Also, while its off, clean the runners out on the top of the lower intake manifold. CAREFUL NOT TO GET ANYTHING INSIDE OF THE RUNNERS OF THE LOWER INTAKE, AND DO NOT USE THE BRAKE CLEANER FOR THIS! Like the dirt you will notice has accumulated in the crevices between the upper and lower intake-there will be a ton of it. Also, careful with the gasket! It is a wire mesh gasket so if you keep it intact you can reuse it (some would argue this, but I did and I have no leaks). You do not need gasket maker to put it on. All that will do is make is hard for the next person to get off. Have fun, this will take a while. Do all this while the battery is unplugged.
---On a side note, don't attempt to remove the fuel rail to clean the injectors unless you have de-pressurized your fuel system, or like the feeling of gas in your mouth and eyes. This is easy, before any disassembly, open your trunk, behind the drivers side tail light is the inertia switch (shuts off the fuel pump if you are in a bad enough accident to trigger it). Unplug the switch. Start the car. When it dies, the fuel system is de-pressurized. Remember to plug it back in or your car wont start.---
7)Replace the pcv valve-about $5
8)Replace vacuum lines
9)check emissions codes, you may need an O2 sensor or something.
If it still idles rough, have your fuel system serviced. Still idles rough-do a compression/leakdown test. Take it somewhere for this, the $100 is worth it.
I wouldn't recommend anything that I wouldn't do. That being said-I have done all of this accept the fuel system service and compression/leakdown test. My car went from your situation to running like a champ.
Hope this helps
Hapkidoan
