CODE READING # 3 MISFIRE ON A 1999 FORD F150 HAVE ALREADY CHANGED PLUGS BOTH COIL PACKS AND WIRES IT GOES AWAY AND RUNS SMOOTH FOR ABOUT 20 MINUTE THEN GOES BACK TO RUNNING ROUGH AGAIN

you have a bad spark plug wire on cylinder 4 or your ignition coil
(coil pack) is bad.

here is a good guide to testing coils and wires if you need it
How to Test the Coil Pack Ford 3 0L 3 8L 4 0L 4 2L

Whats the code u are getting, and what did u tried so far Let me know if u still need help . First thing u need to try is take all spark plug wires out and inspect if they are ok take ignition coil out check that for cracks , if there is no cracks check coil with multi meter for ohms primary and secondry ohms let me know if u have not checked that i can tell u how to do it if dont know

I would check your coil packs and plug wires. These newer motors even the spark plug gap will cause problems. Change your fuel filter. A easy way to check for misfires have your codes read but if there is no codes it has to be something simple like plugs or wires.

looking elsewhere for the problem

measure resistence between companion outputs. they should be marked on the coill pack.12 ohm nominal. mesasure the resistence of the input terminal. 3 ohm nominal. i recently had a coil pack go bad on a '97 crown vic where the number 8/2 tower was reading good resistence when i kept the meter probe steady. if i moved it around while still making contact, it would change. i was getting a cyl. 8 misfire and found a bad wire and the plug was worn funny, so i changed the plugs and wires. this did not fix the underlying problem. the bad coil pack. the vehicle was misfiring at cruise but upon acceleration no misfire was apparent. what are the symptoms the truck is displaying

I have a 1999 ford escort 2.0, its got a miss on cylinder 3 and 4. I changed the wires and the coil pack but its still missing. When scanned I get code p1506, p0303 and p0304

Has anyone found the problem? My 2003 has #3 cyclinder misfire and I have replaced the plugs and wires. It runs rough at idle but smooths out at highway speed. Anyone have any knowledge on this? This sounds like a common problem.

Could be the coil pack should be read in ohms about 7 to be good

sme problem had to have it put on computer and codes reset

You didnt mention having changed the spark plug wires, so that is another item that could be culprit as well. As well, it could be a coil pack that is not working properly.

I cannot recall at the moment if that model year uses coil on plugs or if it uses coil packs. Either way, if it uses a coil on plug set up, take the one for the #3 cylinder and move it to say #4 or anyone of your choice and see if the misfire follows

If it uses coil packs, there is likely one coil pack that controls two cylinders. In that case just swap the coil pack with an adjacent one and see if the problem follows. Pay attention to where the spark plug wires go because it will be an issue if you get them mixed up. You just swap the coil packs around.

I would, however, start with the spark plug wires if you have not changed them already.