I have a friend with a 2007 MKZ 3.5L. He was getting a flashing engine light with a notable engine misfire. The air conditioner stops working also when this happens. He was told it was a coil pack and had one replaced. It worked for the first ride home then after about thirty minutes, he went to the store and it came back. He decided to replace all of the coil packs along with all six plugs. Again, it worked fine for the first test ride to the local Wendys. When we started it after lunch, it started missing again along with a flashing engine light and rough idle. Any ideas as to what this could be are greatly appreciated.

 

Before we jump and start throwing out explanations and cures more info is needed.
Who told him it was a coil and where did the coil come from.
Then where did all the rest of the coils come from.
This info is important because if someone bought economy priced coils these results can be expected. If there was a code provided, which cyl was the code for the first time and was it the same cyl again? Don't laugh when I ask what brand of spark plugs were installed, there are very good reasons for asking that also.

 

Also, to add to what "HANKY" had said, it's not just what brand of plugs, but what type of plugs. You'll probably find that the vehicle came with NGK iridium plugs as the OEM plugs as iridium plugs take less energy to fire and are easier on the ignition system.

 

Thanks for responding Guys, First off, The shop he brought it too coded the #3 cylinder so they changed it. He bought Ford Motorcraft plugs and the coils from Ebay for around $200 for all six. He did some calling around and was told that if the shop that changed out the #3 coil did not reprogram the computer, it probably fried the new coil pack as the computer has a contactor the defaults and need to be reset. Not sure if I believe this. He also looked in the glove box and found a history of coil packs being changed by the Ford dealer along with doing a computer reprogram. He is now looking at buying a pre-programmed computer using his vin info as he believes that due to the history of "coilpack - reprogram" that the computer might be frying the coil packs. Again, not sure if I believe this...... What do you think??

 

Just as an addition, his car history includes being at the dealership twice for a coil pack and computer reprogram visit. Once in 2009 and once in 2013. When we started the car right after we changed all of the coils and plugs, it ran fine (no missing at all with air on and multispeeds). The engine light stayed on and never flashed. We though that maybe we neded to bring it to Autozone to have them reset the light. We stopped after about 20 minutes to eat lunch and came out to start it. It immediately started missing as soon as he started the car and the engine light started flashing just like it did before we changed out the packs and plugs.

 

Few other things to check. Has the fuel pressure ever been checked to see what it is when cold and hot? May have a bad injector that shows its face after being heat soaked. May also have a tired fuel pump that is showing is face with a borderline injector that happens to be for cylinder #3. What about cold and hot compression checks? #3 could be a low compressioned cylinder. Usually about 50% of "BAD" coils replaced are actually good.

 

I have never heard of a computer wiping out a coil, but it is not uncommon for a defective coil to wipe out a PCM (computer). Voltage spikes from a defective coil can do that.

 

if # 3 is low on compression what's my next move.? Thank you

 

This is what I would do.
By the way most folks don't have access to a leak down gauge set.
What a leak down test does is, the cyl that is suspect is brought up to TDC on compression stroke and air is injected through the spark plug hole into the cyl.
If you hear air leaking into the crankcase , there is a piston/piston ring sealing problem.
If you hear the air leaking out into the intake manifold, a possible intake valve problem.
If you hear air leaking into the exhaust, exhaust valve problem.
If you notice air into the cooling system , good possibility of a head, engine or gasket problem.
Might be a good idea to have a qualified shop check this out for you.

Also, the camshaft should be verified good as far as lobe wear on camshaft.