well, here are the symptoms -

> car starts and seems like it is running on only 7 cylinders - it
doesn't matter if the car is hot or cold when started. however,
there are times when it runs well, although more often than
not is runs poorly.
> rotors and caps replaced at mechanics suggestion - also
checked spark plug wires - could find no arcing
> if i put the car in drive and sit there with the brake on, the
problem is worse than just idling in park. in other words, its
worse when under load.
> just noticed this today - problem is aggravated by running
blower fan on high when ac is on. no problem when just using
the fan without the ac on - did not try heat to see if it does the
same.

can anyone shed some light on what the nature of this problem might be? would a bad alternator cause this to happen? could it be the wiring harness? thanks in advance for your help.

When were the spark plugs changed last time? You might want to check if the spark plugs themselves need changing.

the spark plugs were checked before we replaced the cap and rotors. they looked to be ok.

What exactly is the problem? It just does not go.

very comon that the coils are failing under load, Lots of problems with that in the 400e and 400s. Try replacind both coils. I would do and resistance test across coils. But I have seen so many do what you are explaining. Even though engine runs, the load can break down the spark

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thanks very much for the info - i will try replacing the coils and see what happens.

I had the same problem twice:
Once was due to bad spark plug connector on the pspark plug. The other time it was bad fuel back pressure regulator that sits on the fule rail for the injectors. You can change that if you do not have away to check the fuel pressure. The part is inexpensive.

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2010 ML350 Bluetec
2012 Mustang Convertible

Thanks for the reply rafi - i will see first if the coils are shot, then check each spark plug wire to see if any of them are bad. it sounds more like an electrical issue than a fuel issue, although i haven't ruled out that possiblity yet.

Yea, that is what I thought too but it was not the coil. I am willing to wager that it is not the coil.

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2010 ML350 Bluetec
2012 Mustang Convertible

i guess i may try replacing the regulator first - it sounds as if it will be less expensive than replacing the coils. i guess it's just a process of elimination - i'll find it sooner or later - hopefully
sooner.
thanks again for your imput