>The last several months my wife's '99 Windstar with the 3.8 V-6 (60K miles)
> >has been experiencing pre-ignition knock, mainly low speed, under load. At
> >first using a higher octane fuel helped but eventually even that made little
> >difference. I was about ready to turn it over to the dealer and hope I
> >didn't get my wallet emptied, but first I did the 60K service. In addition
> >to the oil / filter change, I replaced the air filter and the knocking is
> >GONE. The old air filter (replaced at 30K) was dirty but not excessively so,
> >but apparently it was plugged enough to mess up the fuel mixture and
> >possibly caused the computer to change the timing?!? Whatever - I'm just
> >glad its OK now.
> >
> >Just goes to show you, it is all too often the simple things.
> >
> >
> It could have been the dirty air filter was screwing up the MAF sensor. The
> dirty air passing over it could have caused the MAF to give false readings to
> the computer.
Isn't the MAF beyond the air filter? So how would dirty air get to it? As air filters get dirtier they filter better, but flow less (i.e. smaller holes for air/dirt to get through). If anything I'd think a dirty air filter would make the mixture richer which would reduce knock/ping and decrease gas mileage.