Just bought a late '99 F350. During a diagnostic drive with a Ford Mechanic, the mechanic pointed out that I had a fuel nock. Said it was probably caused by low fuel pressure, but that it was definately a fuel nock and would not hurt anything. Only does it at low RPMs. Guy purchaced from just put injectors in it a year ago, all the same part #. I have read that #8 was supposed to be different and some say that it doesn't matter. Not sure if nock is related to this. Any one run across this???