rattle spark ping possibly

The differences between 87 and 89 is marginal at best. Octane rating is simply a measure of when the fuel detonates or "sparks up" when in the combustion chamber. If your octane is too low, yes you will get a ping because it is detonating too early. Give 89 a shot, it won't make much of a difference or a difference at all as far as the ECM is concerned.

Rusty67 said:

Use a higher octane ? I've been wondering about this. Isn't the factory engine managment calibrated to use 87 ? On a stock car wouldn't using a higher octane be a waste of cash ?

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Under most conditions, 87 octane is fine. Under adverse conditions, knock will occur. The 4.6 SOHC has no knock sensor for spark retard. Higher octane gives you more protection from knock. Given the small price difference between 87 and 91+, I'd just run a full tank of 91 or higher octane and see if the problem persists. If not, you have two choices - start running higher octane regularly, or get it checked out. I'd recommend getting it checked out by a Ford tech either way.

FWIW, I have been running either 89 or 91 in my Mustang for years. The price difference is minimal. If you buy 500 gallons a year and pay an extra $0.10 for gas, you are paying an extra $50 a year. Only an extra $100 a year for shelling out an extra $0.20 a gallon for premium. You could make that up by simply taking it easy on your tires and making them last an extra two months. I get better mileage out of premium gas (1-2 mpg) as it is, so I fail to see it as a "waste of money." I consider it cheap insurance.

yep, every car is different. Some GT's like 89, some are fine with 87. My b1tch needs 91+