1999 Ford Taurus Engine Code 1518

Possible faulty idle air control valve. If replacing this doesn't cure problem, dealer is probably needed.

This control is driven electronically. The computer opens and closes the valve. Sludge buildup will cause it to stick open, if IAC valve is not the problem.
The computer uses proprietary software, which seems to be unavailable to small auto shops, and do-it-yourself people. Take the vehicle to a Ford dealer where they can re-program your computer, telling it to cycle that valve during driving, to prevent the build up of sludge.

Hope this helps.