One problem related to gas recirculation valve (egr valve) has been reported for the 1999 Mercury Cougar. The most recently reported issues are listed below.
As soon as I bought the car used with 49000 it wouldn't start. Replaced the starter, still wasn't right. Had to replace the flywheel because the old and new starter never matched properly with flywheel. Then, the engine light came on. The egr sensor was bad. Replaced the sensor. All of this within one month of owning the car. Then a few months later coolant light wasn't just coming on, but staying on, had to replace coolant tank due to shorting of the sensor in the tank. Now, six months after owning car, engine light is back on.