1994 Ford Tempo

What kind of smoke? What does it smell like? It's likely either steam or burnt oil. You didn't mention the outside air temperature.
When was the last tuneup? Have your vacuum lines checked for cracks/breaks/leaks. Is the check engine light on? Does it work? Does this problem occur both when the engine is warm or cold? There is a air control for idle speed called the IAC valve. It's electrical/mechanical located up behind butterfly/throttle assy top rear right side of motor by windsheild. It bolts on with 2 little bolts.
The idle speed on car is totally computer controlled using sensors and the IAC.
The egr valve can also effect idle if it is dirty/sticking/leaking.
Also the TPS can be faulty, it senses the gas pedal position. Which determines fuel/air ratios which if 2 lean/rich can lower the idle speed. Idle meaning your foot is not on gas pedal.

These cars are notorious for valve cover leaks. The big silver thang right in the middle top of motor where you add oil. The cheap GOOD fix is silicone. Remove cover and clean edges of any oil and clean ledge on motor where it fits and use a bead of silicone all the way around to reseal it on. Take out your plugs and look at there color. If 1 is dark and wet looking it's dead. This will cause ruff running. The plug wire could also be bad. Was that included in tuneup. Did the car run well after tuneup? Having to add water means your losing it somewhere? If there is no external leak then have compression check as you could be losing due to head gasket leak. That cylinder would likely have less compression and make car run funny/rough. Watch exhaust during/after warm up for excess water drippage steam. In dry weather when warmed up no steam/water should be coming out of tailpipe.