98 taurus engine idle problems

98 taurus SE duratec engine. 115k miles. this just started two days ago. i turn my car on and it seems to be running fine, but when i put it in drive, the rpm goes down between 500 and 750. when i accellerate, the engine gets back up to 2,000 rpm fairly quick, but when i slow down and am stopped at a light, it goes back down below 1,000 rpm. it aggressively shakes sometimes when it's idling that low. i haven't heard any odd noises coming from the engine. any ideas?

Sounds like you have either an engine misfire or a vacuum leak. I suppose it could be a fuel issue, but I wouldn't expect the engine to rev readily then. How recently has the car had a tune-up? That may be a good investment, have the spark plugs and plug wires replaced, along with the fuel filter and air filter. Put a bottle of fuel injector cleaner in the fuel tank while you're at it.

While under the hood, inspect the vacuum lines (there should be a vacuum diagram in the engine compartment) and replace any that appear dry, cracked, brittle, or obviously damaged.

Make sure the Mass AirFlow (MAF) sensor is plugged in and the air intake hose is connected. Disconnect the hose at the throttle body and crack the throttle plate wide open. If there is a bunch of brownish-black sludge in there, pick up some STP Throttle Body Cleaner and get the back of the throttle place and surfaces of the intake manifold cleaned up. This will make the car a bit difficult to start and it will run rough until all the loose stuff gets burned off.

-Rod