In my 03 Mercury Sable, I had the same problem and it was the PVC elbow hose. It had a huge hole in it and I saw online that this is a problem with the 24V sable.
Idling in neutral.
The automatic transmission fluid is checked idling in neutral.
you have a fault code on emission systems
Yes, check it on a level surface with the engine running and the gear shift selector in neutral or park. Check it after the engine has warmed up.
Check the condition of the accessory drive serpentine belt, belt idler pulley and belt tensioner pulley bearings.
Check engine light usually indicates a problem in the emission control system - have vehicle scanned to determine the problem
The idle is not adjustable.You have other problems if it's idling to fast are to slow.The throttle plates are calibrated / preset at the factory.Is the check engine light on ?? If it is, then you need to have the engine scanned with a engine scanner to check the emission system and sensors. It will also help your idling problem after you repair the problem that made the check engine light come on. If you don't find your problem in that area. then PLEASE ask your question again.
On a 1997 Mercury Grand Marquis : Check the Vehicle Emission Control Information ( VECI ) decal in the front of the engine compartment , for the type of spark plug used and the spark plug gap
check the engine mounts
Engine idling in park
check engine light means a problem with the emission system
Check the Vehicle Emission Control Information ( VECI ) decal in the front of your engine compartment . It will show what type of spark plug to use and the spark plug gap