What would cause a rough Idle. on 1994 Mercury Villager

The van runs fine and has plenty of power on acceleration. Idles at normal rpm, Idel air control motor is working good, egr valve works good by manual depression. Vacuum is steady at 15 inches of mercury. Inspected spark plugs, no problems found, cleaned rotor, distributor cap and inspected plug wires. No emission codes. It's not to bad in park but is very noticeable and sounds like it's going to die at a stop in drive.

Asked by for the 1994 Mercury Villager
15 inches of vacumm is not acceptable. You don't clean a ignition rotor. When was the last time you changed the plugs? When was the timing belt changed?
When was a fuel ignition last performed?
This is a 94 mercury villager and an article I have from alldata says that 14 to 22 inches of mercury is normal at idle. The distributor cap was cleaned of build up on the contacts. Plugs are 25000mi old and are platinum, they have some beige color buildup on the insulators. I don't think the timing belt has ever been changed. What is fuel ignition?
When you clean a ignition rotor you widen the gap between the rotor and distributor cap. Hence requiring the ignition system to jump a wider gap which means it requires more voltage. You say you read that 15 inches is acceptable, it is not. Your villager is 18 years old, I strongly suggest that you change the timing belt before you have to change the engine. It is an interference engine. What I also meant to say was a Fuel Injection Service.
it's almost july.I wonder if he bought another motor yet? ..15 inches of vacuum is acceptable if you have restricted exhaust or retarded timing (for any number of reasons lack of maint. eh?
Found the problem, Intermitent knock sensor. back to normal. Thanks for your help.