98 Winstar rpm hunting

Hi all,
Installed new plugs recently at 111000 miles. Engine rpm fluctuates + - 100 rpm. Can feel it at idle, stop light and during cruise. Any ideas where to start troubleshooting as it has no CEL.
Thanks

did it start after you installed new plugs?

Yes it has always run but the engine is not smooth like it used to be. When I disconnect the battery the problem lessens but does not go away entirely and gradually gets worse.

I'm trying to find an idle problem of my own at the moment. However, if it started after the plug change someone may make the assumption of a disconnected vacuum line. There is a vacuum regulator near the coil pack that distributes vac around the engine compartment and into the cabin via some small rigid and colored vac hoses. There is also the breather to valve cover from the air cleaner housing. I'm going to guess if it is vac it would be the backside of the engine where it is harder to see.

Windstar plugs are hard to change and some things do get bumped so that may be a place to start? At least to verify nothing is disconnected.

This problem was before the plug change. That's why I changed plugs - which were worn.

So the problem was before the plug change and persists after the plug change then.

Ya, I'm not sure because I'm having similar problem. For me it is the worst when I am in drive with my foot on the brake and it pulsates fairly regularly. On the scan tool it is only moving 30 ~ 50 RPM but that is with sensor refresh at 5ms. It may be more than that because the frequency is too fast. I'll have to play more.

Either way it is frustrating me too but I'm in the middle of moving so don't have time to mess with it. I did discover that the evap vacuum line is completely wasted away. That is, the evap vac from manifold to purge and purge to the steel line. When pressed further back the same type of vac line is used (and corroded) right back at the cannister.

If anyone has other ideas where to start we'd both love to hear them....

Guys, check the IMRC actuators (valve). It's a VERY common source of vacuum leakage, and the symptoms you have described fit the bill perfectly. There are several threads here detailing the process. I am trying to decide how to deal with the problem right now on my '98 Windstar 3.8L...

(EDIT)
Here is good post on diagnosing and dealing with the problem
http://www.automotiveforums.com/vbulletin/showpost.php?p=4435446&postcount=5

Sorry, should have mentioned mine is the 2001 which has the electric actuator (just one for both sides). So no vac leak there, but I still suspect the source in my situation may be a vac leak. No codes, don't have the isolator bolt issue (as far as I know because I have the original new part valve cover), seem to have a good IAC, and no real clues. But vac is always a good first start.

I disconnected and plugged both vacuum lines. I wired the butterfly valve actuator arms in the open position. I took it for a mile run – so far it seems to have cleared up the hunting rpms. I will update tomorrow or Monday when I have taken a longer trip to see if the problem went away completely. I do hear a sucking sound when the engine is running but for now I am not sure if that is in the intake or hoses behind the engine. No sounds to the front.
Thanks for all of your help and assistance.

So far so good. My rpm hunting seems to have been corrected with the disconnecting of the vacuum lines to the IMRC actuators and wiring the butterfly valves open. No noticeable affect on engine performance that I am aware of. Thanks to all for your assistance.