I replaced plugs, wires and ignition coil about 6 months ago because of rough idle. the ruff idle continues and now there is a swishing sound as it idles on the drivers side of the engine. I have seen multiple possible reasons for this, and I am trying to narrow down what to attack next. It definitely sounds like a vacuum issue, but the question is, what and where is it?

Explorer has 230k miles on it. V6 4.0 L SOHC

 

Start with the simple stuff like the vacuum lines. There's usually a vacuum schematic under the hood that shows where the vacuum lines go. Have you noticed a change in how your power brakes or the climate control works? If your brakes are stiff after the engine has been off for a while, then suspect the brake booster or the vacuum line, but it could also be a bad check valve. Similarly, the damper door and water valve should operate off of the stored vacuum in the system (there's usually a reservoir/accumulator), but that could also be a bad check valve. The evap system also uses vacuum.

Also check your air intake snorkel. Maybe it's not tight. The PCV valve and the EGR are both connected to manifold vacuum so they are potential suspects.

I'm probably missing stuff, not to mention I haven't included things like injector o-rings and intake gaskets. Start with the easy and quick stuff first. An EVTM (electrical and vacuum troubleshooting manual) would be helpful. Documentation is good.

 

Mikeman!!! thanks for that!! going to go through this tomorrow morning and see what I can discover! Great feedback, will update you as i make progress.