This issue has baffled me for over a month now and I am still no closer to discovering what's causing it.
I have a 93 Ford Taurus GL with a problem with rough idling (to the point of stalling if gas is not applied to keep it at a high enough speed) and, of course, stalling. The problem has persisted after I've changed the fuel pump and fuel filter, the EGR valve, the Idle Air Control Valve and the spark plug cables. Code readouts point all over the place, if the car generates codes at all. Codes, when they are logged, point to a vacuum issue. The rough idle problem does not always start first thing.
In the mornings, I can sometimes run the car halfway to work before it misses a lick. Sometimes it can go all day and not miss a beat. And then there are some days I don't get out of the driveway.
There is a whistle when the engine is idling in the driveway but, in the location I hear the whistling, there are no vacuum hoses to be seen. So, if there is a vacuum leak, I can't locate it. Not only can't I locate it, it is beyond intermittent.
Performed a leak test around the intake manifold and there was no leaking there. I have even cleaned the carbon from the butterfly valve on the intake and it has not helped the problem any.
I'm nearing my wits end here and, if I take it into a mechanic, I'd like to point to something more specific than 'it has an intermittent problem, possibly related to the vacuum'. Any ideas or help that you can offere would be most appreciated.