My 92 Explorer XLT started making a whistling noise followed by low idle at stops. It turnes out thre is a whole vacuum hose missing but I have no idea where the second end is supposed to connect!

Its one of the small connections on the bottom of the "christmas tree" that comes off the block. The few vacuum chart diagrams I've found only show the 3 primary lines - none of the minor ones.

I am in California and it may be smog stuff - any ideas?

 

I know that on my '92 there is at least one line on the christmas tree that is simply capped. It could be that you lost the cap and there is no other line to connect. I had a problem with mine when it spit back as I was cranking the engine and let off the starter at just the wrong instant resulting in the engine spitting back through the intake and blowing the cap off. I simply replaced the cap. I've also read about some folks having problems with the modulator valve on the transmission allowing transmission fluid to be pulled up into the intake. This will result in a swelling and softening of the hoses which tends to make them more susceptible to coming off.

 

I thought it may have been capped too but when I plugged it, the engine choked out so that's not the case. And I haven't had any problems with transmition fluid.

 

At the Christmas tree, you should have the following vacuum lines:
1. PCV valve
2. Power Brake booster
3. Cruise control
4. Transmission modulator valve (if automatic)
5. Fuel Pressure regulator
These are the main ones I can think of.
The line to the fuel pressure regulator has a "T" that feeds off to the inside of passenger compartment for some vacuum motor functions in the heater/AC duct works. There is at least 1 port on mine (and possibly 2) that are just capped off on the tree.
Dialtone

 

One thing to consider if the engine choked down when you plugged the tree is that your IAC valve is probably stuck. Even if you hook up a hose, it will not have a continuouse flow of air, it will only pull the air out of the vacuum cicuit until it is evacuated. I don't see the fact that the engine stopped when the tree limb was plugged to have a relationship to whether it's a cap or a hose. If the IAC valve is stuck shut, the engine can be getting enough air to run at idle through the open line. I would suggest looking at the IAC sticky (which is now labled "Read this first" at the top of this forum.