Where are the vacuum lines on a 99 mustang 3.8

My throttle position sensor was out of range even on new one had to slot the screw holes to allow proper idle speed .
Also vacuum leaks
'86 mustang 302ci irregular idle speed.
Loose throttle body or intake creating vacuumleaks.
Need to use a scanner to try to pin it down.
Dirty or faulty map or maf sensors
But I think you'll find rotten rubber vacuum lines.

Vacuum leak somewhere. Also there's a check valve in the vacuum supply line that is probably leaking.

Open hood , passenger side firewall area, look for a thin,black vacuum line, follow it and look for any breaks or bad connection to its vacuum source. repair as necessary.

Could be several diff things, might be as simple as a leak in the air condition duct work, turn off the air when you start it and see if it goes away. Also there is a vacuum line under the dashboard, you will have to remove the glove box to trace it down. GOOD LUCK!

If not the switch, you need to check vacuum lines near the firewall inside the engine compartment and under the dash. Just in case. I had problems with mine blowing only through the defrost because of a broke vacuum line. Good luck

First thing I would check is all of your vacuum lines. I had same problem and turned out there was a split in one of my vacuum lines. Once I repaired it, it worked perfectly again.
thanks,
Lee

www.autozone.com, free online repair manuals and reference materials.

You should have a thing that looks like a fuel filter close to the rear of the trans. called a transmission modulator, should look like this. .

normally, there is a diagram of the vaccuum lines on the underside of the hood for the emission controls. look there.

What exactly is it doing or not doing for that matter. Check vacuum lines going to the unit and see if you have any broken hoses. I'll try to get a schematic to you. It's best if you give me your email address so I can send it to you.