RE: 3.5 High Rev Problem
 

More info on the idle problems with the 3.5. Compression is good in all cylinders. Plugged in a code reader and it indicated a bad idle air sensor. Could that cause a lack of vacuum from the upper plenum?

RE: 3.5 High Rev Problem
 

It is what sends the signal to the PCM to open the idle air valve. Most definitely could be the cause of the entire issue.

FYI, you don't have vacuum because your engine is running at high RPM. Vacuum is diminished as RPM increases. (700 rpm = Maximum vacuum, 4000 rpm = Almost no vacuum) The idle air valve being open because the sensor told it to will cause this. It also happens when you put your foot on the throttle. The motor and the vacuum (mechanical parts, not sensors) don't know the difference. While we control most of these parts with computers and electronics, the parts themselves must still conform to the laws of physics.

Replace the sensor, AND check the idle air valve. It could be stuck open.

RE: 3.5 High Rev Problem
 

What exactly was that code? I don't think there is such a thing as an idle air sensor. There is an idle air control motor that gets its instructions from the engine computer. The computer uses engine coolant temperature, gear position, battery voltage, ambient/battery temperature, vehicle speed, throttle position and manifold absolute pressure to determine aproper setting for the idle air control motor. The idle air control motor is located on the throttle body, just under the intake/throttle area.
The sensors that control this generally have self-checking routines, and will generate their own error codes, so your problem may be isolated to the IAC (idle air control motor). Make sure it's plugged in properly. Consider replacing it.

RE: 3.5 High Rev Problem
 

Just replaced the idle air control valve with no better results. I am leaning towards a big vacuum leak somewhere. Do the vacuum resevoirs go bad? I'm getting frustrated!!!!!!!!!!!!!!!!!

RE: 3.5 High Rev Problem
 

Was that code reader you applied capable of reading Chrysler codes? You should have gotten a code for "Target idle not reached" - P1294 if the engine would not achieve a proper idle speed.
If you have a significant vacuum leak, you should have gotten a P1299 code.

There is a vacuum reservoir, it operates off the vacuum going to the brake booster and provides vacuum for the speed control. The check valve for the wholevacuum system is located at the booster. You might try disconnecting and plugging that line. If that solves your problem, then the trouble is going to be found back up somewhere behind the plugged location.
The reservoir itself has no check valve. It's located in front of the battery behind the front bumper fascia, so it's not real easy to get to.
Also check the EGR line as that's another possible source of a vacuum leak.

Keep us posted. We share your frustration and will share your joy when this is finally fixed.

check your pcv