Chrysler Repair: Idling issue at stop when in gear, throttle position sensor, chrysler cirrus

Question
I have a 1995 Chrysler Cirrus and when I come to a stoplight in gear the Idle will drift from 750 rpm down to about 600 rpm.  I normally never noticed this problem, but recently installed a new oil sending unit for the old was bad and my oil light now will flicker on when the rpm drops down to 600 at idle.  When I have the car in nuetral or park the idle goes back up to 750.  Also when the AC is selected, the RPM is normal.  I have replaced the IAC with no difference.  One other symptom I've noticed is the rpm will drop lower and engine almost stall when I turn the steering wheel when not moving.  I'm considering that I may have a bad Power steering pressure switch but did not want to replace unless I was sure that was the problem.  I have recently replaced the EGR valve due to a check engine light and code for egr solenoid short or open circuit fault.  I also have replaced the PVC valve and cleaned out the throttle body.  There are no fault codes or check engine lights which appear now.  I hope you can help me out with this issue.  

Sincerely,

Jim Higgins

Answer
Hi James,
Several possibilities come to mind. First, I would take a look at the vacuum line to the power brake unit and verify that it is tightly clamped at both ends. Then try leaving the car in drive while at idle but use the parking brake to hold the car in place rather than the foot brake and see if the idle still is too low. If it was then that would eliminate the suspicion about the power brake unit, otherwise there could be a vacuum leak in it that is causing the engine to falter when you step on the brake pedal.
The other idle control factors are the the throttle position sensor and the neutal-park safety switch. The powertrain controller uses both those inputs to adjust the iac for the idle condition. You could verify that the connection between the black/light blue wire at that safety switch (mounted on the transmission in association with the gear shift rod) is connected electrically to pin 76 of the powertrain control plug, and that the switch is alternately grounding or letting that pin float depending upon the position of the gear selector as it should. The TPS should read in the range of 0.5 to 1.0V (orange/light blue wire) with the ignition switch "on" (but the engine not running) with the valve closed (at idle positon) and the voltage should rise smoothly as you open the throttle valve and return to the previous closed value when the valve closes.
The control of idle is complex because it uses data from the engine coolant temp sensor, and the TPS, while the controller makes adjustment in timing and injector pulse width, and of course the IAC motor's position controls the air input. You have eliminated the IAC as a problem by replacing it (unless you have a vacuum leak which should usually cause a higher rather than a lower idle), so the balance of the sensors, switch input, and the vacuum leak issues are what is left to consider. The ECT sensor might be off calibration, for example, without causing a code to be set. It should read across its pins 700 to 1,000 ohms at 200F, or alternatively the voltage on tan/black wire should be 2.4V at that same temp. No sure answer here, but try checking those out.
Roland