84 CJ7, 99 4.0 slow to idle down

[melman]

I have an 84 CJ7 with a 99 model year 4.0 runs great but is slow to idle down, listed below is what has less than 30 hours
62mm throttle body and spacer
Throttle positioning valve
MAP sensor
Idle control valve
Cold air intake
16 office injectors
7mm wire and shields
New cap and rotor
Temperature sensor
Jeep runs very strong and clean just won't idle down quick enough for a smooth gear change.
Any thoughts

 

[TSB8C]

Since the butterfly opening/closing in the throttle body is how engine speed is controlled (excepting idle control with the IAC), a first test is to start the engine, press the accelerator pedal to rev up, and then have someone watch the butterfly in the throttle body and see if it is slow to close when the pedal is released. I don't recall on a '99 year engine, but is that a drive-by-wire? Or do you have a mechanical linkage from the accelerator pedal to the throttle body? In a drive-by-wire, the pedal has an APS (accelerator position sensor) that tells the TPS (throttle position sensor) that the pedal has been moved, and the TPS actuator in turn moves the butterfly in the throttle body. In a system with mechanical linkage (usually a cable) where the pedal physically moves the butterfly open/shut, the TPS (throttle position sensor) simply tells the ECU when and how much the throttle has been opened/closed. In my case, I have a cable between the pedal and the throttle body. If that cable were to bind for some reason (wear, friction, etc) it might be slow for the spring on the throttle body to retract the cable when the pedal is released. Of course there is also a programming or setup procedure to program the readings between the IAC (idle air control valve), TPS, and if a drive-by-wire, the TPS actuator all together. This is commonly referred to as a "throttle body adaptation" to set the ECU compensation rate. That rate should be around 10%-12%. A high or slow to return idle would indicate a rate much higher. Do you have a scan tool to check such readings?

Now to some more involved programming if none of the above helps. Most ECMs have a dashpot function built in that allows for a softer landing to the actual idle speed. Dashpot is usually shown as bypass airflow from the IAC at closed throttle versus engine speed. You'll want to contour this function so the engine speed drops quickly once the throttle is closed at higher RPM, but slow its rate of change as idle is approached. Too fast and the engine drops right past idle speed and stalls; too slow and the engine hangs at higher RPM when lifting off the throttle. Changes in throttle body size, camshaft design, and intake manifold design and volume have noticeable effects on required dashpot. Lower dashpot values at medium to high engine speeds can be used to add engine braking when the throttle is lifted. Care should be taken however to make sure this does not interfere with normal cruise throttle position, which may be relatively low for larger throttle bodies. If you have a programmer, check and try experimenting with this setting.