Weird problem has been occurring only when in reverse. Almost always occurs when backing up an incline with a trailer or heavy load. I run the engine up to 1,000 RPM or so to get moving and the second I let off the throttle the RPMs dip almost down to 200 before the IAC compensates and the engine returns to normal idle. It does this in (D) sometimes but only down to 500 RPM before the IAC opens slightly to compensate


There are no codes in the computer, manifold vacuum is 19 in Hg at idle, fuel rail pressure 30-35 PSI. Transmission fluid is clean too in case this might end up being a transmission issue.


So, what can cause this to happen?

 

Has the IAC been cleaned or replaced lately? Sounds sticky.

 

I cleaned it within a year with some EFI throttle body cleaner. I'm pretty sure it's the original unit. Over a year ago I tried swapping it out with a BWD and got a higher than desirable idle speed, so I went back to the original. In hindsight I didn't reset the KAM for the computer to relearn the new valve ...

But if it was the IAC wouldn't it do it 100% of the time?

 

Maybe it is the "higher" load(more throttle position) the motor is seeing, then dropping to idle that it is not responding quick enough? You mention it does it in drive, but not quite as bad. Can you powerbrake it some in Drive and see if it drops more? That would simulate what it is seeing in Reverse?

I've dismantled mine completely. Remove the valve from the solenoid(two phillip screws). Shake the solenoid, it should rattle freely. If not, then it is sticky. Use some brake clean and blow it out. Don't put any oil/lubrication on it as that will make it sticky(been there, done that).

Reinstall and try again.