I have a 1989 honda civic and it wants to hestitate when you come to a top and only in reverse and don't know what to do, need to know what the diffenece in the IAC unit for a manual transmission and one for a automatic trans. I have had this problem for a few weeks and don't know what to do cause this is my main transportation, how ever I do own three of these but the other 2 have manual transmissions.:confused:

Check this: have the hood up, have someone put in reverse while you're standing beside the car. Have the e-brake set, and have him/her rev it slightly against the brake.
If you hear a big gulp of air, then the intake hose is split causing unmetered air to enter the throttle body.
The engine torques in a different direction in reverse, this is why only in that gear.