I have a 86 Toyota van with automatic. Idles fine in park/neutral and also in REVERSE. In drive it is very rough at idle when the vehicle is not moving. I am guessing that there may be an idle-up when in gear that possibly may not be working in drive. Why reverse is working I don’t know. I believe it is more likely that there is a vacuum leak in something that only engages in drive but I have no idea what that would be. The idle roughness does feel more like a vacuum leak than a low RPM problem.
Thanks for any help
Steve