My 1989 240 does the weirdest thing. It has a slightly high idle. It's most noticeable when I'm coasting down hill. It doesn't always do it either. It's almost like there's a pattern. The car will coast with a normal idle and then it will go back and forth to idling too high. I've been chasing a Lambda issue recently. 2-3-1. I replaced the O2 sensor (OEM) and I cleaned the throttle body and replaced the gasket. The gasket was destroyed. I haven't seen a check engine light since doing this but I'm sure it's coming. I wonder if the old O2 sensor struggled with the mixed messages. I think it was the original sensor and probably needed to be replaced anyway. Once the check engine light is on and the car is in limp mode, it doesn't do it anymore. I don't even know if it's related to the check engine light. For all I know, replacing the O2 sensor fixed that issue. But for some reason, when the car goes into limp mode the idle issue would go away. I've also been looking for vacuum leaks and haven't found any.