My daughter was driving our '86 240DL today, and said that the throttle seemed to stick open. She had just accelerated to about 80 kph/50 mph, then took her foot off the accelerator to pull into a gas station, but it didn't coast down to an idle. She didn't think to put it into neutral, but instead rode the brake, turned off the ignition, and coasted to a stop. When I arrived, I started it up, and everything seemed perfectly normal. I drove it home without incident.

When I got home, I checked the throttle pulley, linkage, and cable, and there was no evidence of binding; return to idle seemed perfect. The pedal seemed fine as well. I thought that maybe the pedal had caught under the floor mat (it's a generic one, with a cutout for a gas pedal having a floor hinge), but when I tried to make this happen, the force of the pedal's return spring easily broke it free.

It's been quite cold here for the last couple of days (-20C/-10F), but I'm not sure how this might play into the problem. I didn't see any evidence of ice or water, either around the throttle, or the pedal lever inside. The car had just been driven for about half an hour, with about half an hour of rest, just before the incident.

Does anyone have suggestions as to what else might cause symptoms similar to the throttle being held open? I'm fairly convinced that it's not actually the throttle plate.

Even if the IAC valve stuck wide open, I wouldn't think that there would be enough airflow to cause a significant "throttle open" symptom. If an injector was stuck on, without an equivalent amount of air available, I would think it would would run rich with a bit more power, but not enough to run at 50 mph.