I have a 1992 240 with 148,000 miles. When I broke down, I was sitting at idle at a traffic light. The car sputtered a little and stalled. When I got it to start, it missed badly and I was only able to go about 50 yards to the side of the road. Had it towed home since it wouldn't re-start. It ran great until this happened. No other problems. It thought it was the AMM, since it had a code 121.

I got the car started tonight, after replacing the AMM, but it idles extremely rough, is blowing white smoke through the tailpipe, and the exhaust has a terrible burning smell! Once it warmed up a little, it would not run. Obviously, it wasn't the AMM!

I checked for signs of water on the dipstick and cap, nothing visible. It appears to me that the head gasket has failed and is allowing coolant into the combustion chamber.
Can a head gasket do this? Give no symptoms and just blow when sitting at idle? The car went from running like a top to this in a matter of a second.