My trusty 1989 240 was running rough at idle like a bad spark plug. I changed the plugs but problem persisted. I read on the Brickboard it could be an intake manifold leak. I took some carb cleaner with the little tube attachement and sprayed a burst around the various locations of the intake manifold. I found a spot where the engine speed changed. This is where the leak was! There was a gap in the gasket and a hole not much but it was sucking air. I cleaned the area around it with cleaner and a tooth brush. I reached into my tool box for my favorite tool "epoxy putty" I mixed a small dab and jamed it into the small crack in the gasket and then fixed a cup of coffee and waited for it to set up.
I started the engine and it ran smooth as glass, problem solved. I will replace the gasket but this is a great quick fix. I thought I would pass it along in case someone else has this problem. The key is to get the area you are patching clean for the putty to stick. I guess after 263.000 miles I could expect the gasket to dry up and fail. once more the Brickboard helped me look in the right place to find the problem.