Thanks for the suggestion on the hot inlet pipe. I'll have this checked out when I bring my car in for the next service next week.
I've not given up on bringing my gas mileage back up. I've seen a number of mechanics, including at Fletcher Jones Mercedes, regard a constant CE light as just normal on an old 124. But I have the same compulsiveness you noted in your earlier posts. Fixing a problem like this just takes time and patience, which is tough. But I know that, eventually, I will prevail. BTW, I've also found the help on this forum to be excellent.