"Tune Up" is a pretty ancient term. In the distant past(which I am plenty old enough to remember). This usually referred to points and plugs, adjust the points and timing, and however deep you wanted to go. There were the terms minor and major tune ups.
With the 124 car, as with most modern cars with high energy electronic ignitions, the plugs last forever and the primary side needs very little if any attention. On these cars the distributor cap and wires, unless they fail will probably be on the car when the engine finally gives up for whatever reason.
If you want to pay it some attention, I would suggest replacing the plugs(don't use platinum), air filter, fuel filter, check the drive belt, replace if necessary and check all filters. Oh yeah, I forget about this because mine is a manual transmission, service the transmission frequently. And you can't change the engine oil too often.
It wouldn't be a bad time to check the brake pads for adequate thickness either.
Of course, you can look at your owners manual/service records and see what's due when.
As for the car not running well, injector cleaner may help, if it's bad enough that fuel mileage is dropping off for real, and it's not just your foot getting heavier with time, you should probably take it to someone who can diagnose it properly. These cars with their injection and bulletproof ignition system don't just start running poorly because they don't have fresh plugs, there's probably something wrong.
'01 C Class, Six Speed
'84 Euro 240D, 533K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in
[This message has been edited by LarryBible (edited 10-12-2000).]