Mercedes Coupes/Sedans/Wagons 1974-1984 Repair Guide



Beginning in 1968, various modifications were incorporated on Mercedes-Benz engines to meet Federal emissions control regulations. Since 1968, these modifications have been continually updated and improved.

The following emission controls and modifications were used on 1968 carbureted engines:

Modifications to the Manifold Air-Oxidation System.
Changes to the ignition timing and distributor advance curves to provide better combustion in the middle rpm ranges.
A Port Burning System, which uses a belt driven pump to force in air directly behind the exhaust valves, creating an afterburning effect.
Modified carburetor jets to provide a leaner carburetor mixture.

Fuel injected engines required no modifications in 1968.

In 1971, the design of the combustion chambers was changed and the spark plugs were set deeper on 6-cylinder engines. Better cooling is accomplished by adding more cooling jackets. All 4 and 6-cylinder engines are equipped with a Fuel Evaporation Control System.

In 1972, the engine compression ratio was reduced to 8.0:1 on all engines except the diesel. Automatic transmission shift points were modified. The fuel evaporation control system remained unchanged, although the evaporation control valve was redesigned and relocated under the rear seat.

The fuel evaporation system remained unchanged in 1973, but the 2-way valves previously used were replaced by switchover valves that are identical in appearance. To be able to distinguish the function of the individual valves, the covers are color coded according to valve function, as follows:

WHITEadvanced ignition valve
REDretarded ignition valve
GRAYthrottle opening valve
BROWNexhaust gas recycling (EGR) valve

It is important that the vacuum line always be connected to the center connection, whether it is on the top or the bottom.