The fuel accumulator is part of the fuel pump assembly and is located near the right rear wheel underneath the vehicle. It looks like this:


It's been awhile since I looked at the 420 engine, but I seem to remember that it's quite similar to my 560. In that case, it's on top of the engine, between the valve covers, towards the front. You might be able to see it with the air cleaner on, but you'll want to remove the air cleaner to work on it. It looks like this:

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Mike Heath
1988 560SL Black/Palomino
1988 300SEL Black Pearl/Burgandy
1984 500SEC Anthracite Grey/Palomino