I have a 1989 420SEL and I can get the left valve cover off without removing the fuel lines. The removal is tricky but achievable. I just had it off yesterday.

It is difficult to explain how to do it. Basically, after removing the 4 screws, try to rotate the cover when it is snug up against the fuel lines. Keep rotating until it is against the firewall foam, at the rear end. Then move it foward. Once it is clear of the last cam tower, it is easy going.

If you want to disconnect the fuel lines, do so at the fuel distributor. Be prepared for lots of fuel. I watched a pro do it once and he used a home fabricated hose, to connect the fuel line to the fuel return line, because the fuel just keeps coming. I guess you could use a couple fuel line clamps, one on the rubber part of each line to eliminate the flow. It will be hard to start, if you let the fuel flow out.

Also, you may choose to have spare valve cover gaskets available in case the old ones are hard or leaky.

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1989 420 SEL