Very common problem for that year. The biggest reason is that the catalytic converter is in the engine compartment. You need to maintain fuel pressure to combat the additional heat.

If you don't have external check valves on your fuel pump(s) (I think you only have one), then you should add one. Make sure none of your injectors are leaking. You'll need a pressure gauge to attack this problem. 1975 should be an EFI system.

Depending on the laws in your state, you may be able to bypass the cat.

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