I always thought the pump needed at least 11.5 volts to work right. Try jumping 30 and 87 on the FP relay and see if that improves the feed to the pump and the running problems. If so, then that would mean the relay is going bad.

You should also check the EGR valve. They can cause a massive vacuum leak when the engine warms up.

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Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe