I have an 85 380SE which required replacement of the fuel pump. It was dead. After pump replacement I've found that with a cold engine, it starts immediately then dies. It will do this 2 or 3 times before it just turns over without starting.

I've also found the only way to get it running is by pouring a few ounces of gas in the air intake. This gets it going but it runs rough for a few minutes. Even after it smooths out, it takes even longer before it can be accelerated without bogging down.

Eventually it runs perfect, just as it did before the pump went out. If turned off, it restarts easily for 10 or so minutes. At that point although it easily starts and remains running, it shows signs of bogging down when accelerated. If turned off an hour or two... you're back to using gas to prime it to get things going.

It runs perfect when warm, and has new plugs, rotor and cap. I've tried shorting the relay pins to force the pump to run but this has no effect. The replacement pump is NOT a Bosch.

Is a check valve failing allowing fuel to return to the tank? Why does 'priming it' get it started while 'cranking it' several times does not? All suggestions are greatly appreciated!