I have a 1982 US model 928 with 133,000 miles and auto trans. It hadn't run for over 10 years when I got it last year. I got it to start right up and run OK, not great just OK, without the fuel pump running. As soon as I turn on the fuel pump the engine instantly dies. So far I had the fuel tank cleaned, replaced the fuel pump, filter and all fuel lines at the tank. Replaced the sparks plugs and all vacuum lines, and cleaned the fuel injectors. I ran a switch from fuel pump relay location so the pump can be manually switched on and off. Any ideas?

 

There's not a whole bunch of ways that a 928 will run without a fuel pump...

However, on a car with a cold start valve (like yours), if the cold start valve is stuck open, the intake vacuum could suck enough fuel out of the fuel rails for the car to run.
And if you turned the fuel pump on, that cold start valve would be like a fire hose spraying into the intake.

I guess that the same thing could occur with an injector stuck wide open, but I'm thinking that would make one cylinder crazy rich.
However, I can see If all the injectors (or most) are stuck open, enough fuel could be sucked out of the rails to make the car run.

On any 928 which sat that long, I'd pull the injectors and the cold start valve and send them all out to be cleaned and tested.

 

Thanks for the advice. I will check and verify the cold start valve for proper operation.
BTW, went to your web site, very impressive. Your attention to complete cleanliness shows you as a top of the line professional.