Okay, probably asked before so forgive me, I can't figure out the correct word combination to get the answer I need.

Is there a way to regulate the fuel pressure from the fuel pump on a 1989 928 S4?

Thanks,
Paul G.

P.S. Almost light ----- come on sun.... been up since 4 am..... feel like child at Christmas.

 

With a regulator after the injecters to maintain constant pressure in fuel rail by bleeding off excess pressure back to tank.

 

Paul, the answer is "no". The FI fuel pump is responsible for volume at it's disigned pumping pressure. It has no internal mechanism for the consideration of pressure.

Generally, pumps that also govern system pressure do so by virtue of a pressure sensing switch that turns the pump on-off. This creates pressure spikes in the system that would be a problem for EFI control. This could be mediated by an accumulator in the system, but that's a lot of parts and control sophistication when a steady-state down-stream-bleed variable-orifice regulator is so simple a solution.

 

Actually the answer is YES. The fuel pressure regulator does this but it’s not adjustable. Preset at the factory and does a good job on a stock or near stock 928.

 

Unless you have an adjustable unit installed.

 

All 928s pump more fuel to the injection system than it can use the excess pressure volume is returned to the gas tank via the top of the fuel level sending unit. All elecronic injection has one or two fuel pressure regulators which set the pressure and dump back the excess. So much fuel is returned to the tank that the gas tank was getting hot so Porsche added a fuel cooler on the firewall using the cold freon to chill the fuel BEFORE it was sent back to the tank. The fuel pressure regulators have a vacuum line attached and the change in engine manifold vacume under full throttle increases the fuel pressure because of the change in the pull on the internal diaphram inside the regulator. So fuel pressure is set by the regulator but should increase depending on vacume. For example 1987 with fuel pump bridged engine not running no vacume the pressure should be 3.8 bar plus minus .2 bar then engine running the fuel pressure at idle is aprox. 3.3 bar because now the pressure regulator sees vacume. This automatically increases the fuel pressure when the engine is under high load low manifold vacume and all the fuel injection fuel maps were based on this CHANGE IN PRESSURE. You can increase fuel pressure at idle simply by unhooking the vacume line to the pressure regulator not that that will do anything good simply make it run rich at idle and waste fuel !

 

Thanks Jim. I was out of the loop on mechanicals for a long time before getting into sharks, so I have had a lot of catching up to do. With posts like yours, (and others) eventually I will understand the systems on this car.

 

Perhaps I mis-read the question. My answer was to explain that there is no pressure control BY the pump......

 

perhaps the question is WHY ? .."Is there a way to regulate the fuel pressure from the fuel pump on a 1989 928 S4?"