Working on 1987 928S4 that will start and idle (for awhile) but not accelerate, and will eventually die. Suspect old fuel in gas line has limited flow. Have added fresh gas and am trying to circulate to improve flow. When putting a jumper in place of the fuel pump relay, the fuel pump DOES run. However, the 15A fuse for the fuel pump gets very hot, so I stop the process. I have cleaned the ground for fuel pump, and tried again. Still the fuse gets hot. I assume the pump is working so hard to move fuel through the clogged lines that the fuse is getting hot. Is that a reasonable conclusion?

I have a few questions about the process of using a jumper.
1.) Will the car start with the jumper in place? I have tried once, then decided to stop almost right away. Thinking I better ask first.
2.) Is it even okay to turn the key, without starting the car? I ask because, once I did turn the key and while listening to the sound at the back of the motor, I discovered that if I move the throttle position, the sound (flow) increases substantially. And while I liked the idea of increasing the flow, I worried that the increased flow might be going places it shouldn't because the throttle is open. Again, I thought I better ask first before doing that some more.

Thanks for the replies. By the way, this Porsche 928 S4 is for sale.
Joe

 

On my GT, I could jumper the fuel pump relay and the fuel pump would make sounds as if it were pumping fuel, but there was no fuel pressure. The fuel pump motor ran, but there was no fuel being pumped. Check your fuel pressure.

 

Also, these pumps tend to slow down and draw more amperage as they are failing, hence the hot fuse...your solution may be as simple as replacing the pump and filter.

 

Xcellr8,
1. Yes, you can run the car with the fuel pump jumpered at the relay socket 30 to 87.
2. No, it will not hurt your car to turn the key to the run position but not start it.
I assume you have at least a quarter tank of gas? If so, if you jumper the fuel pump (car off) in a quiet place and put your head down near the gas tank, you should be able to hear "sloshing" sounds as the return line is returning gas to the tank. Or stick a long screwdriver at the gas cap inlet to open up the door and listen there. Heck, just standing at the engine bay you will here the gas rushing through the lines, diaphrams, etc. If you don't hear anything flowing, John may be right that your pump is "spinning" but not pumping. Obviously, the best thing would be to attach a pressure gauge at the test port and know for sure what pressure you are getting.
-Jason

 

if the fuse is getting hot then it indicates a poor connection at the fuse.

NOTE put some solder on the fuse legs to thicken them then install it into the CE panel,
put deoxit 100 on the fuse contacts in the CE panel ,
monitor the fuse.
The fuel pressure should be about 58 PSI

 

I can hear the gas returning to the tank! Yea. Thanks for that suggestion. I will say however, that the sound starts off strong initially, but soon becomes more faint, making me think the flow is slowing/stopping. I have been running the car on and off trying to keep the fuel lines clear, but still cannot rev the engine past 1300 rpm. The more I push the pedal at that point the more the engine bogs down without black exhaust, so I suspect an air problem on the side of the air/fuel continuum. Suspecting throttle position switch, but open to your thoughts and ideas.

 

Well, my next step would be to check fuel pressure. And then fuel pump delivery rate.
Is your fuse still getting hot? Have you measured voltage at the fuel pump +/- while it is jumpered?
If you want to just start throwing expensive parts at it (which I prefer as the last choice after all testing is performed), you can put on a new fuel pump and filter and see if it changes.
(It could be that the failing fuel pump is putting the extra amperage draw and causing the hot fuse--just a guess.)
-Jason

 

I burned out 2 relays because of poor fuse contact, but they were the bullet style, not blades. Still, clean and apply deoxit. My pump, when last checked pulled 7A, engine not running. On my car, ignition on, not started, the coil pulled >5A, so I would not let it sit on but not running for any length of time, unless you disconnected the coil(s). If your pump is struggling, check to fuel delivery rate (in WSM). I have had good results by adding some MMO or Berryman's oil to the tank, jumpering the pump relay socket, and letting the pump circulate fuel for an hour or more. Then change the fuel filter.
jp 83 Euro S AT 57k

 

If you don’t know when the fuel filter was last changed, then change it and retest. No point in testing flow if you don’t know you have a good filter.

And then, while you’re there you can test the pump’s flow rate.

 

Fuse getting hot is always due to poor contact between the fuse and the fuse holder - clean that up (both sides). Maybe new fuses if you are unsure about provenance (cheap Chinese fuses are usually junk - get known brands).

Alan