Wisconsin Joe

They are marked on the relay itself. There should be a little schematic on the side that tells you what is what and what goes where.

dr bob

Joe --

The fuel pump relay also heats the HEGO (oxy) sensor in the exhaust. There's a connector in the cable that runs across the bottom of the CE panel area. Unplug that and see if the symptoms go away. The LH will stay in open-loop mode otherwise no issues at least for testing.

Tony Harkin has a fuel pressure gauge for the 928. NW of town a little. Borrow it if you can catch him between flights. You will have it on the car while you test, remove it when done. If you get in a bind I can send mine. It's a kludge of a Harbor Freight gauge, has a drilled-and-tapped 928 fuel rail cap. If you can get a spare cap, I tapped the nut 1/4-28 for a standard GM test connector. I used a 1/4" brass tube fitting ferrule instead of the ball bearing inside. The cheap gauge setup is under $20 at H-F. I bought the fitting used for about $10 from Mark IIRC. The ferrules came from Home Depot.

Reason for using the jumper in the relay socket is so the pump will run with the engine not turning. Make a jumper with a toggle switch in the middle of a longer wire loop, so you can turn stuff on and off easily.

From your original description of symptoms, it sounded like the "problem" started very soon after you put gas in the car. Don't know if that's totally correct or a coincidence. I forget the rule number for "there's no such thing as a coincidence". For sure that has my focus on the fuel system components. Pressure and the WSM fuel-delivery test would start the testing process for me. The H-F tester has a hose with a quick-disconnect on it that's perfect to do the delivery test, with a measuring cup and that switched jumper. Verifies all the fuel system right up against the injectors.

jeff spahn

I'd comment but all my knowledge has been expressed above. I can ship you my spark tester that lets you see if you're getting spark at each cylinder. Want me to bring it to third coast or ship it to you?

polecat702

I have a real Dr.'s stethoscope left to me by my Uncle who was an MD. I put it on the internal, and external fuel pumps. Both were running. I let the car run till it quit, and it's on the lift where I can raise it if I need too.

This is a first, The #42 fuel pump relay, oxy sensor fuse blew. I replaced it and the car started and died within 2 seconds and the fuse blew a 2nd time.

Would a bad oxy sensor, cause the fuse to blow? It's dark here and my lift is outside, so I knocked off for the evening.

Mrmerlin

Joe if the header pipes were moved for any reason,
its possible that the O2 wire harness has been damaged .
Per Dr Bobs suggestion, it is a good idea to unplug the sensor at the bottom of the CE panel this should remove the sensor from the fuse blowing issue.
NOTE if the fuse still blows then possibly the pump harness wires are grounding. or one of the pumps is damaged or plugged

Of course make sure the relays you worked on are put back into the correct slots,
the fuses that are empty should also be empty and the ground and power wires at the CE panel should be looked at for shorting after you had the Ce panel down

Asiandude

Have you ever changed the ignition switch?

dr bob

^^^ What Stan said above ^^^

Verify that the wiring you reattached to the pumps isn't chafing or bound up or grounded pinched or damaged by the covers.

The oxy sensor connector is a three-pin doohicky in what looks like a piece of black TV coax laying laterally across the base of the CE panel. You may need to pull the bottom part of the cover (two nuts) to see it there. Just unplug it. I need to go look at the wiring diagrams too -- there's something gnawing at my brain saying the oxy sensor heater is un-fused. I'll be more awake in the morning.

Dave928S

Joe ... here's a pic of a switched jumper (dr bob mentioned) that can be made easily using twin cable and a light switch, for plugging into the relay socket terminals 30 & 87 to run your fuel pumps for testing. Always handy to have, as it allows you to test the pumps and turn on and off quickly, without anything else running, including testing for leaks after you've done anything on the fuel system, without risking a carbecue.

Using this you can run the pumps for a while without actually driving the car .. and getting stranded.

polecat702

No

polecat702

The oxy sensor was swapped from Jeff's car to mine when I bought the supercharger from Jeff. I don't know why, but I guess they're different.

I'll ask Jeff, tomorrow.

Both of the pumps have different size terminals, so getting them crossed isn't likely. They only go back on one way. I'll check them for chaffing and being pinched or touching something that would cause a short.


My other cars don't do this. Does the ignition switch need to be turned all the way off, back to the lock position to restart the car? When it dies I have to turn the switch all the way back to where you can remove the key.

dr bob

There are some tips-page hints about fuel pump wiring, like how to install the pump with the positive terminal away from possible short-to-ground spots. Those hints must be there for a reason.

It's a 928 "feature". It keeps us from engaging the starter motor when our relatively-quiet-by-Porsche-standards low-revving high-torque V8 engine is already running. Also locks the steering column as you are frantically attempting a flying restart while hurtling towards the guardrail and the bottomless canyon.

polecat702

I have the wide band O2 sensor, with the electronic boost gauge. It's working as it should, it's giving accurate readings on the gauge.

I'm gonna bite the bullet, and buy the correct fuel pressure tester. I'll loan it out if it's needed.

Mrmerlin

might you have 2 O2 sensors?

NOTE the wideband to monitor engine operation and the factory narrow band thats running the engine?

dr bob

Joe --

Double-check the wiring for the sensor and make sure it isn't faulting to ground. It passes into the tunnel just aft of the CE panel, originally through a grommet, then above the heat shield to the sensor itself at the rear of the header pipes/front of the cats. It's really easy to mis-route the wire there. dress the cable poorly, or have that grommet push through and expose the wire to the sharp edge of the hole in the tunnel.

polecat702

I only have one O2 sensor. It's fine.