bryce.hanson

Here's a little update video:

Mrmerlin

to be clear with this jumper,
are the original wires also connected to the fuel pump as well as your added positive wire?

Are you absolutely sure that all of the relays are plugged into the correct slots? ,
once they get pulled its common for them to get mixed up,
your car has different fuel pump relay so this needs to be checked for correct part number

bryce.hanson

Mrmerlin,

The fuel pump relay is a genuine relay from porsche. I also have an extra one that doesn't work either. They both work fine in my friend's 928. Currently I have the fuel pump plugged directly into the battery with a toggle switch for testing, and it has the same result as if it's going through the fuel pump relay. The car won't start with the relay plugged in, and it won't start with the fuel pump toggled on

bryce.hanson

As for the other relays, yes I've verified that they are the same by checking my friend's 928 that runs fine.

Mrmerlin

Ummm did you use a fuse relay chart to verify your CE panel ?
using your friends car may not be a good idea unless its the same year.
do you have any empty fuse slots in your panel? you should.
I suggest to get a fuse relay chart and go over the CE panel and re verify each part number is correct and the slot is also correct and that you have the correct part number CE panel in your car.

read through this it may help,

https://rennlist.com/forums/928-foru...se+relay+chart

GregBBRD

Obviously, a fuel injected car will not run without a fuel pump....so are you saying you have to start the car without the pump running and then turn on the pump....because if that is the case, I'd have to think that you have a complete circuit (both negative and positive sources), at the cold start valve, pouring fuel into the intake.

Disconnect the cold start valve and rig up some sort of light bulb to the two terminals to check for voltage. Crank the engine, get it running and see what the light bulb tells you.

It's possible to have a bad thermotime switch, have a short in the loom, or even have some plugs mixed up.

Kiln_Red

In the first video, the cold start injector is said to be unplugged.

I am also battling VERY similar issues with a US '84 right now, also. The only difference that I am seeing in our symptoms is that, with the car that I am working on, it will start and run but it is super rich. However, with the fuel pump fuse removed(fuse #13), the car will start and idle smoothly until there isn't enough fuel left to pressurize the rails.

Notable things that I've done since chasing:
Replaced FPD's and FPR
Replaced injectors
Swapped with known good AFM
Swapped main engine harness
Cleaned all grounds
Disconnected cold start valve
Replaced temp II sensor
Replaced plugs, wires, cap, rotor, coil, and green wire
Replaced o2

At 75 degrees ambient, here are the resistance values of my fuel injection circuit from the plug at the brain:

Air Flow Meter
Terminal 6 & 9 - 557 ohms
Terminal 6 & 8 - 360 ohms
Terminal 8 & 9 - 200 ohms
Terminal 6 & 7 - 121 ohms
Terminal 7 & 8 - 321 ohms
Terminal 27 & 6 - 1.73k ohms

Temp II
Terminal 5 & 13 - 1.65k ohms

Camshaft grounds were verified at 0 ohms. What I found most interesting was that there was that each of the checks for my injector circuits yielded no resistance(closed). Also, I did discover that the idle switch on the side of the throttle housing was never making a contact because the throttle mechanism never pushes it far enough. Unsure if this could be the culprit, I closed the contact manually while a helper restarted the car. The results were no different whether the contact were made or not. The WOT switch is working as it should.

I am more than willing to bet that our issues are from the same cause. Start by checking resistance from the injection brain plug. Record your findings and report back so we can compare notes.

Kiln_Red

Below are the references you'll need to check your injection circuits. Set your multimeter to Ohms, disconnect the plug at the brain in the passenger footwall and go to work. Power is constant. So leave the key off, but th battery does have to be connected and hot.

Hope this helps the both of us.

bryce.hanson

Update!! I routed my fuel pump to an external gas can (instead of the fuel tank) and guess what? It runs! Not super rich either. Fuel pump plugged into relay and everything. The car starts right up.

Time for me to drop da fuel tank.

Kiln_Red

Yep. Sounds like an appropriate diagnosis to me. I won't discount the possibility with the '84 that I'm working on now, although I'm still favoring the ignition control module as the likely culprit based on my findings. Thanks for updating and good luck!

bryce.hanson

Thanks kiln_red! Let me know if I can help at all with yours! (Although it sounds like you know what you're doing). I'll try to update with more results as I progress into this project.

WESTER928

bryce, was dropping the fuel tank and cleaning it the answer to your issues? I have an 84 928S US model with the EXACT same issues but when we looked into the fuel tank it looks clean... Any tips/recommendations would be greatly appreciated.

dr bob

Reading through the solution post a few above this, the problem was a clogged return into the tank. OP states that he routed the return circuit to a gas can and the symptom went away. Note that this would show up as high fuel pressure at the rails, since with no or partially clogged return line flow the pressure regulator would have no bypass/return path.

The workshop manuals give guidance on correct fuel pressure, the decay rate on shutdown, and the delivery volume needed. The delivery volume test has the return line disconnected at the regulator or cooler so you can capture the fuel in a cylinder or measuring cup. While that's disconnected you can blow through the return line to the tank and know immediately if there's an obstruction, with no further disassembly needed.

To check or inspect the return line going into the tank, remove the carpet and padding in the luggage area. Lift the inspection cover mid right side area over the level sender. You'll see the top of the sender, secured with the ring nut, and the hose that connects the steel return line to the tube into the tank at the sender. Be careful not to disconnect that hose if the tank is completely full, as the liquid level in the tank will be higher than the fitting. Same warning with the ring nut holding the sender itself.


And... Welcome to Rennlist!

WESTER928

Thank you dr bob, now I'm not so sure this is the answer because we don't have high fuel pressure at the rails - around 36psi with the relay jumpered. I will try it anyway just to be safe.

Billu

Check all resistance readings from the controller to the engine. High reading to either temp sensor will cause too much fuel. I found, after a lot of dead ends, a PO had cut the temp 2 wire and inserted a potentiometer (hidden up in back of the glove box). Once removed and wire spliced, problem solved.
No one I’ve talked to about this remembers this as an accepted mod.