You can solve your problem by donating your car to Jim at 928 International.
You have heard of them. They are not the Polly Klass Foundation, but they will use your car to help other 928s live. :)
What it sounds like you have is a bad fuel pump. Here is the story. A marginal fuel pump will draw more current than a good one and the extra draw burns up the points on the relay. The longer the pump runs the more current it will draw until it gets to the point of locking up. Leave the car for a while and the pump will start to work again. I had this problem with my 78 and it drove me nuts until the fuel pump finally died.
There is some good news here. The pump may be fine just gummed up from old gas residue or even moisture sitting near the bearing or impeller for long periods. I find running a pint of Marvel Mystery Oil through the fuel system will clean up bits of contamination while providing some needed lubrication for the pump and injectors. I add a pint to the gas about every six months. Afterwards you will find the fuel pump quieting down to a point where you can hardly hear it.
As an emergency measure when getting stuck with a bad fuel pump try running it in reverse by reversing the wires for a few seconds. This will clear anything stuck in the impellor and get the car home. For further treatment you can try removing the fuel pump, squirting a bunch of Marvel Mystery Oil and even some Liquid Wrench with Teflon in the pump and running it a couple of minutes in both directions to get it cleared. Make sure the pump is empty (The pump holds a good quantity of gas) when doing this and make sure you do it outside so any sparks will not flare the gas fumes from the pump.
One other issue to be aware of is the tank screen. Inside the gas tank is a fine screen that keeps contaminates from reaching the pump. The tolerances inside of the pump are very tight and even a small bit of dirt can lock up the pump. Thus, it is important to filter or strain the gas before it reaches the pump. In older cars, these screens start to deteriorate and fragment bits like a grenade into the pump. This can cause a successive number of new pumps to fail. Since your car is an 86 it is old enough to consider replacing the tank screen. The procedure is covered on my web site. The only caution is to be careful not to strip the tank bushing when removing the screen.
Dan the Pod Guy (Dan Brindle)
Okay, here is another question, On an S4, should the fuel pump relay and fuel
pump come on for a couple of seconds when you turn the key to the run position
like they do on many other cars to build up pressure in the system?
I thought that I has a bad fuel pump relay but I replaced it and nothing has changed. Sometimes I think that I hear the pump come on prior to starting and others times I do not. When I do not, the car just cranks and won't start.
On the LH cars, the fuel pump relay is activated by a signal from the EZK ECU, so the fuel pump should run only when the engine is turning over.
Thanks. I am starting to wonder if I may have an intermittently bad ECU because there are a couple of run ability issues also that seem to come and go.
Sometimes it fires up nice and smooth. Sometimes it seems like it is rough and loaded up but the plugs don't show it. Sometimes it bogs down if you are sitting still and rev it up in neutral or park. Sometimes after sitting for a few minutes after driving, it seems to run on less than 8 cylinders and then clear up after a couple of miles and run great. Sometimes, it just runs fine.
This is a safety feature. If the pump sometimes fails to run while the engine is turning over, it can be a faulty relay, or a LH driver problem.
(one of the known LH issues)
1992 928gts Midnight Blue (2006-)
1988 928s4 Cherry Red (1999-2006)
Really? I thought I've read in the past that the pump will active for a second or two when you turn on ignition, to build pressure. After it will only run once the engine is turning over.
But I haven't really examined this properly. Just trying to repeat what I've read (I think).
That is the case in the early cars - the LH is different.
Carefully check and clean all of the coil wire connections. Not the only possibility, but a common problem.