The basics of the fuel delivery system are:
- fuel tank
- in-tank filter (or screen)
- fuel pump
- fuel accumulator
- fuel filter
- fuel distributor
- fuel injectors
- cold-start injector and thermo-time switch
- frequency valve
For descriptions on each component, refer to Jim's Basement Workshop:
Components
Now is probably a good time to point out that all years models don't always have the same set of components. Here is another link to Jim's Basement Workshop that shows a table of what components are used in each year model:
Parts ID by Year for US models
General guidelines for troubleshooting fuel delivery problems:
1)
Lack of Fuel Supply. Is there gas in the tank?
2)
Defective fuel pump. Get underneath the car and locate the fuel pump. Have someone try to start the car. Can you feel the fuel pump hum or vibrate? If not, check the fuel pump fuse and relay. You can alternatively use the method outlined above in the section on the fuel pressures test (post #6) to run the fuel pump without the car running. If the fuel pump runs with the switch but not when the relay is installed, the relay is bad (you do keep an extra fuel pump relay in the glove box don't you? If not, you should to keep from getting stranded someplace). You can use a test light connected to the positive side of the fuel pump connector and the ground wire. If the light turns on, then it is receiving power and is grounded. If not, check the ground wire to the fuel pump to ensure it is grounded properly.
3)
Restrictions in the fuel lines. Common places are the screen in the tank filter, the fuel filter in the engine compartment and the return line to the tank. If you suspect there is varnish or other blockage, try running some fuel injector cleaner through the system such as Techron or BG44K. If this doesn't help, replace the fuel filter (do you know when the fuel filter was replaced last?). If the car has been sitting for a long time, there could be water and/or sludge in the fuel tank. If this is the case, the tank will need to be emptied, removed and cleaned.
4)
Leaking or restricted fuel injectors. The best way to determine if the injectors are functioning properly is to perform a simple test like this:
http://forums.pelicanparts.com/porsche-911-technical-forum/574957-excellent-way-test-injectors-78-911-a.html You want to see a fine mist pattern with no drips. Another possible way to see if you have any injector issues is by looking at your spark plugs. If you have any doubts about how to "read" your plugs, pull the plugs one at a time and lay them down on a piece of paper being careful to write the cylinder number down to each plug. Take a picture of all the plugs together on the paper (in good light) and post it on the forum. Another thing to note is if the injectors are spraying an equal amount. Using the method listed above, let the injectors spray into the container for a few seconds. The volume of fuel in the containers should be fairly equal.
5)
Problems with the fuel distributor (FD). The fuel distributer (as the name implies) is responsible for distributing fuel to the injectors. The FD is made up of two chambers - upper and lower - that are separated by a diaphragm. The pair of chambers is sometimes called the differential pressure regulator because there is a pressure differential between them. If the FD develops leaks, it won't be able to pressurize fuel properly. Further reading:
http://forums.pelicanparts.com/porsche-911-technical-forum/725101-fuel-distributor-leak.html The FDs are no longer available for sale new but they can be sourced elsewhere and there are several places that will rebuild them as well.
6)
Problems with the cold-start injector (or the cold-start valve). The cold-start injector is also sometimes called the cold-start valve. Two of the most common problems with the cold-start injector is that it either doesn't work (making it hard to start the engine when cold) or that it leaks (causing overly-rich running conditions). The cold-start injector receives voltage whenever the starter is engaged and is grounded by the thermo-time switch when the engine is cold (below 95 F). If the engine is cold, the TTS grounds the injector allowing it to run (and only for a few seconds - 10s or less - to avoid flooding the engine).
7)
Problems with the fuel accumulator. The accumulator does two things: it evens out the flow of fuel coming from the fuel pump and it helps maintain the fuel pressure. To do this, it has a diaphragm inside. If the diaphragm fails, the system pressure will drop. If the car is running when this happens, it might shut off and not restart.