This is the most "bang for your buck" test you can do. Most people who attempt to answer your questions will want to know the results found from performing these tests. If you are looking for help, it is practically impossible to diagnose CIS problems without doing this test. Yes, the gauges are not cheap. If you are keeping the car for any length of time, this is a must-have tool in the toolbox. If you are really in a pinch, post your location in your profile. Maybe you will get lucky and a helpful forum member will either let you borrow a set or better yet might help you perform the test. Note: pizza and beer have been known to lure fellow Porsche enthusiasts.
What you will need:
1) a set of CIS fuel pressure gauges
2) digital multimeter
3) fused switch (details below)
4) open-end metric wrenches
5) rags
6) eye protection
7) gloves
8) stopwatch or video camera
9) thermometer
10) fire extinguisher(s)
11) shop light
12) Bentley manual
For this test procedure, it is best to let the car be completely cool (such as sitting overnight). You will be disconnecting the fuel lines so if there is still pressure in the fuel system you get a spray of fuel. And if you get a spray of fuel, you don't want that happening around a hot engine.
A word on safety: I'm not the kind of person to belabor this stuff but you are working on the fuel system, ok? Please make sure you keep a fire extinguisher, preferably two, around just in case while you're doing this. Wear some eye protection and gloves and keep some rags handy to catch any excess fuel that spills. If you don't tighten some of the connections enough you might create a fine mist of fuel leaking at the connectors, so check, check and check again during the test and after. And it's always a good idea to keep a fire extinguisher in the car with you, especially after working on the fuel system.
Let's say you're going to do these tests the next day after letting the car sit overnight. There are some things you can do the day before to make things go faster the next day. I'll try to break things down that way to make it easier, but you don't necessarily have to do it exactly this way. The main thing is that the engine needs to cool and the fuel pressure has dropped throughout the system.
The Day Before:
Disconnect the electrical connection to the Warm-Up Regulator (WUR). Take your digital multimeter and attach it to the male spade connectors using alligator clips. Set the multimeter to read resistance (Ohms) and record the result and the ambient temperature. Example: a type -090 WUR should read 25-26 Ohms in warm ambient temps and ~9.5 Ohms when cold. If the resistance is too low, it will cause the mixture to lean out prematurely.
The next thing you'll want to do is make a switch to use to run the fuel pump without having to start the car. This will involve a trip to your FLAPS (Friendly Local Auto Parts Store) for a few simple things. You'll need to buy a switch rated for about 10A or more (my store only had a 50A rated switch at the time), about 6 inches of 16 gauge electrical wire, an in-line fuse holder and 8A fuse, a couple of ring terminals and a couple of bullet-style connectors. You can either solder or crimp your connectors and shrink tube is optional but not necessary.
Assemble the switch like this:
bullet terminal, in-line fuse holder, ring terminal, switch, ring terminal, wire, bullet terminal
I used a pair of needle nose pliers to slightly bend the end of the bullet connector to make it fit in the female end of the fuel pump relay terminals. Some people may say this isn't necessary but I did it so that the bare wire wouldn't come loose while I was trying to perform these tests.
Remove the red fuel pump relay from the fuse panel in the front luggage compartment. (Or the boot or the frunk or whatever you prefer to call it.
)
Install your new switch across terminals 87 and 30a of the fuel pump relay and make sure the switch is in the OFF position.
Last thing. Leave your thermometer out in the garage overnight. The charts in the Bentley manual refer to certain pressures at so many degrees Celsius, so make sure you either have a thermometer that reads Celsius or use the following equation to do the conversion:
C = 5/9 * (F - 32)
The Day of the Test:
At this point, the engine should've sat overnight and be cold. Make sure the electrical connection on the WUR is disconnected before starting the test.
The first step is to install the fuel pressure gauges between the WUR and the fuel distributor (FD). Disconnect the port on the WUR going to the FD. This is where you will install the gauges. If your particular set of gauges has the on/off valve on one side or the other of the pressure gauge, make sure you install the gauges with the on/off valve on the WUR side so that the pressure can be read when closed. Your gauge set likely includes the appropriate adapters but it is a good idea to verify beforehand.
Once you have the gauges installed, the first test is the
SYSTEM PRESSURE test. With the valve in the CLOSED position, turn the ignition key to the ON position (the last position before starting the car) and turn the switch you made ON to start the fuel pump. You should hear a whirring sound that indicates fuel is being pressurized and sent through the system. Check the system pressure reading and record the result. You can now turn off the fuel pump. Compare your result to the table on page XXX of the Bentley manual. (note: need to fix page reference)
The next test is the
COLD CONTROL PRESSURE test. With the valve on the fuel pressure gauge in the OPEN position (and again with the ignition key in the ON position and the fuel pump running), record the fuel pressure and the ambient temp. In most cases, this should be about 1 bar. (Note: I'm not sure the Bentley manual states what the cold control pressure should be.)
Next, you will check the
WARM CONTROL PRESSURE. It is called warm control pressure because the electrical connection provides voltage when the fuel pump circuit has power. This applied voltage causes the bimetallic strip in the WUR to slowly move which changes the control pressure. A lower pressure (~1 bar) means a richer mixture and a higher pressure (~3.5 bar) means a leaner mixture. It is for this reason that the WUR is sometimes called the Control Pressure Regulator and this is probably a more accurate term since it regulates pressure all the time and not just when the engine is cold. For this test you will need something to record the elapsed time. You can use something as simple as watching the second hand on a clock, a stopwatch or even the video function on a smartphone. Leave the test running exactly as before from the control control pressure test. Find the electrical connection to the WUR and plug it in. The control pressure will begin to rise. Record the time it takes for the control pressure to rise and reach it's peak value. In most cases, this should be about 3-5 minutes. Compare the peak value to the table on page YYY of the Bentley manual. (note: need to fix page reference)
The final test is the
RESIDUAL PRESSURE. Turn off the fuel pump and the turn the ignition back to the off position. Record the pressure reading on the gauge at the following intervals: 5 min, 15 min, 30 min, 60 min. Compare the results to the table on page ZZZ of the Bentley manual. (note: need to fix page reference)
Now you can disconnect your fuel pressure gauges and replace the fuel pump relay. If you need help interpreting the results, post your question.
Also make sure you double-check your fuel connections afterwards. The last thing you want to do is to drive the car later and discover a fine mist of fuel the hard way! And again, make sure you keep a fire extinguisher with you!
If you need to post your test results on the forum for help in interpreting your results, here is a simple template to follow to make sure you post all the necessary information. The conversion factor for psi to bars is 1 bar = 14.5 psi, so a pressure reading of 30 psi is ~2 bars.
1. Year of engine:
2. US or RoW (Rest of World):
3. WUR model number:
4. Ambient temperature at time of test (in degrees C):
5. WUR Resistance (in Ohms):
6. System Pressure (in bars):
7. Cold Control Pressure (in bars):
8. Warm Control Pressure (in bars):
9. Time delta for Cold -> Warm Control Pressure (in minutes & seconds):
10. Residual Pressure @ 5 min (in bars):
11. Residual Pressure @ 15 min (in bars):
12. Residual Pressure @ 30 min (in bars):
13. Residual Pressure @ 60 min (in bars):