can someone tell me what a fuel pressure test really does?

benzat
1982 380 SEC

I am no expert in this subject but as no-one else has replied I will attempt to help you.

A Fuel Pressure test tests that the fuel pump is working.

I just did fuel pressure test on my 92 400E.

I bought a fuel pressure gauge from local automotive stop, then attached the gauge to the test port on the fuel rail of the 400E, as soon as I turned the key to on position, fuel pump worked for about 1 second, the reading from the gauge showed 60 PSI(3.2 bar, right at the middle of the fuel pressure regulator's spec.), then I started the car, the reading slightly changed a little bit, but still near 60PSI, then I shut down the engine, the reading held it self at 60 PSI for quite a long time, longer than 15 mins, this showed my fuel system was working properly.

This test does not only test the fuel pump, but also tests a lot of things in the fuel circuit:

1: Fuel pressure regulator, a defect fuel pressure regulator will either release fuel pressure sooner let fuel pressure low or release pressure late let fuel pressure high, the correct fuel pressure should be within the spec. of your fuel pressure regulator, in my case, 3.1 to 3.3 bar for my 400E.
2: Fuel injectors, fuel lines and fuel pump check valve, if there is a leakage somewhere, after you turn off the engine, fuel pressure will not hold for a long time.

__________________
99 BMW 540i 6-speed 110K Km
03 SAAB 9-5 wagon 80K Km
92 400E (Sold) 245K km
Still missing the days with the Benz, it kept me busy.

In addition to checking for proper running pressures & leakdown problems as already noted, on an ’82 with (lambda) CIS injection, you should also check your control pressure. Control pressure affects proper mixture throughout the different temperature ranges by controlling the counter pressure on the center plunger in the fuel distributor. Control pressure drops at lower temperatures via the control pressure regulator, such as during the warmup phase.

If you are having a problem with mixture at idle, don’t forget to check your frequency valve for proper functioning (on-off ratio).

RTH