fuel pressure & pump action
 

hello forum

on the 72 350sl i've got most all the wiring replaced that was showing signs of cracking or failing in the ignition. the wire (+) pos from the coil to the distributor had 2k ohms of resistence by itself. the new wire virtually zero.

anyway

my question?

I installed a pressure gage in the fuel line (ring). activated the pump and the fuel pressure went to 20psi (appx 1.4 bar) then dropped to 0 psi. did this repeatedly. you could hear the different solenoids/relays tripping but the fuel pressure just went back to 0.

this 72 has electronic fuel inj. the book calls for used injectors to have at least or a minimum of 43 psi to open. new injectors require 50 to 58 psi.

i'll be going exploring tomorrow for the fuel filter and pump.... but can anybody tell me why it would rise then fall to zero?

the way i see this is the pump made the initial pressure to 20 psi but a back door must open up for the pressure to drop back down.

can anybody shed some lite on what is taking place?
tks
craig

Craig,

Sounds like you are turning the key on to check fuel pressure in which case the fuel system is doing what it is designed to do. When the key is first turned on the pump only runs for a few seconds if it doesn't get a signal from the computer that the engine is running. If the car is in an accident where the engine quits and the fuel system springs a leak the pump gets shut down rather than continueing pumping fuel at about 3 bar in a potential fire situation. To check fuel pressure you will need to jumper the fuel pump relay.


Peter

peter

thanks for the advise. that's what i shall do. never thought about that.

are you able to confirm the pressure. i know my 88 4x4 p/u has 70 psi. but can you (or anybody) confirm the 43 psi is the minimum on the fuel pressure.

thanks for your help peter

craig

im not to good on old fuel systems,but do you know if you have a fuel accumulator?,is located under the vehcle near the fuel filter,kit looks like a fueal pressure regulator,if this is defective it will give you a low reading,and its purpose it to send fuel to the main line,once its primed it should release the fuel back into the system,if you disconnect the line from the back of it,crimp off the line so no fuel leaks,then prime he fel system,you will see a very quick squirt,thats it,if it continues leaking fuel,the fuel accumulator may be defective,please look up the proper procedure n how to do this,before you take my word for it,just an opinion.also make sure both pumps or just the 1 are putting out full pressure,i just had too replace both pumps and filter on my 89 330ce.