Mercedes-Benz Forum banner

Fuel system bleeding '91 420SEL

4734 4
Bleeding fuel on 420SEL.
I changed my fuel filter, vent valve, and rubber shocks (buffers)
for the fuel pump/filter assembly.
What I can not find in the manual is bleeding the fuel system.
I do have the factory manual on CD and have read the part for replacing the filter but have not found the general instructions for bleeding the fuel.
I should point out that I have done some extensive engine work as in replacing the heads, cams shafts, timing chain, etc. so my fuel injection system has been off for about 8 months.
I put the spark plugs back in last night and am down to bleeding the fuel for the initial start after 8 loooooong months of not being able to drive my car. My pumps run for about 5 seconds and then shut down and I assuming that they feel the pressure is good enough so they must be seeing air. I would further assume that there is a pressure switch that must be temporarily bypassed for this task? The line is disconnected and pointed into a can for bleeding but no fuel.
Any remedial points for this topic would be greatly appreciated as I am an aircraft mechanic but am not schooled on the Mercedes product.
Thanks in advance.
1 - 2 of 2 Posts

Proud owner of an Blue-Black (Charcol Gray) '91 420SEL
704 Posts
Discussion Starter · #2 ·
I received the below reply from another forum. Just for comparative thinking I would assume that the relay also powers some sort of a pressure switch that must be bypassed for this procedure?
What would be a safe size of fuse to use for this? I would want to protect the circuit even though my pumps are o.k.
Locate the fuel relay at firewall area , relay has number like 003 545 25 05, its not the relay that's next to it that says KLIMA, pull relay out locate adjacent pins 30 and 87 they are directly across from each other bridge those , preferably with a fused wire, don't short those to ground, or try and jump with bad pumps you can melt the wiring! You will see with a test light that pin corresponding with 30 will have voltage , key on or off I believe , pin 87 sends power to fuel pumps, once you supply power you can disconnect fuel line at fuel distributor and pump it out, careful a lot of fuel comes out. Be safe.
1 - 2 of 2 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.