Curious if you ever got this resolved. It is very difficult to purge air out of a modified Mercedes fuel system.

Do you have this fuel configuration:

tank > pre filter > filter > 3 way valve > priming pump > IP ?

It is just about impossible to purge air out of the system using the priming pump in this configuration with the FSM method. A Mityvac on the return won't work either since the banjo bolt is plugged and you are pulling the pressure relief valve closed.

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85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

I did solve it this weekend.

My system is indeed as you described. (Other readers this is a WVO conversion - all of this doesn't apply to a normal fuel system)

• I replaced and re-seated the secondary filter. I used the mity vac to pull fuel at the inlet to the three way valve to make sure there was no air leaks in either the primary or secondary filters.
  
• I also used 5/16" vinyl tubing to replace fuel lines as needed to visually verify there was no air.
  
• I then used the mity vac to suck down air from the return line. I could see air was leaking in and vacuum was being lost at that point.
  
• I re-seated that banjo and fit new Aluminum crush washers on it - problem solved.

Lesson learned - priming the fuel system with the mity van before the first valve (before the lift pump) is useful for modified systems. I am no longer going to rely on the primer pump for that job.

Second lesson re-learned: Changing diesel side filters really stinks on modified systems. I am tempted to put a return valve setting on my WVO system that pushes VO back through the diesel side to prime it via pressure. Yes, some VO would go back into the diesel tank, but not much, and if i only do this job in warm weather the vehicle will be no worse for the wear.

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'85 300D, 'Lance',250k, ... winter beater (100k on franken-Frybrid 3 Valve Kit)
'82 300D, 'Tex', 228k body / 170k engine ... summer car
'83 300TD Cali Wagon 210k, wife's car

Maybe

3rd lesson learned: Install a vacuum gauge to tell you when to change filters. If you install it at the common of the 3 way valve, it will tell condition of both the diesel and VO filters. I am going to say that you wasted time and money by changing the diesel filter since you probably have less than 3k diesel running miles through your diesel filter in 40K of VO miles.

Don't run out of fuel!

Having an external lift pump (to the IP) can pump a lot of air into the IP very quickly when you run out of fuel or have air leaks in fuel lines. If you are not prepared and have all the tools to purge air, you are stranded.

Didn't your Frybrid manual have instructions on how to purge air out the modified fuel system for Mercedes?

__________________
85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked