I have an '84 300D that runs diesel and WVO. It runs great on WVO but sucks air on diesel. The fuel select solenoid is just before the the lift pump and injection pump. Since I know everything is fine on WVO side and the main lines I'm troubleshooting the diesel side. I've re-tightened all the hose clamps, replaced hoses, re-tightened the secondary diesel filter and replaced the compression washers between steel fuel line (which runs under the car) and the fuel select solenoid valve but this hasn't helped. I'd beginning to think the air leak is somewhere closer to the tank.
Are there any connections between the tank and the engine compartment that might introduce air?
Could this be something else I haven't thought of yet?

 

Any connection from tank to selector valve that can introduce air will leak diesel as the joints are below fuel level near the tank. If you have a Pollak valve, they have been known to fail under long periods of heat (even if the oil was under Pollak's stated heat range). You may want to switch the oil and diesel supply at the selector valve to check the valve. Are you looping veggie and returning diesel?...The loop will cause a pressure or lower vacuum so the line between the selector valve and lp may be under pressure or less vacuum when looping not letting air in while a higher vac under return can introduce air.

 

Thanks for the reply. I've installed a Frybrid kit (www.frybrid.com) so I'm not worried about solenoid failure. However, I have not checked the torque on the brass fittings leading into the solenoid. I'll do that this weekend.

I also like your idea of swapping the WVO and diesel at the solenoid. That will at least verify that the leak is/isn't at the solenoid fittings.

Frybrid uses looped return for diesel and WVO so I don't think I have a pressure problem there.

I just had an idea... what if there is something partially blocking hole where the diesel leaves the tank? This would create great suction on the diesel line and perhaps cause my leak? I dunno.

 

All,
It took quite a few hours of troubleshooting but the problem was the secondary fuel filter. I doubted that was it. I just assumed it was something I had done. Just faulty workmanship on the filter. I replaced it and the air disappeared.