PD Lift Pump Questions

After a bit of assistance please! This is a mk5 golf PD lift pump. I'm using it in something else but need to get my head around how the surge tank mechanism works.



The pump itself has 2 outlets, the larger goes out the top of the housing and obviously goes to to the fuel hard lines. There's a smaller outlet though that is routed to a one-way valve at the bottom of the swirl pot. What is the purpose of this? I'm guessing maybe some sort of pressure relief valve.



I'm trying to figure out the method using for filling up the surge tank however. As far as i can see, fuel just enters naturally through a one way valve at the bottom. I was under the impression that these tanks fill up to completely submerge the fuel pump?


I just want to get my head around how these operate as i'm transplanting the lift pump into another vehicle.



Thanks!

I replaced the one in my MK VI years ago and tried to figure it out. Iirc the return line from the engine goes to the surge pot so fuel warms up faster on a cold start.

The original design had a bypass that allowed the engine to run without the lift pump. Is the second ‘outlet’ that bypass? Pretty sure there is a thread that describes how it works, sorry can’t be of more help.

Hi, thanks for this, I actually found the thread you're talking about. it's actually a much more complex system than I first thought.

In some earlier swirl pot designs the second outlet provides enough flow for a venture which draws more fuel into the pot. The thread I found does suggest it can be used as a failsafe so fuel can still be drawn even if the main lift pump fails.

I do have a question about the actual lift pumps though, and it's regarding the rated flow. I've seen Ryan from Darkside suggest that the flow of the PD lift pump is in the region of 80gph, this is obviously extremely high.

I've been through the VDO catalog and they don't even make a pump that flows this much. Unless it's 80gph free flow rather than 80gph at 0.5bar (supply pressure).

Does anyone have the actual specs of the PD pump? the VDO part numbers are either 993762120 or 993762094 (from the pump casings themselves).

Thanks

For reference, this is the thread I mentioned I my post above: http://forums.tdiclub.com/showthread.php?t=225672