Understanding the 300tdi fuel supply system

**Plutei** · 25th October 2020, 01:09 PM

Trying to understand as much as I can about the 300tdi fuel system as I have been getting air in fuel at low fuel tank levels.

At first my theory was that there was something wrong with the baffles or pickup in the tank whereby at lower fuel levels the fuel would starve if I went up hill. I'm now thinking I have a leak that air is getting into and the air intake is exacerbated as the tank runs emptier or I drive uphill as there is a greater vacuum effect. So I need to check for leaks /replace fuel lines but I do have a few questions.

1) If I open the bleed screw while idling should fuel come out?
2) I understand the system is self-priming, but is it actually self-bleeding?
3) Where does air go if the system is self-bleeding? Injected into the cylinders?
4) My lift pump lever normally has zero resistance through the whole stroke in most engine positions (lobe engaged). If the lobe is disengaged then most of the stroke has no resistance except the last 10mm, which will have resistance and actually pump, but never more than 10mm. Is this normal?

Thanks!

**Red90** · 25th October 2020, 01:23 PM

1) Yes. If not then the lift pump is not working.
2) Yes
3) To the tank

**Plutei** · 26th October 2020, 10:54 AM

OK, when engine is idling and I open the filter bleed screw mine starts the suck in air and engine starts to splutter. So lift pump is supplying fuel at a lower flow than the injection pump is using which is wrong. Lift pump has done this since it was replaced so clearly a dodgy unit.

Time to get an electric pump at the fuel tank I think. Aside from bypassing the lift pump is there anything else that needs to be considered with this?

**Red90** · 26th October 2020, 11:53 AM

That shows the lift pump I’m not working at all. The engine will run with just the injection pump. The lift pump is not mandatory.

Fix the lift pump.

**Plutei** · 26th October 2020, 03:32 PM

Great, thanks Red90.

**Blknight.aus** · 26th October 2020, 08:02 PM

you dont have to bypass the mechanical lift pump (but I advise you do) run a facet pump off the fuel cut solenoid for the Injector pump, by taking the lines off of the mechanical lift pump, putting 3/8th hose over them, hose clamping and routing to the facet pump. bolt the facet pump on 2 resilliant mounts on the inner part of the wheel guard.

never worry about priming again.

**Plutei** · 26th October 2020, 09:32 PM

Thanks, Is there much benefit to mounting at the back near the tank pick up or not really?

Originally Posted by Blknight.aus

you dont have to bypass the mechanical lift pump (but I advise you do) run a facet pump off the fuel cut solenoid for the Injector pump, by taking the lines off of the mechanical lift pump, putting 3/8th hose over them, hose clamping and routing to the facet pump. bolt the facet pump on 2 resilliant mounts on the inner part of the wheel guard.

never worry about priming again.

**Blknight.aus** · 27th October 2020, 05:05 AM

Originally Posted by Plutei

Thanks, Is there much benefit to mounting at the back near the tank pick up or not really?

the facet pump will prime quicker and if you have a leak it may be easier to find as more of the fuel system is upstream of the pump.

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