I have to hook my primer/ lift pump output strainght into the IP input to get the overflow valve to open up. It primed it perfect. No leaks no air anywhere else when it’s hooked up correctly, but will only prime and run like this. I just loop the in and out of the filter housing. If i hook it up correctly all pressure bleeds off through return line.
This is a swap into an ‘88 4runner. But as for as I’ve been able to see i do have itbdouted correctly. Will add pics in a bit.
What keeps the pressure from just dumping into the return?
Again, when main filter is integrated “correctly”, pressure bleeds off to return and it will not bleed or run

I am not sure from you description what you did but the Fuel Pressure Relief Valve is the Over Flow Valve. It is what in normal circumstances is supposed to hold the pressure inside of the Fuel Injection Pump Housing.

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84 300D, 82 Volvo 244Gl Diesel

So it’s not the overflow that has a problem. If i hook the lift pump output fuel line straight to the injection pump return, it will prime the pump perfectly, buzz, flush air, run and drive.

The problem is when i hook up the mail fuel filter, the pressure from the primer is lost to the return line. The fuel will just cycle and never enter the injection pump, following the path of least resistance.

There must be something in the return system that I’m missing or isn’t hooked up correctly, this mystery mechanism must create a greater pressure than the overflow in order for the fuel to cycle through the IP, and not just flow right back into the tank via return line. Does that make sense?
Ask away. I’ll go get a picture

Wellp... The pics won’t load... Any questions? Really just need to know if there is a check valve or something or something in the return system, i feel like it would t be far from the banjo bolt for the fuel filter considering that’s what let’s the fuel from the filter Flow straight into the return to the tank

The banjo bolt on the back side of the IP (return line) should have a spring-loaded check ball in it. Everything else is just a standard hollow banjo bolt. The filter housing should have a bleed hole in it to burp the air out, but the majority of fuel should be cycling through the IP.

Check your hose connections to make sure they're routed correctly.

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Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Here is a picture of what Dieseasel300 is describing. The Fuel Pressure Relief/Overflow Valve.

When you are pumping on the hand primer and air is going through it that valve will not make that buzzing squealing noise. So it could simply mean that when you go back to the main filter you still have air that needs to be processed out of the system.

On some versions of the Fuel Pressure Relief/Over flow Valve instead of the screwed in cap #5 on the diagram there is a Ball Bearing pressed into the end of it.

In the 2nd pic the red arrow points to where the Fuel Pressure Relief/Over flow Valve is on the Fuel Injection Pump and the green arrows show the return fuel going back to that junction on top of the Spin On Filter Housing and on the way back to the Fuel Tank.

Attached Thumbnails

   

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84 300D, 82 Volvo 244Gl Diesel

#15 is the fuel from the Lift Pump going into the filter housing. There is actually arrows pointing the direction of the fuel.

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84 300D, 82 Volvo 244Gl Diesel

The Fuel Pressure Relief/Overflow Valve is the only one way valve in the fuel return part of the system.

Of couse the Fuel Supply/Lift Pump has 2 valves inside of it.

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84 300D, 82 Volvo 244Gl Diesel

Thank you guys.
I totally understand the overflow concept. That valve works fine. IF, i have the lift pump hooked up straight to the IP
My fuel routing is correct but when i try to prime the filter, the pressure that should cause the fuel to enter the IP and force air out is being lost through the return hose. I can hear fuel dumping back into the tank because this is the path of least resistance.
One thought, are they separate systems?
Does the banjo bolt connect only the return system or does/should the hole at the bottom allow fuel to flow straight from the filter to the return? If it shouldn’t, then maybe it was overtightenwd without washers at some point and now does?

To say this more concisely, fuel pressure from the lift pump is going straight back into the tank. Should that be possible? If so, why would the fuel ever be able to overcome the IP backpressure when the path of least resistance is the free flowing return...
If that doesn’t make sense please ask questions. I did the swap so obviously I’m competent haha. Thanks again.

If the Fuel Filter was totally plugged up or the central bolt was clogged fuel would likely not go into the fuel injection pump.

What if the lower O-ring on the central bolt on the Filter was gone or damaged and not sealing enough to let the fuel pressure go by?

The wrong filter bolt in the housing? There is 2 types of filter bolts for 2 types of housings.

Loosen banjo bolt 7 and pump with the hand primer and see of fuel comes out.

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84 300D, 82 Volvo 244Gl Diesel

No there is nothing blocking fuel from going anywhere. That’s the issue. The fuel enters the filter and disperses. It doesn’t go back up the injector or overflow return lines for obvious reasons. It does enter the low pressure side of the injection pump.
The issue is that the primer cannot build enough pressure to force the fuel through the high pressure side of the pump because the fuel just leaves through the tank return line unobstructed.

That's not possible if you have everything plumbed correctly. The filter housing has a TINY air bleed hole in it. Otherwise, all fuel has to go through the IP to get to the return line. Something isn't connected up correctly, or you're missing a rubber O-ring or gasket somewhere.

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

I have not notice which engines fuel injection pump you are speaking but there is a fuel supply system that is what you are priming with the hand primer and on my year and model when the engine is idling you get about 8 psi and at 3000 rpm you get about 18 psi of pf pressure held in side of the Fuel Injection Pump Housing.

That supplies the Elements/Plungers and Barrels with fuel and it is those Elements that supply the high pressure fuel to the Injectors. You cannot prime with the hand primer though the Elements up to the Injector because each Element has a Delivery Valve (one way valve) between the Injector and the Element. I believe the spring on the delivery valve is stronger then your hand primer can cause to compress.

So you have a fuel supply system and a high pressure fuel system (the one that injects the fuel).

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84 300D, 82 Volvo 244Gl Diesel

I can't help without being there and you live too far away even if I had the desire to go.

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84 300D, 82 Volvo 244Gl Diesel