The picture below is typical of Datsun/Nissan 720 pickup trucks having the Bosch inline injection pump.
Incorporated into the OFV ("over flow valve") banjo bolt is a small spring loaded fuel pressure relief valve with a small orifice. If stuck closed (and even when it operates through the orifice) the fuel line pressure will be higher than spec (20 psi) and accumulated air will make its way to the injection pump. From the injection pump, the only way air can exit is through the injectors. The exception here would be manual air bleeding using one or both of the bleeding bolts on the injection pump. See thread title: Fuel Filters
This OEM pressure regulator / air bleed is not perfect, partly because of its small flow rate relative to pump output. So air will still reach the injection pump when the fuel tank level is low. When this happens (nearly always associated with running the fuel tank at or near the Empty mark on hilly terrain or frequent traffic stops), the engine will run rough at idle until the air is passed through whichever injector injested the air.
Last edited by philip18 years ago, edited 10 times in total.
-Philip Passed 08May2008
My friend, you are missed . . .
1982 Datsun 720KC SD-22
So ... I've been experimenting with this solution.
Basically, move the lift pump pressure control location from the top of the secondary filter to the injection pump itself. This relocation allows any air reaching the injection pump a path back to the tank instead of being pumped through the injectors which causes misfiring. The injection pump is now self purging. This modification requires an aftermarket (adjustable) pressure relief control bolt (smaller thread size too) and a smaller double outlet banjo fitting.
16x1.5mm engine oil drain plug and washer used.
-Philip Passed 08May2008
My friend, you are missed . . .
1982 Datsun 720KC SD-22
philip wrote:Basically, move the lift pump pressure control location from the top of the secondary filter to the injection pump itself. This relocation allows any air reaching the injection pump a path back to the tank instead of being pumped through the injectors which causes misfiring. The injection pump is now self purging. This modification requires an aftermarket (adjustable) pressure relief control bolt (smaller thread size too) and a smaller double outlet banjo fitting.
So how is this mod working for you? Any problems? I like what you did cause I was looking into an aftermarket filter to replace the secondary unit but that over flow valve was holding me back.
82 Datsun 720 King Cab Diesel - FOR SALE !!!!
85 Chevy Monte Carlo CL
90 Toyota Pickup -SOLD
05 Kia Spectra 5
02 Dodge Dakota Quadcab
-OOOOOOOOHHHHHHHHH YEAH!!!!!
philip wrote:This modification requires an aftermarket (adjustable) pressure relief control bolt (smaller thread size too) and a smaller double outlet banjo fitting.
philip wrote:Basically, move the lift pump pressure control location from the top of the secondary filter to the injection pump itself. SNIP.
So how is this mod working for you? Any problems? I like what you did cause I was looking into an aftermarket filter to replace the secondary unit but that over flow valve was holding me back.
Tank above the 3 gallon low has been working fine. Most noticeable when fuel tank is down to a few gallons combined with stopping and starting on hills. Instead of misfiring for up to a couple of minutes after coming to a stop, the misfire clears up in a few seconds. Also, this redesigned fuel manifold pressure operates the fuel line in the 15-19 psi range because this new pressure control bolt has a much larger flow capacity. The OEM OFV bolt with its tiny orifice permits line pressure to exceed 30 PSI quite easily.
Last edited by philip18 years ago, edited 2 times in total.
-Philip Passed 08May2008
My friend, you are missed . . .
1982 Datsun 720KC SD-22
philip wrote:This modification requires an aftermarket (adjustable) pressure relief control bolt (smaller thread size too) and a smaller double outlet banjo fitting.
Parts source?
My diesel man (Edgar) at DSI evaded that very question. Since the pressure control bolt and banjo were new pieces, both must be from something Bosch uses currently.
For the search engine:
DSI Diesel Specialists, Inc.
Bosch * Stanadyne * CAV * Nippondenso * Roosa Master
Fuel injection pumps & injectors
Edgar Sibiglia
350 E. Orangethorpe Ave #12
Placentia, CA 92870
714.528.8442
714.528.8464 fax
Last edited by philip18 years ago, edited 1 time in total.
-Philip Passed 08May2008
My friend, you are missed . . .
1982 Datsun 720KC SD-22
I'm having fuel-related issues with my truck (fuel leaks and hard starting) which means that it's time to replace all of the stock fuel hoses. While messing around with the fuel system, it would be a good time to do this mod, but I've got some questions . . .
I notice that the stock OFV location, right at the top of the secondary filter, is at the highest point of the fuel system (on the entire vehicle), which seems to make sense, as one would think this is where the air in the system would collect.
Philip--do you think the improvement that you noticed on your vehicle was due to 1) a stuck OFV, and/or to 2) moving the OFV to the side of the pump? It seems to me that if the OFV was working properly at the secondary filter, no air would ever get to the injection pump (although the output from the secondary filter is also at the very top, so maybe air gets in due to this). Am I wrong in thinking this?
Let's discuss!
1982 Datsun 720 King Cab, SD22, 86K miles (sold)
1981 Rabbit LS 4-door, 1.6D, 130K miles (sold)
1996 Passat TDI 4-door sedan, 197K miles
redmondjp wrote:I'm having fuel-related issues with my truck (fuel leaks and hard starting) which means that it's time to replace all of the stock fuel hoses. While messing around with the fuel system, it would be a good time to do this mod, but I've got some questions . . .
I notice that the stock OFV location, right at the top of the secondary filter, is at the highest point of the fuel system (on the entire vehicle), which seems to make sense, as one would think this is where the air in the system would collect.
Philip--do you think the improvement that you noticed on your vehicle was due to 1) a stuck OFV, and/or to 2) moving the OFV to the side of the pump? It seems to me that if the OFV was working properly at the secondary filter, no air would ever get to the injection pump (although the output from the secondary filter is also at the very top, so maybe air gets in due to this). Am I wrong in thinking this?
Let's discuss!
The OFV was not stuck ... easily moved off seat with a wire poke. If you notice, the orifice is only about 0.20" whereas aftermarket pressure control bolt has about an 1/8" port (not shown). Even though the OFV pop-open pressure is supposed to be 20 psi, I got fuel line psi much higher readings because the OFV is restricted due to the vent orifice.
Last edited by philip18 years ago, edited 1 time in total.
-Philip Passed 08May2008
My friend, you are missed . . .
1982 Datsun 720KC SD-22
After a few months of the "new" pressure control bolt arrangement, a shortcoming needs to be dealt with.
The OEM "OFV" was partial pressure control at the secondary filter whereas the first revison placed full pressure control on the IP gallery ... which still allowed air to reach an injector or two so but vent it quickly.
If this second revision fixes things entirely, I'll post a few photos.
-Philip Passed 08May2008
My friend, you are missed . . .
1982 Datsun 720KC SD-22
asavage wrote:Am I right in thinking that the lift pump is a fixed-volume, positive-displacement pump?
If that's true, then downhill running at high revs must really push the pressure-side lines to high pressure.
The cam opening ramp provides the suction stroke while the piston spring provides fuel pressure. Lift Pump
I never measured lift pump output BEFORE the pump overhaul. With the original OFV arrangement and rebuilt lift pump, the gallery pressure would EASILY peg a 30 psi gauge not far off idle (measured at the IP). NOT desireable. Version-I solution limits gallery pressure to 20 psi tops and improved air purging ... at the pump instead of the secondary filter. So the occasional gulp of air a low fuel tank levels just got purged in a couple of seconds.
Improvement in Beta.
-Philip Passed 08May2008
My friend, you are missed . . .
1982 Datsun 720KC SD-22
does anyone have a price and part # for the over flow valve?
82 Datsun 720 King Cab Diesel - FOR SALE !!!!
85 Chevy Monte Carlo CL
90 Toyota Pickup -SOLD
05 Kia Spectra 5
02 Dodge Dakota Quadcab
-OOOOOOOOHHHHHHHHH YEAH!!!!!
Considering the way the OFV operates, I chose not to use the Nissan OFV at all. The OFV does not -limit- fuel pressure from the Lift Fuel Pump ... through the 2nd Fuel ... to the injection pump. All it does is bleed off some fuel / air and sends it back to the rear tank. The OFV -starts- at 15-20 psi and only passes through that little pin hole (est 0.015").
-Philip Passed 08May2008
My friend, you are missed . . .
1982 Datsun 720KC SD-22
philip wrote:Considering the way the OFV operates . . .
In your picture, it looks as if that's a slide valve (shuttle valve). The hole bleeds a little air, and when pressure comes up the whole inside gizmo moves to reveal a larger passage.
[later]
Hmmm. I don't have a 1982 720 FSM here at work, but in the '82-83 720 FSM on pg. EF-30 under "TEST" for the Overflow Valve, it says:
1982-83 720 FSM wrote:Attach a pressure gauge to fuel filter discharge port, and check valve opening pressure by operation priming pump. If pressure is not within range of . . . 16 to 21 PSI . . . replace overflow valve.
philip wrote: The OEM OFV bolt with its tiny orifice permits line pressure to exceed 30 PSI quite easily.
It's a regulating valve. It should hold pressure to maximum of 21 PSI. If it doesn't, it's broken.
