Over Fuel Valve (OFV) operation

SD diesels were widely available in the US in the 1981-86 Datsun/Nissan 720 pickups, and in Canada through '87 in the D21 pickup.

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philip
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#16

Post by philip » 12 years ago

asavage wrote: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.
Correct. The liquid/air mix enter the orifice .... then open a ball/spring starting at "16-21" psi.

What the Nissan fails to make clear is: The OFV orifice pass is MUCH LET LESS fuel .... BOTH than .... the ability of the Lift Pumper pressure pressure and volume. (I still have short speech) I first become aware AFTER rebuilding the Lifter Pump.

"16-21" psi only means when the OFV orifice opens ... it does NOT limit the fuel line (lifter to injection pump) to "16-21" psi.

The new Lifter Pump present (don't know the Original lift pump) resulted the fuel line pressure exceeded "16-21" psi... passed over 30+ psi (my dial pegs 32 psi with only a slightly RPM aboved idle.).

I corrected this first design installing using a "adjustered pressure vent" .... that has about 1/8" capillary and a adjustabled ball/spring. This has shown capable Lift Pump 20 psi and flow vent and at the injection location.

Note: the external seen ball/cylinder is physically move as needed.
Note: I've come up with a later version better ... but haven't not shown it here. :wink:

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-Philip
Passed 08May2008
My friend, you are missed . . .

1982 Datsun 720KC SD-22

"Im slow and I'm ahead of you"

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asavage
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#17

Post by asavage » 12 years ago

philip wrote:"16-21" psi only means when the OFV orifice opens ... it does NOT limit the fuel line (lifter to injection pump) to "16-21" psi.
So what you're saying is, it begins to bypass more fuel at 16-21 but even wide open (or full capacity), it cannot pass enough fuel to limit pressure to <21 PSI?

Hmmm. I think I'll have to do that test on mine, too.
Note: I've come up with a later version better ... but haven't not shown it here.
Well, get on the stick, I want to see it -- and a part No. this time, too ;)
Regards,
Al S.

1982 Maxima diesel wagon, 2nd & 4th owner, 165k miles, rusty & burgundy/grey. Purchased 1996, SOLD 16Feb10
1983 Maxima diesel wagon, 199k miles, rusty, light yellow/light brown. SOLD 14Jul07
1981 720 SD22 (scrapped 04Sep07)
1983 Sentra CD17, 255k, bought 06Jul08, gave it away 22Jun10.

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TooManyIdeas
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#18

Post by TooManyIdeas » 12 years ago

asavage wrote: 16413-G7000 Valve,overflow, $12.85 + $10 shipping

I hope you appreciate this, it was a lot of work.
Thank you Mr. Savage. :D
philip wrote:Considering the way the OFV operates, I chose not to use the Nissan OFV at all.
I would love to run the setup you are philip, but I can't get any info on the parts you used.

Image

My interest in this has to do with concerns over filtration. If I'm reading the diagram right, then fuel goes through the IP before going through the water separator. That scares me a little looking at what crap can make it through that screen.

Hypothetical: What would happen if you eliminated the OFV altogether and just ran a line from the #4 drain back to the tank?

Also could you run the water separator before the screen?
82 Datsun 720 King Cab Diesel - FOR SALE !!!!
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philip
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#19

Post by philip » 12 years ago

asavage wrote:So what you're saying is, it begins to bypass more fuel at 16-21 but even wide open (or full capacity), it cannot pass enough fuel to limit pressure to <21 PSI?
You've got the idea. :wink: Even with a restricted OFV orifice opened (returns to rear tank), the Lift Pump line flows higher PSI to the injection pump ... which may ... assist a little to increasing the fuel rack movement.
-Philip
Passed 08May2008
My friend, you are missed . . .

1982 Datsun 720KC SD-22

"Im slow and I'm ahead of you"

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philip
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#20

Post by philip » 12 years ago

TooManyIdeas wrote:SNIP- My interest in this has to do with concerns over filtration. If I'm reading the diagram right, then fuel goes through the IP before going through the water separator. SNIP
You are not "reading the diagram" as shown ... long ago. Look at the little white arrows. Also notice Lift Pump vs. the Injection Pump .... on the same assembly. :wink:
TooManyIdeas wrote:Hypothetical: What would happen if you eliminated the OFV altogether and just ran a line from the #4 drain back to the tank?

The fuel excessive out of #4 tube is ... small drips. Look at the insides of an injection ... times 4.
-Philip
Passed 08May2008
My friend, you are missed . . .

1982 Datsun 720KC SD-22

"Im slow and I'm ahead of you"

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TooManyIdeas
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#21

Post by TooManyIdeas » 12 years ago

Ok, walk me through my dysfunction

* Fuel gets siphoned from the tank into the screen (yellow up)

* The "filtered" fuel then gets pulled into the lift pump (yellow down)

* Fuel is then pushed into the water separator (orange up) (after going through the lift pump)

* It flows through and out the separator back into the injection pump (red down)

* The fuel is then pumped from the IP to the cylinders (not shown)

* Excessive fuel is pushed out the #4 drain tube to the OFV (green up)

* Passed the OFV the fuel goes back to the tank (green down)

If I'm interpreting this right, the fuel isn't completely filtered before reaching the IP. Also the water separator runs after the screen.

And if the OFV regulates such a minimal amount of fuel why bother with it at all?
82 Datsun 720 King Cab Diesel - FOR SALE !!!!
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90 Toyota Pickup -SOLD
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asavage
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#22

Post by asavage » 12 years ago

TooManyIdeas wrote:If I'm reading the diagram right, then fuel goes through the IP before going through the water separator.
The fuel is sucked from the tank, through the Primary filter (basically a wire screen) on the firewall, then through the lift pump (which is not the IP, but is mounted on the IP), then to the Secondary filter (which is what I think you are calling the water separator, since it has the Water-In-Fuel warning light sensor in the bottom (1982-on)).

Unfiltered fuel does not get into the IP, but it does go through the lift pump, which is bolted to the side of the IP.
Hypothetical: What would happen if you eliminated the OFV altogether and just ran a line from the #4 drain back to the tank?
That would work for the injector leakage return, but now how do you control overpressure of the IP feed by the lift pump? You control it by bleeding pressurized fuel . . . somewhere. Do you recirculate it or put it back to the tank?
Also could you run the water separator before the screen?
Sure, lots of diesels do. There's no real reason for dual filters. But all pumps "push" better than "pull", and a sluggish fuel (cold temps) will not "pull" through a filter nearly as well as it can be "pushed". See my sidebar in my post on the wrong Primary filter for more info.

The Primary filter (wire screen) keeps the bugs and boulders out of the lift pump's check valves. But you may be able to reroute the lines to pull fuel through the Secondary filter. My feeling is that "stock is best" as it works rather well, IMO. Nissan had almost two decades experience with the SD22 when it was put into the 720; they may have had a good reason for filtering fuel the way they do.
Regards,
Al S.

1982 Maxima diesel wagon, 2nd & 4th owner, 165k miles, rusty & burgundy/grey. Purchased 1996, SOLD 16Feb10
1983 Maxima diesel wagon, 199k miles, rusty, light yellow/light brown. SOLD 14Jul07
1981 720 SD22 (scrapped 04Sep07)
1983 Sentra CD17, 255k, bought 06Jul08, gave it away 22Jun10.

moose60
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#23

Post by moose60 » 12 years ago

TooManyIdeas,

A quick read through of your diagram interpretation seems like you have it right. But your conclusion seems wrong.
If I'm interpreting this right, the fuel isn't completely filtered before reaching the IP
Even in your interpretation, all fuel goes first through the primary filter (screen in your desc.) then to the lift pump. All fuel is then pumped through the secondary filter (or separator in your desc.) before arriving at the IP. Some seems to bypass the sec. filter, and return to the tank, but this does not see the IP. All fuel at the IP is filtered, or the IPs would be short lived.
Also the water separator runs after the screen.
Yes, the secondary filter/water separator is after the primary filter. I believe this ia due to the much finer media in the secondary filter. I bet that the lift pump can only suck medium-hard, but it can clearly put out 30+ psi (via Philip). Or, in other words, the LP can probably push through the sec. filter, but not draw through it.
And if the OFV regulates such a minimal amount of fuel why bother with it at all?
Most diesel fuel systems have a provision for allowing small amounts of air to escape. If this was not the case, everytime someone had a tiny air leak on a suction line, their rig would start to run like crap. Kinda like Philip's when he drives around below the "E" mark in hilly country, well before his super OFV install anyways.


Philip, I agree with Al. Let us see v. 2.0. :D
Byron

82 Datsun 720 KC SD22

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philip
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#24

Post by philip » 12 years ago

TooManyIdeas wrote:Ok, walk me through my dysfunction

* Fuel gets siphoned from the tank into the screen (yellow up)
Close enough. :wink:
TooManyIdeas wrote:* The "filtered" fuel then gets pulled into the lift pump (yellow down)
We refer it as "screened (primary) filter." Otherwise many have mistakenly used paper filter that restricts the fuel particularly when the fuel nears 35 degrees. :wink:
TooManyIdeas wrote:* Fuel is then pushed into the water separator (orange up) (after going through the lift pump)
Referred here as the "2nd Filter" primarily which finely stops dirt above 10 microns (meaning, Nissan uses filters smaller than 10 micron). You do not have to have a water here ... such water would then just sit at the bottom (which you should anyway).
TooManyIdeas wrote:* It flows through and out the separator back into the injection pump (red down)
"Back" was not there before in your "injection pump." You should say "Fuel is pushed through the 2nd Filter and then moved into the injection pump. Fuel having left the 2nd Filter next enters injection pump.
TooManyIdeas wrote:* The fuel is then pumped from the IP to the cylinders (not shown)
The fuel line from the 2nd Filter fills to the Injection Pump Assembly. WITHIN are four (in this case), four "plungers" (having several inter mechanisms).
TooManyIdeas wrote:* Excessive fuel is pushed out the #4 drain tube to the OFV (green up)
PRIMARILY ... these "plungers" vent unused fuel back to the "plungers" opening (all within the injection pump assembly). The -needed- fuels are pump at highly (1600 psi) to the injection nozzels. Your "#4" (called "fuel split tube") is only tiny-tiny lost pass the injection nozzles.
TooManyIdeas wrote:* Passed the OFV the fuel goes back to the tank (green down)
Tiny drips from the split tube is very tiny but ... availed anyway.)
TooManyIdeas wrote:* If I'm interpreting this right, the fuel isn't completely filtered before reaching the IP. Also the water separator runs after the screen.
And now? :wink:
TooManyIdeas wrote:* And if the OFV regulates such a minimal amount of fuel why bother with it at all?
Tell me ... what happens to the engine when a solid liquid fuel suddenly becomes foamy? :wink:
Last edited by philip 12 years ago, edited 2 times in total.
-Philip
Passed 08May2008
My friend, you are missed . . .

1982 Datsun 720KC SD-22

"Im slow and I'm ahead of you"

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philip
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#25

Post by philip » 12 years ago

moose60 wrote: snip- I bet that the lift pump can only suck medium-hard, but it can clearly put out 30+ psi (via Philip). Or, in other words, the LP can probably push through the sec. filter, but not draw through it.


Here is how thisLIFT PUMP works. :wink: Think it through.
Last edited by philip 12 years ago, edited 1 time in total.
-Philip
Passed 08May2008
My friend, you are missed . . .

1982 Datsun 720KC SD-22

"Im slow and I'm ahead of you"

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philip
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#26

Post by philip » 12 years ago

asavage wrote:The fuel is sucked from the tank, through the Primary filter (basically a wire screen) on the firewall, ....
Well technically ... :wink: ... it is atmospheric pressure that moves fuel toward the lift pump only when the LP reduces the inlet pressure. Right? :wink: Those Germans say there is no "suck" or "vacuum" . There's just pressure adjustments ... high to low. :wink:
Last edited by philip 12 years ago, edited 1 time in total.
-Philip
Passed 08May2008
My friend, you are missed . . .

1982 Datsun 720KC SD-22

"Im slow and I'm ahead of you"

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asavage
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#27

Post by asavage » 12 years ago

Pressure differential. That's what pumps create.

However, the non-technical term "suck" is a single-syllable shortcut for saying, "the lower pressure side of a pressure differential in a hydraulic system." I vote for "it sucks the fuel . . . " :D
Regards,
Al S.

1982 Maxima diesel wagon, 2nd & 4th owner, 165k miles, rusty & burgundy/grey. Purchased 1996, SOLD 16Feb10
1983 Maxima diesel wagon, 199k miles, rusty, light yellow/light brown. SOLD 14Jul07
1981 720 SD22 (scrapped 04Sep07)
1983 Sentra CD17, 255k, bought 06Jul08, gave it away 22Jun10.

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philip
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#28

Post by philip » 12 years ago

asavage wrote:snip- I vote for "it sucks the fuel . . . " :D
So did Datsun! (EC-32)

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-Philip
Passed 08May2008
My friend, you are missed . . .

1982 Datsun 720KC SD-22

"Im slow and I'm ahead of you"

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TooManyIdeas
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#29

Post by TooManyIdeas » 12 years ago

Ok so the fuel only goes through the lift pump "screened". (Thanks for the English lesson Philip :wink: ) But I've seen finer wire mesh in screen doors. Wouldn't this damage the lift pump at the very least?

I really want to run Philip's setup or at least figure a way to relocate the OFV so I can install one of these units in place of the "Secondary Fuel Filter"

http://www.rcipurifier.com/Products/Fil ... lters.html

p.s. There's something ever so naughty about that service bulletin :twisted:
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!!!!!

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#30

Post by davehoos » 12 years ago

The pump dont suck. Atmospheric pressure forces the fuel into the pump from the tank. Screen stops foreign body's getting in the pump valves.
philip wrote:Those Germans say there is no "suck" or "vacuum" . There's just pressure adjustments ... high to low.
English is a germanic language and the same rule applies to the Queens English. Mechanics in the past have been lucky to be literate.

How many poor translation have survived for many years.
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