Over Fuel Valve (OFV) operation

[philip]

But notice the 1981 Nissan Lift Pump operation we've had over a year (Apr 27, 2006).

I thought that was a 1982?

1981 Datsun Product Bulletin. Sorry.

[philip]

I will verify Philip's pressure measurements with the stock OFV. After cleaning mine and verifying it moved OK with a wire, and even pulling off the line going back to the tank and directing it to a bucket (to eliminate possible restriction) mine would still peg a 30 psi gauge when revved up in neutral. So I tried taping the gauge to the windshield and going down the road with the same result.

Thank you "Phil".

The true pressure limit is due to the spring under the Lift Pump's piston. As you will see, only 28 psi is expected. Either Datsun lied or the Lift Pump piston has been replaced years ago. Seems the aftermark Bosch part has a stronger spring. Oh well

The OFV vent leaves only a relative small loss due to its pin hole.

[Nissan_Ranger]

Considering the way the OFV operates, I chose not to use the Nissan OFV at all.

Mine plugged last winter and managed to build enough pressure to blow a hole in a fairly new high pressure rubber fuel line. A few days later, another plugging caused the pressure to climb to where it managed to blow a hole through a rust spot on the secondary filter... Enough of that crap; I punched out the valve altogether and haven't had a problem since. I'll likely have to block the return line (or at least restrict it) if I ever have to bleed the fuel system. No problem. That beats trying to fix a fuel line problem at -20 in the dark on the side of the road with nothing but a pair of needle nose vice grips and a pocket knife.

N_R

[philip]

The OFV does ... not ... control fuel line control pressure. You could pinch off the OFV line right there ... and it would not matter. It is the lift pump's -spring- piston that limits fuel pressure.

Agreed ... normal lift pump pressure is applying to the OFV which in turn overcoming an old line rubber hoses and / or cracked metal lines.

As for your "fairly new" rubber line, you used gasoline, vapor, or some other soft rubber that quickly failed on diesel fuel. Fuel line pressure is in the range of 30-40 psi.

[Nissan_Ranger]

The OFV does ... not ... control fuel line control pressure. You could pinch off the OFV line right there ... and it would not matter. It is the lift pump's -spring- piston that limits fuel pressure.

Don't know where the pressure came from or why, but the plugged OFV ...was... causing OR ...was... part of the problem; punching it out solved the problem.

As for your "fairly new" rubber line, you used gasoline, vapor, or some other soft rubber that quickly failed on diesel fuel.

No. The line I used is fabric coated and rated 100 psi max for use in diesel fuel systems. Wish I could get more of it, but the dealer I got it from is out of business since farming is no longer my area's main industry. Pinholed and nicely sprayed fine mist under the hood. Having been a licenced mechanic here in Ontario since 1979 after the usual 5 year apprenticeship has taught me to specify beforehand what the intended purpose of hoses, lines, belts or whatever ...before... ordering them.

N_R

[philip]

We've been around this OFV and the Lift Pump matter long ago. We've also gone around diesel vs biodiesel fuel hose.

I suggest the hose was material defective. And it is impossible to make line pressure above the lift pump's piston -spring-.

RE: "...to blow a hole through a rust spot on the secondary filter."

Then the problem is the rusted secondary filter ... not the Lift Pump pressure ... which is only 30-40 psi. Those filters are supposed to be replaced every year if not more often.

[Nissan_Ranger]

Me bad.... I promise not to do it again

[philip]

Me bad.... I promise not to do it again

This might be helpful about fuel hose.

[chrwei]

great thread, especially to those new to diesels like myself.

one thing I'm not clear on though, the IP is actually a pump that provides increased pressure to the injectors, or is it just a "distributor" and the lift pump does all the real pumping work?

also, is it worth adding a pre-filter on a '81?

[asavage]

one thing I'm not clear on though, the IP is actually a pump that provides increased pressure to the injectors, or is it just a "distributor" and the lift pump does all the real pumping work?

The lift pump, which is bolted to the side of the IP but is not really part of it IMO, does the low-pressure conveyance of fuel from the fuel tank, through the Primary filter (which is a pre-filter, it's only a screen), to the lift pump, then out to the Secondary Filter, and finally to the IP's main housing. It tops out at around 20 PSI.

The Inline IP's four plungers pick up this moderately-pressurized fuel and further pressurizes it (in spurts) to around 1900 PSI.

also, is it worth adding a pre-filter on a '81?

Take a look at the links above, you'll see that the firewall filter is adequate to the task of keeping gravel out of the lift pump. The (very good) Secondary filter gets the rest. Why complicate things?

For balance, member TooManyIdeas did go overboard on fuel filtration.

[chrwei]

because, the '81's have no primary filter, as mentioned earlier in this thread. there is a steel pipe on the frame rail to the tank, and a 1.5 ft hose from there to the LP, no filter. So, should I get some extra hose and add a primary filter or not worry about it since this is a factory setup?

[asavage]

Oops, sorry I missed the "81" part.

Yes, I'd add a pre-filter. Do not use a paper element though.

[exsimguy1]

Al,
WHere did all my posts go?
Terry


Never mind, found V2.0 OFV

[TooManyIdeas]

overboard?! why i oughta... nyuk nyuk nyuk

[Tracteur Tom]

Hi all, new member here so be gentle !

This thread is very informative, I've read it several times ! I'm in the process of converting a TD42 from a rotary to a in-line IP and wonder whether the issues raised by Philip (RIP Philip, your posts were brilliant and inspiring) were ever resolved ?

Having mulled the fuel plumbing and OFV valve operation over in my head numerous times, I'm of the opinion that the original Nissan set-up is the way to go. To do Philip's mod I think you loose the auto air pocket venting that the original set up does in the filter head. The mod relies on a full flow though the IP to vent air pockets, OK, but I can see air being trapped in the IP maybe, so whats the point ?

Any further thoughts guys ? I know the use of an electric pump is probably a good way to go, but - as said in an early post, keep it simple, there's not much wrong with the Nissan set up !

Thanks

TT