The 1981 720 diesel has no primary filter, it has only one filter, and I've covered that down in the Secondary filters section.
This is the small, plastic filter on the passenger firewall. It is used in 1982-86.5 models, and comes in two styles: 1982 (and '83 w/SD22) and 1983-on (w/SD25).
> Is the Fram p6415 a correct primary filter for the sd25? i know there has
> been some discussion on the possibility of getting one for a gas engine that
> is wrong.
The correct primary fuel filter for 1982 (from Jul-81) and 1983 w/SD22 is Nissan 16401-36W01. I have not found an aftermarket substitute that is identical to it. Both the inlet and discharge lines are on the side of the top end. Philip took a comparison photo of the wrong (left) and correct (right) filters:
In 1983, for the SD25, Nissan changed to a filter that is nearly identical, but whose inlet/outlet are top/bottom. OEM No. 16401-29W00 :
Apparently, the Fram P6415 and the AC Delco GF630 are available aftermarket replacements.
AC Delco GF630:
I would usually never recommend a Fram product, but this may be one of those instances where quality truly doesn't matter (much). Anything that the pre-filter (primary filter) passes will get caught in the (much more expensive) main filter.
It appears, from the pics, that these filters would probably work fine in '81-82 trucks. But I think I'd rather go the other way: use the early filter in the later trucks, because the crap shows up very nicely in the early filters, and I'm not sure it would, in the later ones.
The OEM filter for 1982 costs about $11 from the dealer. The AC Delco GF630 for '83-on seems to cost approximately the same (RockAuto).
[tale of woe]
The firewall-mounted plastic filter seemingly isn't available from anywhere except the dealer. I had been fighting this fuel system off and on for about a month, and finally figured out why I was still having problems: that little plastic filter.
While Wix does not have a listing for the firewall primary filter, the Wix 33214 (note the '1' in the part number) is physically identical to the OEM version, and that's what I had been trying to use. Unfortunately, diesel does not readily pass through the media in it, which is designed for gasoline use. With new fuel and a new Wix filter, I pulled 8-11" vacuum out of that filter by the lift pump! Just for reference: that's a LOT of vacuum for a lift pump. Not surprisingly, this truck will run for about two miles with cold fuel, and then it'll slowly starve. On a warm day, I can run it 50 miles (I know, because I did it last week), but in the morning it is two miles, tops.
The vac test identified the problem, and on a hunch, I drove 60 miles to the dealer and bought the OEM filter (see first picture, above). The OEM filter's body is translucent, and you can see plainly that the OEM filter media is wire mesh! Obviously, it flows diesel a lot easier than a paper element, which is what is in the Wix unit I was using.
Summary: do not substitute a gasoline fuel filter for the OEM primary fuel filter: it won't work! The Wix secondary filter (spin-on) is fine, but there's a reason Wix doesn't list a replacement for the primary filter.
[/tale of woe]
[Edit, added Aug2013]
The Wix 33972 filter is a plastic mesh filter that appears to be an acceptable non-Nissan filter for this application, and might be a drop-in replacement for 1983-on:
[end Aug2013 edit]Wix wrote:Principle Application: Plastic Universal In-line fuel filter w/ 5/16" lines - can be used w/ gasoline or diesel fuel.
Style: Fuel (Complete In-Line) Filter
Type: Full Flow
Media: Plastic Mesh
Outer Diameter Top: 1.828
Outer Diameter Bottom: Closed:
Burst Pressure-PSI: 58
Max Flow Rate: 1 GPM
Nominal Micron Rating: 50
This is the larger, metal spin-on filter on the passenger fenderwell.
1981 uses the Wix (33393/NAPA (3393):
They're pretty cheap (about $15) and fit other rigs like Isusu NPR, Nissan UD, so they're in stock most everywhere. No "Water in Fuel" sensor hole in the bottom. Because the 1981 720 has no primary filter, and this filter is smaller than the 1982-on filter, this might make necessary more-frequent fuel filter changes.
1982-86 uses the Wix (33224/NAPA (3224) (NLA Aug2013):
This filter is used ONLY on '82-86 Nissan SD diesels, so availability is sparse; plan on ordering it up in advance. Pricey: about $30 is list, often you can get it for a bit less than that, depending upon the store.
This later Secondary Fuel Filter has provision for a Water-In-Fuel sensor, hence the threaded hole in the bottom. The black o-ring shown in the left pic above is the sensor seal, and is often included with a replacement filter. The "sensor" is merely a steel probe. The real sensing is done by a module under the dash, that measures the resistance of the probe to the filter shell: water at the bottom will conduct (a little) from the probe to the shell; fuel will not. There's no float switch, as in a brake fluid reservoir.
There's a decent tutorial (with nice line drawings) at an Autozone page, for the diesel fuel filter change procedure. It includes the fuel system bleed procedure, with drawings.