Installing A Donaldson Fuel Water Separator 

Last time, we explained how we were going to install a fuel water separator from Donaldson Filters on our Isuzu NPS. At that stage we hadn’t decided where or how we were going to install it. Now it’s installed, here’s an update.

Where To Install It?

As we saw last time, the fuel water separator needs to be mounted between the dual fuel tank switching valve and the manual primer. This diagram from last time shows the location.

Diagram of our modified fuel system. Donaldson Filters.
A diagram of our fuel system, showing where the Donaldson fuel water separator will be installed.

We’re limited for space. The fuel water separator is quite long and has to be mounted vertically. Also, we wanted to keep the fuel hoses short. In the end there was really only one option… make up a bracket and mount it to the underside of the cab.

We found a spot which is tucked in behind the front mud guard. This is important. You obviously don’t want to mount the unit where it could get hammered by stones kicked up from the front wheel.

Fixing the separator to the underside of the cab is okay because the dual cab doesn’t tilt and conveniently there are solid gussets under the cab. We made up a bracket from 40×6 galvanised mild steel flat bar and painted it black for extra protection.

Then we drilled four dia 7mm holes in the cab gussets and gave the exposed metal a coating of Gal-Met galvanised paint for protection from corrosion. Next step was to mark and drill the flat bar bracket, then bolt it on with M6 bolts and Nyloc nuts.

Mounting bracket for Donaldson fuel water separator.
The flat bar bracket straddles two strong gussets under our dual cab.

Now the Donaldson fuel water separator already has two ¼” UNC threaded holes in the aluminium housing. So we used these holes and added a small dob of Loctite to make sure they wouldn’t work loose. Don’t over-tighten these bolts, as the Donaldson housing is cast aluminium and you don’t want to strip the threads.

Running The Fuel Hose

With the filter mounted, we then screwed in the supplied brass barb fittings after applying thread tape. Teflon thread tape is compatible with diesel… make sure you use thread tape or sealant that’s compatible with diesel.

The barb fittings are sized for ⅜” fuel hose, so we bought a length of 10mm fuel hose from our local Isuzu truck dealer. A box of stainless steel hose clamps completed the picture. It’s worth spending a bit more on the hose clamps. The last thing you want is a 50 cent hose clamp failing in the middle of nowhere and bringing you to an unscheduled standstill.

We removed the existing hose from between the switching valve and the manual primer. This had a thick protective sleeve, which we re-used on the new hose. So we ran two new hoses, one from switching valve outlet into the Donaldson fuel water separator and another out of the separator into the manual primer.

When you run these, make sure they have a gentle radius. Avoid tight bends, as the hose might kink. Check for potential rub points and protect the hose.

Inlet hose for Donaldson fuel water separator.
Inlet hose has a protective sleeve, is fastened to other components to prevent rubbing and has a nice long radius bend.

As we said earlier, on one hose we re-used the existing protective sleeve. On the second hose, we simply used corrugated split tubing, which you can buy at any auto parts store. Then we found a few places where the hoses might rub and used cable ties to keep them firmly in place. Just nip up the cable ties, you don’t want to squash the fuel hose.

Donaldson fuel water separator outlet hose.
We used split corrugated tubing to protect the outlet hose. Again, we’ve maintained long radius bends to prevent kinking.

Priming The System

Now you’re ready to fit the filter, clear bowl and twist drain.

To avoid having to prime the entire filter cartridge, use a (clean) jerry can to pour diesel into the cartridge before installation. Rub a smear of diesel around the seal with your (clean) finger and screw it onto the housing. Screw it on firmly without over-tightening.

Then you simply crack the bleed valve open on the manual primer (the little black dot at the front of the manual primer in the photo above) and keep priming until there are no more air bubbles coming out of the bleed valve. Then close the bleed valve and keep priming until you feel the plunger go hard.

One handy tip: use the redundant hose you removed earlier to slip over the bleed valve and into a container. This way you won’t make a mess.

The Moment Of Truth

The time has come to start it up and check for leaks.

Our truck started straight away and we ran it on high idle for about 10 minutes. With no leaks in sight, our job was complete! Keep an eye on the hose connections for the next few days to make sure a leak doesn’t develop over time. In fact, keep an eye on it anyway. Add this to your general check, every time you look over your vehicle.

Donaldson fuel water separator mounted neatly under the dual cab.
The separator sits up high, neatly tucked behind the front mud guard (rear door step removed in this photo). It is in the perfect place for a quick visual check every time you step out of the cab.

Summing Up

We now have added protection from dirty fuel with the 3 micron Donaldson fuel water separator installed. We’re now full time on the road and no doubt we’ll encounter bad fuel at some stage.

The best part is, we now have a visual check. Every time I jump out of the driver’s seat, I quickly glance at the clear bowl to make sure all is well with the fuel.

Donaldson fuel water separator is protected behind the mud guard.
After driving on a mix of rocky, dusty and muddy roads, the unit is still clean… almost! So it’s definitely well protected in this location.

This gives me peace of mind and the way I see it, this unit is cheap insurance.

And lastly, how easy is this unit to install? If you have some mechanical knowledge, it’s a piece of cake. Having said this, if you have any doubts at all get a licensed mechanic to install it.

If we’re unfortunate enough to encounter dirty fuel, I’ll fill you in on the details of how the separator performs.

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NOTE: The Donaldson Fuel Filter Kit was supplied to us free, in return for a review. (That said, our reviews are always honest and authentic – we tell it how it is). Thanks to Brett Howe from Donaldson Australasia and Craig Kennedy from Dwyers Truck Centre, Nowra for organising this.

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2 thoughts on “Donaldson Fuel Water Separator Installation | Isuzu NPS Common Rail Diesel”

  1. I bought the kit to do the same for my isuzu npr. Do you know how long the cartridge will last. I bought 4 cartridges to be safe but really have no idea.


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