Fuel System

The fuel system is a dual-feed, high-pressure configuration with two fuel pumps. A canister-style fuel filter feeds the fuel rail, and a vacuum-referenced bypass-style pressure regulator and tank selector solenoid send return fuel back to the source tank. An optional cross-feed feature allows fuel to be transferred between tanks for servicing, weight & balance, or fill-ups.

I used the stock fuel rails in my setup, but this design could be easily adapted to custom rails - just change the fittings at each rail as required. Note that this is usually only required for the secondary rail. Even with a custom intake, the primary rail rarely gets in the way, so it's probably worth keeping unless you have a good reason to change it.¹⁾

Fig. 1: Click to zoom in

Fig. 2: With part numbers

Simple as it seems, it took me years to finally settle on this design. I considered every option when designing it: in-tank pumps, sump tanks, transfer pumps, returnless systems, and even carburetors. In the end, I chose this design because it reduces complexity and pilot workload and contains numerous safety features.

Each tank is plumbed to its own fuel pump behind the rear seat. I rejected in-tank pumps because whatever you might claim about long-term reliability, we composite builders have dirty tanks that can clog pumps when first used. The chance is high that you will need to service a pump at least once or twice in the early hours of running the engine, and this arrangement makes that easy to do.

Fuel lines from the sumps to the pumps are protected from large debris by finger strainers inside the sumps. They're also oversized to reduce the possibility of vapor lock. It's a common myth that vapor lock is bubbles in the fuel rail. Vapor lock is actually when fuel before the pump vaporizes. Because these pumps can only push liquid, this causes them to stall and not pump any more fuel. It's a very dangerous condition with an easy remedy: provide vapor with an easy path away from the pumps.

Finally, isolating the fuel tanks allows fuel source selection. You can use this to keep the aircraft balanced while in flight by simply switching tanks every 10 minutes or so. With the optional cross-feed, you can transfer known-good fuel to one tank and fill up the other. During takeoff, running both pumps reduces the possibility of losing your engine to a failed pump. If the engine stumbles, contamination is likely and you can immediately switch to the known-good fuel. The pumps flow enough fuel to push contamination past the rails within seconds.

This design is fairly expensive (plan on $800 if you follow it verbatim). If you're on a tight budget, eBay and other vendors can be good sources of better prices, particularly for items you can buy in bulk like hose ends. eBay in particular is a good source for pre-made hoses if you know what lengths you need.

If you need to bring the cost down more, consider these options:

1. Eliminate some of the bulkhead fittings. This will make it harder to service the system but could cut 5-10 fittings from the system, and each averages $20 including hose ends.
2. Look for bulk deals on fittings and other items on eBay.
3. Look for pre-made hoses. Sometimes they're cheaper than making them yourself.
4. Use more hard lines, but only where flex/vibration aren't a factor. They take longer to make, but are a fraction of the cost of braided stainless and fittings.
5. Make your own NPT bulkhead flanges. At $7 apiece * 4 flanges, you can save a few dollars doing this. Any >0.125” scrap aluminum is fine here, although 0.25” would be better.

Because builder budgets and design goals vary, and product vendors can change/go out of business over the years, it's worth a review of the critical components used here in case you want to make any changes.

Fig. 3: GSL-392 Dimensions

Fig. 4: GSL-392 Flow Graph

The fuel pump is a Walbro GLS-392 inline pump. At a 50psi reference this pump can flow >40gph while consuming <6A at 12VDC. The pump is cooled entirely by the fuel passing through it, so it does not need to be installed in the tank. (Behind the rear seat backs is the best location.) The two fuel tanks are each serviced independently by one pump with a Tee to connect their outputs. Internal check valves prevent reverse-flow when only one pump is engaged.

Fig. 5: Earl's 230503 Fuel Shutoff

Fig. 6: Mallory 3160 Fuel Filter

It took me a long time to find just the right fuel shutoff valve. Aircraft Spruce sells a “lightweight, low-force” shutoff valve that was neither lightweight nor low-force. It's a quality product… and if you want them I'll sell you mine for half price. I also tried several options from McMaster and other vendors but all were either ridiculously heavy (with solid brass bodies - designed for 1000psi+ applications) or difficult to turn. Finally I found this device from Earl's. It's a bit on the large/long side, but it's very light and easy to turn. It's perfect for the task.

The fuel filter was easier to settle on. After seeing numerous complaints from other builders/flyers about inline filters clogging, I settled on a canister-style filter that I placed just below my oil filter, so I can change both at the same time. This unit from Mallory can handle very high flow and pressure, and filters down to 5 microns - more than we need.

Fig. 7: Aeromotive A1000-6 Regulator

Fig. 8: Hydraforce 3-port Solenoid

The regulator is an Aeromotive vacuum-referenced unit on a sturdy firewall mounting bracket. Two inlet ports eliminate the need to Tee the fuel rails, and the ports are ready for AN fittings to be installed. An 1/8” NPT port in the front face allows the installation of a pressure gauge.

Finally, the fuel return solenoid is from Hydraforce, although it's difficult to find online by that name. Biodiesel conversions use this valve for their tank source selection, and you can find them that way. The valve has an aluminum body, stainless hardware, and Viton O-rings, plus a 12VDC control signal. If you have trouble finding this solenoid, consider the Pollak 6-port valve. I prefer the Hydraforce because it has NPT fittings, but if you put the Pollak in an aluminum box it should be fine.

Fig. 9: Fuel system wiring. Annunciator indicators not shown.

The wiring diagram calls for three switches. The first switch powers the entire system, and ideally would be located under a safety cover. If you do this, the switch should be wired such that the switch is ON while the cover is closed. (That is, the safety cover should make it more difficult to accidentally turn it off.)

The second switch selects the source tank by routing power to either pump. A middle “both” setting allows both pumps to be run, which could be used during takeoff and also is the position the switch passes through when selecting a tank. This is a safety feature. An additional signal from this switch controls the return solenoid, reducing pilot workload (the return feed is automatically routed to the source tank). Note that while the selector switch is on “BOTH”, the return solenoid still returns to only one tank. I selected my right tank for this - either one is fine.

Finally, a cross-feed switch reverses the fuel return solenoid. This allows fuel to be transferred from one tank to another. This is helpful when performing maintenance on a tank or part of the fuel system, as well as for weight and balance. You might also want to transfer known-good fuel from one tank to the other when filling up, then fill only the empty tank (this should be the tank the fuel solenoid will return to when the switch is on “BOTH”). This allows you to take off with both pumps running (eliminating pump failure as a possible problem), then immediately change to the known-good failure at the slightest hint of trouble. By returning to the “unknown” tank, you know that the “known” tank should be a good source of fuel.

Flox/BID tape a pair of 3/8” NPT flanges in the side wall of each sump. They should be as low as possible while still allowing the finger strainers to be installed. Once the BID has cured, use a razor blade to carefully cut open the holes in each. Try to avoid letting any fiberglass dust or debris fall into the tanks.

Next, install a finger strainer in each sump fitting, and then a 1/4” NPT to AN-8 straight fitting in each finger strainer. Connect a pair of 90-degree AN-8 couplers to these but do not tighten them yet. Make sure they do not impact the aileron torque tubes. If they do, you might need to make your own 90-degree bends using tubing.

Unscrew the handles on the shutoff valves, then drill a 1/8” hole about 1/4” from the end of each. Use a nail set to precisely center this hole. When you re-install the handles, install them such that one handle pulls to the left and the other pulls to the right. That way each valve shuts off by pulling forward.

Install the shutoff push-pull cables in your desired location and route the cables back to where the shutoff valves will hang. Determine whether your valves will work better facing outside or in, attach the push-pull cables, and then mount the valves. As above, do not tighten the fittings yet.

Fig. 10: Fuel shutoffs, handles removed

Fig. 11: Push/pull holes, one handle reversed

On each pump, identify the inlet and outlet and mark those locations on the end plates with a Sharpie marker (the foam rubber sleeve will cover the markings on the body). You might also want to mark the + and - terminals because they're hard to see after the pump is installed. Now slide on each pump's rubber vibration damping sleeve and then a pair of mounting brackets. Finally, install an M10-1.0 to AN-8 fitting on each pump inlet and an M10-1.0 to AN-6 fitting on each outlet. Be sure you include the copper or brass crush washers that should be in your pump installation kit.

Position each pump behind the rear seat back approximately centered left-to-right, then mark where the mounting bracket holes should go. Remove the pumps, Dremel out some space in each location, and install a pair of -4 nut plates with a 2 BID pad over the top. When cured, open up the holes to the nut plates and install the pumps.

It's possible to uses braided stainless hose here, but there's very little room left for them and the fittings are large (and expensive). Either way, make up a pair of AN-8 hoses to connect the pump inlets to the fuel shutoff valves, and a pair of AN-6 hoses to connect the pump outlets to the bulkhead fittings. Install the bulkhead fittings near the center keel, about 1.5” inside and up from the floor to give you room to get a wrench onto the fitting. This is a possible air leak location, so even though the bulkhead nut will cover the hole you may want to put a small bead of RTV underneath it just before you tighten it down.

Once the plumbing is complete, connect a pair of wires from each pump to the instrument panel area using the supplied ring terminals and min 18AWG wire.²⁾ The wiring diagram is included above. The system is powered by a 20A breaker or fuse that connects to an ON-OFF switch. That switch feeds a LEFT-BOTH-RIGHT (ON-ON-ON) selector switch to which the pumps should be wired. (The return solenoid and cross-feed switch will be wired later.) Before making the final pump connections, test your switch wiring using a test light or voltmeter.

Install a pair of AN-6 straight bulkhead fittings on each side of the center heat duct. The fuel pump outlets will not line up perfectly with these fittings so you need to locate them far enough away to let the hose flex and make up this alignment difference.³⁾ Make up an connect an AN-6 hose from each pump to its respective bulkhead fitting.

On the firewall, choose a suitable location for your fuel filter. I installed mine just under the oil filter so I can inspect/change both at the same time. Install a 3/8” NPT plug in the inlet port that faces the firewall (this port is difficult to plumb to, and will not be used), and 3/8” NPT to AN-6 straight fittings in the remaining ports. Mark the location of the filter's mounting holes, pilot drill each hole, and finally open the holes up to 1/4”. (You will find stainless dulls drill bits quickly - a sheet-metal step drill is very helpful here.) Rivet four nut plates to a piece of aluminum and BID-tape it to the cabin side of the firewall in the appropriate location. Then install the filter.

Inside the hell hole, make up a line to connect to the bulkhead fuel fittings. The plumbing here calls for a short hose from the right pump fitting to a Tee at the left pump fitting and then a longer hose from the Tee into the hell hole and to the firewall bulkhead fitting. I listed the hose ends I used, but feel free to mix/match straight and angled fittings as required to fit your desired layout.⁴⁾

If you wish, at this time you can test the tank/cabin portion of the system. Put a bucket or gas can below the engine and route an AN-6 hose from either the firewall bulkhead fitting or both fuel filter outlets into it. Then add gas to one or both tanks, and activate your pumps. At this time you can test the pump flow, check for contaminants, and re-test your wiring.⁵⁾

The secondary rail can be prepared for AN plumbing by installing an M12-1.25 to AN-6 adapter in the outlet (be sure to use a crush washer) and a non-threaded O-ring flange to AN-6 adapter in the inlet (be sure to lubricate the O-ring). If you prefer, you could also just drill and tap each end for 1/4” NPT as described for the primary rail below.

It's much harder to find suitable adapters for the primary rail, so just drill and tap those for 1/4” NPT. Be sure you clean up all chips produced by this step, and if you drilled far enough into the fuel rail to intersect the injector holes, use a jeweler's file to carefully smooth any rough edges at the intersection.

Unless you have a lot of room for your intake (and if you're building a Cozy, you don't) the fuel temperature sensor on the secondary rail is going to be too tall to fit underneath. You don't really need it, so you could just buy a plug for the hole - but it's an odd coarse-thread size that's tricky to find a good plug to fill. You could drill and tap this for 1/8” NPT, or you cheat and just cut off the plug from the sensor. It makes a perfect plug if you don't plan to reuse it.

Fig. 10: Fuel rails, with adapters

Fig. 11: Note the temp sensor plug

At this point you can service, clean, and re-install your injectors. You should have a pair of new O-rings and a new pintle cap for each one in your service kit (eBay). The primary rail requires a grommet around each injector housing and a plastic spacer under the rail itself. You may leave out the primary rail diffusers if you wish. The secondary rail requires a grommet between each outlet and its diffuser, so you can't leave those out. If you're concerned about them breaking, just snip off the legs on each and use the top part.

The fuel lines should be routed through the “cool” areas of the engine. It helps if you already have your intake and most of your accessories installed. Ideally you want to support the hose at numerous points along its length. Hose brackets work fine for this - there are plenty of threaded bosses on the engine block that aren't used for our installation that you can choose two or three that are suited to the task. I found that I got the best fit using a 90-degree hose end for the primary rail, and a 120-degree hose end for the secondary. This brought the fuel hoses together near the rails, and I was able to wire-tie them together at intervals throughout their run.

To wire the injectors you can either use OEM connectors or solder wires directly to the pins. This isn't as crazy as it sounds - in combination with silicone sealant for support, this makes for a solid, low-profile connection. I chose a middle ground. I didn't have room for all four connectors under my intake, but I found that I could cut away the outside portion of each connector and just use the center part. I bought a set of 4 new connectors with pigtails already attached from eBay for this purpose. To keep the wiring neat and well-supported you can gather the wires together in a bundle, cover them with high-temp heat-shrink tubing

Choose a suitable location for your fuel pressure regulator and return solenoid, making sure you have enough room to route the necessary lines between them. As with the fuel filter, you should make a pair of aluminum plates with nut plates riveted to them and flox/BID tape them to the cabin side of the firewall. You don't need much strength here, but it makes it much easier to install the devices by yourself.

The fuel regulator uses straight-cut AN fittings with O-rings, not NPT ports. Be careful to install the correct fittings in these ports (and lubricate the O-rings). Then make up a pair of AN-6 hoses to connect the outlet of each fuel rail to the left and right side ports of the regulator. Make sure these lines are shielded from the heat of your turbo/exhaust.

You should also support these lines midway along their length, and leave a bit of slack in them to account for engine movement. If you have the recommended support bracket from the turbo to the water pump housing, this may be a good place to support these hoses. The regulator also needs a vacuum hose to the intake. Standard vacuum hose is fine for this connection - you can double-clamp each end for safety if you wish. The fuel rail feed lines make an ideal route/bundle with this vacuum line if you wire-tie them together.

The return solenoid installation varies by the type of device you obtain. Some valves come with NPT or AN fittings and may be connected directly to the regulator. The Pollak valve uses hose-clamp fittings. To connect it, strip 2” or so of the stainless braid from the end of a standard hose and attach with hose clamps. (Remember, this portions of the system is not under pressure.) If you're concerned about cracks/leaks at the return fittings on this device, you can box in the unit and route a small drain line out the bottom of the cowl. Digikey sells inexpensive aluminum boxes that are ideal for this purpose.

The outlet ports of the solenoid should be plumbed to bulkhead fittings back through the firewall. You could put these ports on either side of the firewall, near each tank, but since firewall space is precious I would recommend putting them close to the return solenoid, and routing the left-side return line inside the spar, or above it.

Flox and 2 BID tape a pair of 1/4” NPT flanged fittings into the upper wall of each tank, about 1.5” below each longeron and 1.5” forward of the rear wall of each tank. When cured, carefully open up the holes in each and install a pair of 1/4” NPT to AN-6 90-degree fittings. Ideally, you want these ports to face aft, toward the firewall. Then plumb an AN-6 hose from each fitting to the appropriate bulkhead port.

Complete the installation by wiring the return solenoid according to the diagram above. It may be a good idea to install a jumper block for your firewall-side electrical connections. This will make it easier to disconnect the entire engine if you need to remove it for servicing.

Disconnect the fuel rail lines and let them hang into a bucket. Fill each tank with 5 gallons or so of fuel, then raise the nose gear until the plane is level (so the fuel can get to the sumps - make sure you have ballast in the nose!) Turn each pump on and listen for its hum. Make sure each is working, and verify that you have a full and steady flow from the fuel filter. Shut off the pumps and examine the fuel in the bucket. It should be clear and clean, indicating the filter is working.

Now remove the fuel rails and slide a piece of cardboard underneath them. Reconnect the lines to the fuel rails, pour the gas back into the tank(s), and do the test again. Once the air is purged from the fuel lines and rails, you should see good fuel pressure on your engine monitor. You should have no leaks on the cardboard - if you do, you have a leaky injector and should fix this problem before proceeding. (Note: the FSM calls for less than 1 drop per 5 minutes.)

Measure the resistance of each injector. It should be 13.8 ohms. Then quickly pulse the injector with 12V from the battery. You should have wired your injectors Be sure you connect the jumper to the injector first, and use a long lead to the battery. You do NOT want to create any sparks at this point! A 1-second pulse is long enough. This should create a smooth and even spray of fuel on the cardboard. Check each injector this way. Finally, carefully inspect the bottom and top of each fuel rail one last time for leaks, then re-install the rails.

Flip the fuel pump selector switch and verify that the return solenoid is returning fuel to the correct tank. Inspect all fittings and hoses for leaks. Then activate the cross-feed and verify that fuel returns to the opposite tank.

Connect

then study your engine monitor or fuel pressure gauge. Adjust your regulator until the system produces 45psi (note: check this).

Next, remove each fuel tank cap and verify that fuel is being returned to the correct tank. Then operate the cross-feed switch and verify that fuel is returned to the opposite tank.

Finally, it's a good idea to “slosh” the tanks by pulling up on each winglet to let the fuel wash around in each tank. As noted above, composite tanks are notoriously dirty when they're first built. Before attempting an engine start, try to let the fuel clean all of the walls of the tanks as best you can, then clean out the filters and finger strainers. Repeat this process until they come up clean.

As noted above, if you have installed the optional in-line filters, they should be removed after you have some engine run-time but before your first flight to eliminate one possible clog risk.

Check the filters every 1 hour during the first 10 hours of engine run-time, then every 2 hours until you've reached 4 hours, then every oil change (or unusual engine behavior) after that. The finger strainers are more difficult because you have to drain the tanks to get to them, but they should still be checked at least once or twice during this period as well as if you have any unusual engine behavior. After that, follow the manufacturer's recommendation for cleaning or replacing the fuel filter - you should do this at least as often as you change your oil.