The XJ6 implemented a dual fuel tank system with twelve U.S. gallon capacity on each side (effective total about 21.5 gallons), an innovative concept but often fraught with leaks or gas fumes in reality and designed for the low pressure (under 10 PSI) carburetor XK engine. The high fuel pressure electronic fuel injection (EFI) GM LSx engine requires a different setup with pressures around 60 PSI. While it is possible to mimic the EFI system of the later Series 3 XJ6 (see illustration below), this complex design still suffers from many Series 1 shortcomings.

Modified fuel plumbing for XJ6 Series 3 EFI operation (single pump)

The Series 3 EFI plumbing eliminates the dual fuel pump setup, but an array of changeover values, dual fuel level sensors, and a significant run of additional fuel lines are still required.

Due to high pressure operation, EFI pumps are generally installed inside the fuel tank (called in-tank pumps) to help cool them down and reduce noise. Perhaps the major objection to the Jaguar EFI design is that it retains an external pump instead of placing the pump inside the tank (as a result, a dedicated fuel cooler is required – see component [4] above); there is no way to modify the XJ6 twin side tanks for an in-tank EFI pump.

Fuel injection is more sensitive to flow interruption than a carb engine, so pumps are often designed with small sumps around them to help contain a minimum of fluid at the pump intake. The XJ6 uses a gravity feed from the bottom on the tank instead of the more optimal sump arrangement.

Types of fuel injection

There are four types of EFI:

  1. Throttle body injection
  2. Multiple Port fuel injection
  3. Direct injection
  4. Sequential fuel injection (GM LSx)

Throttle body injection (TBI)

The earliest and simplest type of fuel injection uses a single-point of entry that replaces the carburetor with one or two fuel-injector nozzles in a throttle body, the throat of the engine’s air intake manifold. Since 2015, the aftermarket has blossomed with an array of throttle body EFI “bolt-ons” that now offer stiff competition to the traditional competition carburetor.

Multiple port fuel injection (MPFI)

Multi-point or multiple port fuel injection locates a separate injector nozzle at each cylinder, right outside its intake port, to shoot fuel vapor — either all at once in a batch or in bank groups — directly into each cylinder. MPFI meters fuel more precisely than TBI designs with superior air/fuel ratios, but fuel vapor may “linger” in cylinders due to non-specific timing of discharges.

Direct injection (DIG)

Direct injection shoots fuel directly into the combustion chambers, past the valves. More common in diesel engines, this method is appropriate for low emission, lean-burn cylinder head designs.

Sequential fuel injection (SFI)

Sequential fuel injection triggers each injector nozzle independently (unlike the less specific MPFI method). Timed like spark plugs, they spray the fuel linked precisely to each intake valve movement. SFI offers an immediate response to any throttle increase as well as high efficient and low emissions. Due to the complexity of SFI, the powertrain control module (GM’s term is PCM, but ECM and ECU also used) is central to its operation and any change in the engine must be accompanied by reprogramming the PCM.

The GM Gen III/IV engines encompassing all the LSx models exclusively use SFI. Thus any modification from stock of the LSx engine must also 1) reprogram the LS1 PCM (by a dyno shop tuning service or with specialized custom programming equipment), or replace the entire GM SFI with an aftermarket product, typically a TBI. TBIs can generate additional horsepower when mated to new cams and heads, but suffer somewhat in efficiency and responsiveness.

GM Performance has responded to the swap/retrofit market by offering a crate engine already programmed for installation into a non-GM vehicle. The alternative is to rebuild a LSx engine and then re-program it. Since the plan is to use a GM crate engine, the re-programming software requirements and costs below won’t apply (but included in the Reference section at the end of this post).


EFI system options

There are two basic options for an EFI retrofit: 1) deploy a secondary high pressure sump pump outside that works in tandem with original OEM low pressure pump(s), or 1) install a single high pressure pump inside a tank expressly designed for EFI operation.


Tandem low/high pumps

As illustrated above at right, the “non-return” configuration utilizes the FiTech Command Center (FCC, part 40003) which consolidates in- and out- pressure gauges, an EFI sump pump, and a pressure regulator (note that a return line is required with the FCC and their marketing “non-return” designation is misleading). The major FFC benefit is the ability to reuse OEM low pressure pumps and original tanks for a relatively simple retrofit procedure. In our XJ6 case, Jaguar low pressure twin pumps would feed into the FFC to boost pressure via the in-house EFI sump pump.

While the existing twin XJ6 tanks were in restorable condition, they need to be completely cleaned out and coated with a fuel tank sealer if put back in service (the left tank was leaking from the bottom due to a poor seal around the drain sump). Both Jaguar pumps are 17 years old and need replacement. When road testing, the fuel level indicators displayed spotty performance, so new fuel level sending units are desirable as well.

Component Cost
Tank sealer and restoration $300
XJ6 fuel level sending units (2) $300
XJ6 Series 2 low pressure pumps (2) $300
FiTech Command Center FFC 40003 $400
Fuel filters (2) $200
Lines and fittings $100
TOTAL $1600

User feedback on the FiTech unit is mixed: customers report leaking fuel, poor venting and fuel temperature control problems leading to vapor lock or sub-optimal performance. Summit Racing, one of the largest online retailers, lists 75 reviews as of May 12, 2017 with a 4-star overall rating. Furthermore, FiTech itself frequently gets negative feedback for poor customer service.

EFI in-tank pump

Given that the FiTech FFC becomes the critical link in any retention of the Jaguar dual tank/pump setup — a system with significant disadvantages even if upgraded — a new EFI replacement tank with an in-tank EFI pump looks like the preferred option.

Component Cost
EFI tank includes EFI pump and fuel level sending unit $1200
in-line filter $100
Pressure gauge $25
Pressure regulator $225
Lines and fittings $50
TOTAL $1600

With costs roughly the same, the new EFI tank option also offers quieter and cooler operation (since the single pump is submerged in the tank), less line plumbing (no dual components to hook up), and less reliance on a single vendor for support or spare parts.

EFI tank fitment

The XJ6 chassis does not permit mounting a fuel tank under the car. The only realistic alternative is placing a new tank inside the trunk tire hold, a lowered rectangular box shape in the center of the trunk floor. Of course, putting a tank in the hold eliminates the spare tire, but larger modern tires don’t fit anyway!

One advantage of a single in-the-tire-hold tank is better balance and a lower center of gravity that should improve cornering. The dual tanks, mounted high inside the rear wings with typically different fuel levels in each tank tends to unbalance the XJ6 somewhat. Also, the confusion about how much gas is in which tank (not to mention the inconvenience of trying to refuel the car on the opposite side) is eliminated. Finally, a single tank setup frees up a center dash panel rocker switch for some other function (the original XJ6 has a tank selector switch).

Tire hold in the XJ6 trunk

XJ6 trunk with spare tire and fuel pumps removed

The XJ6 rectangular spare tire hold is actually an isosceles trapezoid that narrows slightly towards the rear of the car. The bottom isn’t completely level either; a slight taper rises up towards the rear. Thus, finding an exact fit with an existing OEM tank is virtually impossible. Still, a close rectangle shape approximation might be found.

Fill location shown on right side

A second constraint is the location of the fuel fill connection on the tank. The Jaguar twin filler caps (obviously only one will be used!) are located just behind the chrome edging of the rear window.

To avoid the boot lid hinge inside the trunk, the tank filler connection must be on the left or right edge of the tank.

This effectively eliminates any tank with the gas fill in the middle (the angle would be too low for proper flow) or one with the fill connector pointed away from the left or right side.

A third constraint is the original dual pump mount that rises up at the end of the tire hold limiting the tank depth to about 6.5″ unless the tank is supported by tabs so that part of the tank rises above the deck level, or designed so that the tank only extends about 23″ from the rear shelf.

The maximum length is 28″ (tapering down to 26″ at a 22″ width) with a maximum depth of 8″ (at a 22″ width). This volume, assuming a flush mount with the deck, is 3736 cubic inches — a gallon is 231 CU, so capacity is 16 gallons (useable 14.5 gallons).

Mounting above the deck can add 5-6 more gallons. If the tank rises more than 2″ above the deck, however, the top of the tank couldn’t accept a flat (like plywood) cover. Thus the optimal total depth of the tank is about 8.5 inches with a capacity of about 22 gallons, close to the original Jaguar OEM specification (with better fuel efficiency of the LSx engine, range should be at least as good).

Aftermarket tanks

Several vendors, like Tanks Inc., offer “universal” tanks designed for retrofit purposes that are not specific to a particular make or model.

Unfortunately, none of the aftermarket steel products come close to fitting.

Polyethylene (a chemically resistant plastic) tanks come in some different form factors, but these are typically non-EFI and often don’t have provisions for a high pressure in-tank pump.

Polyethylene 14 gallon tank

The best aftermarket candidate is a polyethylene 14 gallon tank with 27 x 17 x 9-1/2″ dimensions.

This tank could be mounted on the tire hold floor (with some modifications), but would rise up above the trunk deck at least 3 inches, conflicting with a flat trunk cover. Furthermore, the tank fill is several inches away from the optimal location which would make the filler tube connection awkward. Also, the 14 gallon capacity is well below our expectation.

Fuel cells

Fuel cells, often in rectangular shapes, offer advantages not relevant to our needs, like the ability to rapidly fill the tank, prevent gas sloshing around in very high speed turns, and anti-spill safeguards in the event of a rollover crash. Good for racing, but not for our restomod application.

Custom fabrication

Without a suitable aftermarket “universal” tank, the alternative is to design a custom tank and have a vendor like Boyd Welding fabricate it. Dave Boyd specializes in aluminum tanks and these often come in rectangular-like form factors, quite applicable to our needs.

Custom EFI tank with side mount tabs


Custom aluminum tank

Our aluminum custom tank has a capacity of about 22 gallons, the same as the combined original twin tanks. But the single aluminum tank weighs about 32 lbs (including the in-tank pump) compared to 38 lbs for the twin steel tanks (without pumps) with an improved lower center of gravity positioned equally along the chassis centerline. This should enhance road handling.

Custom aluminum tank trial fit into the spare tire hold

Aluminum EFI tank final installation


Fuel filler ports

Oval XJ6 gas cover

The chrome locking fuel filler flip-up cover is one of the few external embellishments on the XJ6 that is certainly worth keeping if at all possible. In a single tank configuration, the dual fuel covers on either side of the rear window are obviously redundant, and one of them needs to be repurposed.

Modified fuel cap

To handle the filler tube fitment, the cap was modified to hold an extension pipe securely with an O-ring connection.

Modified gas cap with filler tube extension

Battery cutoff

The driver side fuel cover will be repurposed as a battery cutoff switch; in fact, the battery can be relocated to trunk shelf, an improvement that would keep the battery away from excessive engine bay heat. Since the fuel caps lock, a cutoff switch in this location would, in effect, also act as a theft-deterrent device.

Battery cutoff switch with removable red key

Cutoff switch terminal connections

Modified cutoff key (left)

Aftermarket cutoff switches with removable keys are designed for larger diameter spaces than is available inside the filler cap!

To remedy this conflict, the key had to be cut down eliminating the key ring hole; now the symmetrical key fits the filler hole and remains just as functional.

Filler tube to tank

A more complex problem is the fitment between the end of the filler cap body (that previously connected to the side tank fill opening) and the new EFI tank in the spare tire hold. First, as noted above, the filler tube must avoid a conflict with the boot lid hinge. Second, the tube must make several precise bends and terminate at just the right position on the truck wall to make a 45-degree connection to the tank fill.

Fuel filler tubes and hose connections

The problem can be solved with a combination of 1) a straight down tube attached to the fuel cap mount, 2) a 45-degree stainless pipe, and 3) a 90-degree stainless pipe, all 4) connection with short flexible hose sections.

Final install of filler tube assembly with drain and air vent hoses


Fuel system components

EFI pump

Selection of the optimal EFI fuel pump depends on matching pump capacity to three engine parameters: 1) maximum horsepower, 2) EFI required fuel pressure, and 3) voltage (typically 13.5 volts at normal operation, but it’s a good idea to plan for voltage sag to about 12 volts under heavy load).

The fuel flow requirements are directly related to horsepower, as follows:

  • 300 HP -> 100 LPH (liters per hour)
  • 350 HP -> 115 LPH
  • 400 HP -> 135 LPH
  • 450 HP -> 150 LPH

With our 430 HP LS3 engine that operates at 58 PSI, we need a pump with a capacity of at least 140 LPH, and preferably well over 200 LPH for a margin of error.

The three leading aftermarket pump manufacturers — Aeromotive, Bosch, and Wallboro — have similar operating characteristics at 60 PSI: they all can deliver a flow rate around 270 LPH at the normal voltage of 13.5 volts.

Aeromotive Stealth 340 pump

Boyd Welding Aeromotive 340 assembly with custom top connection plate

The Aeromotive Stealth 340 provides high flow at lower pressures, but falls off more rapidly as pressures go up. In our application, the pressure regulator will keep PSI below 60 and current draw will be 10-14 amps.

The Aeromotive mounts in the tank using a special baffle and cover plate for the 1/4″ electrical connections. The pump outlet fitting is a #6 ORB (O-ring boss).

Fuel level sender

The tank’s fuel level sending unit transmits a resistance value back to the fuel gauge. We selected a 240 ohm EMPTY to 33 ohm FULL calibration that will work well with digital gauges and provide a high level of accuracy.

Fuel level sender 240E – 33F

Pressure regulator

The LS3 returnless fuel rails still require some kind of return line to the tank. For pre-2010 Corvettes, GM designed a combined filter and pressure regulator, GF822, that when placed inline after the fuel pump eliminates the need for a second dedicated return line. Fuel still returns to the tank, but this is managed by the engine computer that varies the output of the pump.

GF822 filter/regulator eliminates a dedicated return line

However, the GF822 is probably underpowered for the high flow Aeromotive pump we use and was designed specifically to match older GM fuel components.

Fuel lines

See also the post about fuel line materials and fittings.

The standard 3/8″ fuel line matches 3/8″ NPT and -6AN fittings. Is this diameter large enough to support a 430 HP engine?

The question revolves around fuel flow, a function of line diameter, pump “headroom” capacity, and system voltage.

Voltage can be adversely impacted by poor wiring design like inadequate wire gauge size and/or excessive run lengths (fuel pumps typically have a long lead wire coming from the fuse box in the engine bay or under the dash). The Aeromotive 340 pump operates at 13.5 volts and draws 10-14 amps. For example, a 12 foot length of wire (say from the dash to the fuel pump) must be at least 12 gauge to support a 20 amp/13.5v requirement. At a 20 amp peak load, a 10 gauge wire is recommended.

We’ve planned for sufficient wire gauges, so the question turns to line diameter and pump “headroom”. In round numbers (the math gets complicated), one gallon of gas flow per hour supports 12 horsepower. Thus, to support 430 HP, we need a 36 gallon per hour (GPH) flow (430/12). Most pumps publish specifications in liters: 36 GPH equals 136 LPH. Pressure drop due to friction is less than 10 PSI (assuming about 20 feet of fuel lines). Therefore, the 136 LPH nominal requirement needs to be 146 LPH to provide sufficient headroom: the bottom line is that with the 340 LPH Aeromotive pump, we have plenty of spare capacity and the 3/8″ line diameter is more than up to the task.


To tune or not to tune

EFILive software tool foro PCM programming

If we needed a reason to choose a crate engine from GM, here’s one: the high cost of tuning software suite to get your engine to run at all.

For example, the “full suite” re-flashing EFILive costs $900 and a “scan-only mode” goes for $500. Or one can visit a local dyno shop with PCM reprogramming typically starting at $200 but often costing much more. This price umbrella has nurtured aftermarket EFI projects, largely TBI, with “self-programming” simple fuel burst throttle body 2- and 4- injector designs … basically, a step backwards.

The PCM world is highly proprietary with complex licensing requirements. Vendors like Diablosport and HP Tuners offer handheld and PC interface devices (typically Windows Vista) respectively that scan data. The HP Tuner VCM Suite ($500), for example, reads PCM flash memory, saves it to a PC binary file, and then flashes a valid calibration written to the PCM.

GM PCM aftermarket programmers include:

  • HP Tuners – 6021/6022 VCM Suite, $500 or $650 pro interface for real time 0-5v signals (MS Windows)
  • EFILive – $900 (MS Windows)
  • Diablosport – Trinity T1000 handheld, $500
  • FAST (Fuel Air Spark Technology) – FAST-FLASH power programmer, $336

But choosing a GM programmed crate engine avoids this entire arena.