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Fuel System

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Site last updated on 15/08/08

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   Fuel injected cars require a recalculating fuel system in which the fuel is pumped around a loop. Most of it returns to the tank, with the rest being used by the engine. The pressure in the pump side of the fuel loop is maintained by the fuel pressure regulator, which also raises the fuel pressure with boost on a turbocharged engine. The arrangement of such a system is as follows:


   If your VW did not have an electronic injection system with a recirculating fuel system (such as the the Digijet / L-Jetronic fuel injection such as the 1986 --> T25 / T3 / Vanagon), then you will need to install one as part of your Subaru conversion. The carburettor with the associated low pressure fuel system is not suitable, and the hoses, pump and filter must be replaced.

  If you are converting a VW which already had a recirculating fuel system , you already have a fuel system (pump / filter / high pressure hoses) suitable for a Subaru engine. As such vehicles are all at least 15 years old now, replacing the filter and the rubber fuel hoses may well be overdue though, so replacing them as you install your conversion is probably a good idea.


Fuel Pump:

  Using the Subaru pump is not practical as it is an ‘in tank’ type. An external fuel pump is needed with the same specification.
  Naturally aspirated injected engines run a fuel pressure of about 2 - 2.5 bar (29 - 36 psi), and require a pump which can produce about 3 bar at the required flow rate, such as the one shown below:  bosch_pump_3   

   The pumps we sell are this type, and are not suitable for use with turbo engines.

   Many specifications of pump are available in this housing design, and these pumps require rubber mounting because they are noisy.

   Turbo engines require a pump which can produce the 2.5 bar required by a naturally aspirated engine plus the boost pressure, plus a safety factor of a bit more pressure. Often turbo injected engines use a pump which can maintain about 4 bar pressure at the required flow rate, which would be OK for up to around 1 bar boost, but engines running a higher boost will require a higher pressure pump. Such pumps are available in the same housing as shown here, as used on cars such as the Ford Sierra Cosworth. A good general rule with turbo fuel pumps is to use one from a car with a similar engine capacity which produced slightly more power than the engine you are installing. Alternatively, aftermarket pumps sold for motorsport are often external (i.e. not ‘in tank’ ) and have detailed specification available. We do not sell fuel pumps for turbo engines.
   This type of pump should be mounted below the tank, or as close to below the tank as possible, so the pump is gravity fed. Many different configurations of inlet and outlet fittings were available on these pumps, to suit the application for which they were designed. Many have threaded fittings, and don’t come with the necessary fittings when bought new. We sell a type which has an inlet to take a 12mm bore hose, and an outlet to take an 8mm bore hose. These are the same end fittings as the pumps used by VW on T25 / T3 / Vanagons, and are the easiest to use for aftermarket installations.


Fuel Filter:

   Fuel filters for injected cars are designed to withstand being installed in the high pressure side of the fuel loop, and have a steel or aluminium body. They typically filter to around 10 microns. The plastic bodied filters used on carburettored cars are not suitable. Injection filters are also much bigger that carburettor ones to allow for the higher flow rates of a recirculating fuel system. The filters are typically mounted near the fuel pump on most European cars with external pumps. Subaru filters are mounted in the engine bay, on the left hand inner wing near the suspension strut up to about MY99. After that they are fitted inside the tank.
   The filter is usually mounted after the pump (as shown above), and therefore in the high pressure part of the circuit. This means that they do not filter the fuel going into the pump. The pump relies on the tank strainer to prevent premature wear due to picking up dirt from inside the tank. If you have any doubt about the strainer in the tank, it may be worth mounting the filter between the tank and pump. If this is the case, a filter to take 12mm bore hose should be used, so as not to restrict the flow to the pump.


   Like the pumps, many different configurations of inlet and outlet fittings were available on these filters, to suit the application for which they were designed. Many have threaded fittings, and don’t come with the necessary fittings when bought new. The filters we sell feature fittings at each end to take 8mm hose, as shown above, as these are the easiest to use in aftermarket installations.


Fuel Return

   As many of the VW’s being converted to Subaru power originally had carburetted engines, many don’t have a fuel return fitting on the tank, as they were never intended to have a recirculating fuel system. However, some of the most popular conversion recipients (the T25 / T3 / Vanagons) usually do have the necessary tank fittings despite originally having a carburettor. The return fitting on a 2WD is at the lower rear left hand corner of the tank (exactly opposite the outlet, on the right), and is often connected to a float bowl vent pipe on the carb. This fitting can be used for the return, but the pipework to us should be replaced with 8mm bore hose. Use high pressure hose despite the return being on the low pressure side of the fuel loop, as high pressure hose is generally much better specification. You really don’t want a leak in a recirculating system, as the flow rate is very high.
   If your VW does not have a fuel return fitting on it’s tank, then some ingenuity will be required. Avoid welding or brazing a used tank, as it’s highly dangerous. If your tank looks corroded anyway, the best solution is to replace it and weld or braze the necessary fitting in before the new tank ever goes near any petrol. Some VW’s have more than one breather fitting, and using one of the breathers as the return fitting is quite common. However, it is far from ideal. The multiple breathers are usually on either side of the tank, and are there so the tank can breath properly even when parked on slopes, etc. Using one as a return compromises this system.
   An alternative way to install a breather without welding or brazing the tank is to fit the breather into something which cane easily be removed from the tank and cleaned of all traces of petrol before welding. Possibilities include the fuel level sender or the filler neck pipe. If you fit a return into the tiller neck pipe, it should be as near to the tank as possible, and should turn 90 degrees inside the filler neck, directing the returning fuel into the tank.


Connecting to the Subaru Engine:

  All Subaru EJ series engines have three horizontal pipes at the right hand end of the induction manifold as installed in a VW (assuming you have not reversed the manifold), like this:

   Fuel Connections

  •    The top pipe is the fuel feed, from the pump, and takes 8mm high pressure hose.
  •    The middle pipe is the return, back to the tank, and takes 8mm high pressure hose.
  •    The lower pipe is for the petrol vapour to return to the engine to be burned. and takes 6mm hose - see below.


Fuel Tank Vapour

   Most VW’s being converted are plumbed to vent any petrol vapour from the tank to the atmosphere, usually via a pipe arrangement which prevents petrol leaking from the vent in an accident. The exception are some late model T25’s.
   All Subaru’s vent the vapour to a charcoal canister. The fumes are stored in the canister until the ECU opens a solenoid valve, and they are drawn into the inlet manifold and burned with the incoming fuel. Exactly how it is plumbed varies, as early Subaru’s had the charcoal canister in the engine bay, and later ones had it near the fuel tank. A system similar to that used on late model Subarus was fitted to some late model T25’s.
   Most conversions do not use this feature, leaving the tank to breath to atmosphere, as VW intended. However, it can be connected using either the Subaru charcoal canister or the VW one (as fitted to some late model T25s), if required. Reconnecting this system may be required if you live somewhere where the details of engine conversions are tightly controlled, such as California.
   If you leave the system disconnected on your VW, you should block off the pipe to the tank, otherwise unfiltered air will be drawn into the manifold when the solenoid opens. Also, don’t be tempted to remove the solenoid valve which opens to let the fumes into the inlet manifold (Subaru refer to it as the CPC, or canister purge control valve), otherwise you will get an error code.

 Inventors of the Subaru - VW conversion bell housing using Subaru flywheel, clutch and starter

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