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Okay, this log entry deals with the issues of whether or not to install a fuel return line.  I happened to be surfing Dan C's web site and came across a log entry he made while plumbing his purge valve and fuel selector where he stated that if he didn't have his tanks already sealed he would have run a fuel return line all the way back to the tanks.  Alright, now I am hooked I have to figure out if I want to put in a fuel return line and/or if I needed one.  I am not a motor-head by any stretch of the imagination so this was going to take some research.

When talking about fuel return lines, they are used for two, but distinctly different, functions:

1. Fuel Return-  If you are installiing a "Subie" automobile conversion engine (Crossflow, or Engenfellner), then a fuel return line is required.  In an automobile, fuel return lines are used to keep a minimum amount of flow through the fuel system and a pressure regulator (or fuel controller) is used to control the fuel system pressure.  It maintains a certain pressure/flow by bypassing some flow from the injector rails back to the fuel tank.  This means that there is always fuel flow through the return line. Engenfellner aircraft engines have their engine installation manual on their web site and chapter 2 details how a fuel return is to be installed to support their engines. 
2. Fuel Vapor Return-  This is a somewhat different function than the fuel return briefly discussed above.  Application to light aircraft is further detailed below.

Allthough, these two functions are different, when researching this concept, I often seen them discussed interchangeably lending to my confusion.

Anyway, the rest of this entry is a little background on light aircraft fuel systems and was taken from the Airflow Performance purge valve installation manual. 

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Airflow Performance Purge Valve Background Info Excerpt

Modern light aircraft fuel injection systems are of the low pressure, constant flow type.  Fuel is metered and is delivered to air bled nozzles which atomize and spray the fuel at each intake valve.  Airflow Performance and Bendix fuel injection systems meter fuel to the engined based on air consumption.  The fuel regulator in both these types of injection systems do not return any fuel to the tanks, therefore oly the fuel that is used by the engine flows through the fuel controller.

Vapor in these types of metering systems cause the fuel regulator to operate erratically and porr engine performance will result.  This is usually evident during hot restarts.  Hot restart problems are a typical complaint of pilots operating fuel injected engines.  After engine shut down, heat in the cowling and engine tends to boil the fuel in the fuel control, fuel pump, and related fuel metering components.  Some of the fuel expands in the nozzle lines and gets forced through the injector nozzles and into the engine.  This leaves hot fuel and fuel vapor throughout the complete fuel metering system, engine driven fuel pump included.

During an attempted start under this condition usually results in the engine starting for a moment then quitting.  The pilot is then faced with the decision of how to proceed with the start procedure.  Flooding the engine then cranking the engine until it starts is usually done.  This procedure is dependent upon battery life which sometimes expires before the engine starts.  Engines which use the high pressure diaphragm fuel pump, experience another problem which influences the ease of restarting the engine.  By their design, the diaphragm fuel pump acts like an accumulator when the engine is shut down.  This keeps fuel pressure on the fuel controller, and leakage in the idle cut off circuit of the fuel controller will allow the fuel to bleed off into the engine.  This can cause run on in idle cut off and flooding of the engine initially after shutdown.

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Okay, if I understand this all correctly....  You don't need a fuel return line to have a properly operating fuel system.  Even if you want to have a fuel return line, it does not necessarily have to go back to the tank.  Dan Checkoway ran his to

The map of fittings for my inboard rib is shown in the picture below.  I used the vent fitting for the left tank to plumb my return line.  I will order two more AN833-4D fittings to plumb this the same way in the left wing tank.

The inside of the tank for the return fitting is installed the same as the vent fitting.  I torqued this fitting to 55 in-Ibs and then covered it in tank sealant.  No additional tubing is required for the return line.  Fuel and vapor will be returned to the relatively cool fuel in the tank which will absorb the heat from the circulated fuel.