Fuel return line

Lots of good info on this thread - and I am now seeing some fuel pressure issues after a warm day pee stop.  After take off + 15 minutes, pressure went to zero - engine ran perfect - not a burble.  Me, on the other hand, yeah, I had a burble. 

I have a mechanical fuel gauge that happens to be the highest element in the fuel system.  I suspect I do not really have a fuel pressure problem so much as a fuel pressure indication problem.  I suspect I am getting vapor up to the gauge causing the low reading.  The vapor in the carb lines is probably getting expelled via the float bowl vent. 

Yes the lines are all fire sleeved - except for the aluminum fuel filters - maybe I should wrap those too.  I also do not have a "proper" gascolator - it is this welded cube shaped thing with a drain port - it cannot be opened and cleaned.  But it is also 6 inches from the muffler - maybe I need to put an aluminum heat deflector around it or some asbestos insulation - or both.

I have the old collection of T's fuel distribution with no return line - over 700 hrs on the engine.  This being a 601 XL - adding a return line to tanks that are not configured for returns would be a PITA - opening the wing skins, adding a port to the tank, changing the fuel selector valve to 6 way, ... PITA. 

I was considering the return to the gascolator trick.  I just went to the Lockwood IMRT 900 series trainings (great course btw) but of course the official word from Rotax is that the fuel return must go back to the tank from which it came.  I understand the reasoning - cool fuel from the wings and all.  But I have to wonder if I really need to be that extreme ...

I was looking at the Andair GAS375 gascolator.  It reportedly has an integral vent to remove air trapped from maintenance activities.  Why would this not also remove fuel vapor coming back on the return line?  Or will the vapor make its way back to the gascolator?  

I am also going to swap out the mechanical gauge for an electronic one with the sensor close to the distribution plumbing.  This will remove the vapor trap to the mechanical gauge.

The distribution plumbing could also get changed out but Rotax wants ~$400 for the distribution block assembly.  Maybe T's are good enough for another 700 hrs.  

--- any advice can be good advice - its up to you to figure out how to use it ....

Hi all...long thread and I must admit I skipped a lot of the comments.  In teaching this for years let us remember that the original wording travels back to the early 1980s.  At that time almost all 912 engines had no return line.  The reason you got away with it was we had better fuel with a Reid Vapor Pressure similar to Avgas.  Since then many things have changed....first we start with Ethanol, this along with automotive fuels has created some issues.  Winter blend fuels have RVP numbers that allow fuels to vaporize much easier for cold weather starting.  This fuel is however a huge problem when it gets hot or you decide to fly at higher altitudes, say over 8000 ft.  Fuel can vapor lock.  The boiling point of the fuel can cause huge problems in summer temperatures.  Avgas has a fixed RVP and is safer however people what to use auto fuel to save costs and it is easier to obtain in many areas.  

Rotax has always called out the use of a return line.  Given that this is a carburetor with a mechanical pump you need to restrict the amount of pressure you would lose if it  was an uncontrolled open line...it will simply pump it all back to tank...or as some suggest to the gascolator.  The mechanical pumps, there are a few different ones, do not have the flow to fill both carburetors and return an open line to tank.  The restrictor is to bleed off vapor and pass extra fuel back to tank to clear vapor that occurs in the system.  

Fuel pumps used to have .4 bar pressure max in the early days..  The original Pierburg pumps and the A/C pumps were rated for .4 bar.  The change in 2010 to the Corona pump saw the limit raised to .5 bar pressure.  There was no change to the restrictor size requirement and the new pump was an improvement to clear vapor from the lines.  

So best advice is to always use a return line on the carburetor equipped engines with a restrictor in the fitting.  If you are not using the factory one then be sure that your system has the return line as high as possible in the fuel line (top of engine) since this is where the vapor collects.  Run your electric pump for a short while before you start the engine.  This clears vapor from the lines and will also fill the float bowls if they happen to be low or empty.  Be aware that a plugged restrictor orifice may lead to high fuel pressure readings, especially with the Corona fuel pumps.  

Cheers.

Just FYI as a comparison, the LAA in UK will not accept the return line going to the gascolator, it has to go direct to a fuel tank. It's only a low flow rate so you don't have to switch it between tanks, just allow for it in your tank use rules.

This notice from Vans for RV-12/Rotax 912ULS may be worth considering:

https://www.vansaircraft.com/wp-content/uploads/2018/12/Notification-2018-07-12.pdf

The RV-12 recommended Facet Cube, PN 40135, delivers Pressure Max/Min 7.0-4.0 psi and 32GPH (US) .

This is well above Rotax recommended Max 4.5 psi

https://facet-purolator.com/specifications/

The RV-12 story mirrors, my Sonex/912ULS,  low fuel pressure warnings, that I have been experiencing on climb out.

I have posted this information elsewhere on the Forum. At this stage, neither Roger Lee or Rotax Wizard are making any comment on this post/ RV-12 advice????.