flyrotary@lancaironline.net Mailing List Archive

Ed,

Do you think your fuel system is somewhat unique with the use of the small header tank?

It seems to me that running two high pressure pumps simultaneously to deal with the possibility of failure of one pump increases the likelyhood of vapor lock in the supply to the pumps due to the doubled fuel flow requirement compared to running only one pump. This increased susceptibility to vapor lock would only exist during the critical phases of flight when both pumps would be running. The procedure used would depend on the operators judgement of which failure mode (pump failure or vapor lock) would be most likely. Like everything else, a compromise.

This may not have anything to do with Sam's system, operating procedure, or problem, just something to consider.

Steve Boese

On Tue, Aug 9, 2011 at 8:06 AM, Ed Anderson <eanderson@carolina.rr.com> wrote:

Further discussion with Sam leads me to believe that the highest probably cause was a "vapor lock" of some type in the fuel line. Sam stated the engine never stopped running (correction to my earlier report - where I stated it had quit), but just continued to sputter and failed to produce power.

The ignition checked out OK as did the rest of the system back in the hangar. The symptom of sputtering engine and no power would put a fuel system problem high on my list of suspects and given the high ambient temps (98F) and the run up before take off -- would leave me to believe those conditions could have lead to diminished fuel flow.

Sam is going to inspect his coarse filter to ensure it was not partially obstructed.

One item of note is that Sam does not have any sort of boost pump between tanks and his EFI pumps. In my early days of testing while doing power-on stalls (high power low cooling airflow) on a hot day, my engine started to sputter (unlike Sam I had plenty of altitude when it happened) and I noticed my fuel pressure fluctuating between 20 - 35 psi. I turned on my Facet boost pump (puts out approx 6 psi pressure) and it immediately squelch the problem. So I now take off/land with boost pump on and have never since had that problem.

We know that any obstruction in a fuel line produces at least a small pressure drop down stream - Given the suction of the EFI pumps and even minor restriction of the coarse filter - a lower pressure region would exist between filter and EFI pumps - elevated temperatures could be enough to cause some of the fuel in this area to flash into a gaseous state - which of course is not pumped very well. Its my opinion that a boost pump is a worthwhile addition in keep pressure in the low pressure part of the fuel system sufficiently high to prevent the fuel from percolating - particularly on HOT ambient days. However, make certain that the boost pump is of a type that does not obstruct fuel flow during a malfuction or when off.