T&T: Vacuum in Fuel Line

[Peter Bennett]

Thursday, June 18, 2009, 4:41:15 PM, john wrote:

jb> Greetings List,



jb> I have been having a vexing problem for some time now that I have not yet
jb> solved. I have a restriction somewhere in the fuel line that causes a vacuum
jb> to develop. It has caused air to be sucked into the line, stopping the engine.
jb> It has also caused fuel starvation that nearly stopped the engine before I
jb> turned on the electric fuel pump I use for bleeding and priming. Of course the
jb> vacuum is no longer present when the electric fuel pump is pressurizing the
jb> line, but I don't want to use the electric pump all the time.



jb> I have three vacuum gauges, one on each Racor in place of the T handle and one
jb> down stream of the Racors between the filters and the Flow Scan sensor. The
jb> gauges on the Racors read 0 inHg most of the time, but the gauge between the
jb> filters and the Flow Scan reads as high as 7 or 8 inHg. It will start out
jb> early in the day at 3.5-4 inHg and build up after 5 or 6 hrs.



jb> My main question is where is the restriction, upstream or downstream of the
jb> vacuum gauge. My guess is that the restriction is in the Flow Scan sensor or
jb> damper which is  downstream of the vacuum gauge. The only other place would be
jb> upstream in the valve or fuel line leading from the Racors into the vacuum
jb> gauge and Flow Scan. I plan to remove the Flow Scan as it hasn't worked since
jb> I purchased the boat but I need to know whether I also need to replace the
jb> valves and fuel lines from the Racors.



jb> I have pictures that I could attach to an email if they would help understand
jb> my plumbing.



jb> Thanks in advance for your help.



jb> John  Brees

A "schematic diagram" would probably be more useful than photos.

I assume that by downstream" you mean "towards the engine".  If so,
the obstruction must be upstream of the gauge - an obstruction
downstream of it won't affect its reading.

If the valve before the gauge is not a ball valve, or other type with
an obvious on/off handle position, I might suspect that the valve is
not fully opened. Otherwise, the fuel hoses may be deteriorated
internally.

-- 
Peter Bennett, VE7CEI    Vancouver, B.C., Canada
Ennos 31 "Honeycomb"
GPS and NMEA info: http://vancouver-webpages.com/peter 
Vancouver Power Squadron: http://vancouver.powersquadron.ca