Berni Breen wrote:

I have been trying to chase down the cause of a fuel issue in my normally aspirated Lancair IV engine.  During a cross country flight my fuel flow indicator began to show fluctuations which I mistakenly believed to be false readings.  Shortly thereafter during the same flight the engine began surging, slightly at first but increasingly worse in short time.  So much so that I began to plan for an emergency landing as I was looking to find the cause and to introduce a remedy.  I turned on my electric (back up) fuel pump and the engine immediately began to run strong and the fluctuation readings stabilized.  

After gaining altitude I turned off the electric pump and the unstable fuel flow readings as well as the engine surge returned.

My initial take was that my problem was caused by a failing engine driven fuel pump.  Even with that belief (once back on the ground) I checked as much of the fuel system as possible looking for potential restrictions.  I looked at and cleaned the screens at the wing root connections as well as the inlet on the GAMI injectors.  I found no restrictions so I removed and exchanged my engine driven pump for a rebuilt unit.

After installing the rebuilt pump the engine tested fine on the ground and fine in the air on a subsequent cross country trip.  Approximately 2.1 hours of flying time later the exact same symptoms returned...erratic fuel flow readings followed closely by engine surge.

I am now at a loss as to what I should now look for.  Could my problem be due to air somehow being introduced into my fuel flow?  Should I be looking at my fuel selector as a possible culprit?  If so how would I test that possibility?


Berni,

I suggest you check two other things. First, be sure the tank(s) you are feeding from are properly vented. A blockage of the vent will cause a vacuum to build within the tank as you burn fuel, eventually keeping the engine-driven fuel pump from being able to draw fuel. (as an aside, Don Goetz, renown Lancair test pilot, told me once that he'd observed the ribs showing in right wing of a Lancair he was flying, and wondered that he'd never seen anything like it before...the wing skin seemed to be shrinking around the ribs. Then the engine quit, and the cause became obvious)

Second, check for an air leak in your fuel supply system. Test this by capping the fuel lines at each wing tank and apply a slight vacuum to the engine fuel pump inlet line. This little tester from Amazon is inexpensive and very useful for this...as well as pitot/static system troubleshooting. http://www.amazon.com/Actron-CP7830-Hand-Vacuum-Pump/dp/B0009XQUK2/ref=sr_1_1?ie=UTF8&qid=1294676784&sr=8-1  

Switch tanks with the control valve to test both positions...and the connecting lines. The fuel selector valve is a known issue for leaking air into the fuel lines under a very slight vacuum, while testing OK under positive pressure. Overhaul kits are available.

Good luck, and let me know what you found.

Bob Pastusek