Maybe you can help me with this riddle. I just completed the annual on my 300 hour 7-A with ECI IO-375 and Precision Silverhawk injection. I installed a fuel pump shroud with blast tube to see if I could improve persistent vapour locking with mogas. (I keep avgas in left tank for TO, landing, and backup to electric boost pump). I damaged the original fuel pump by installing the push rod beside the pump rocker arm instead of on top, and replaced the pump with a new Tempest unit installed correctly. I had an experienced mechanic check firewall forward before flight
The new mechanical pump puts out 35 psi, about 3 psi more than the old one, but well within the specified 20-50 psi required for the servo. My Airflow Performance electric pump continues to work well.(stock Van's setup, except filter is replaced with firewall gascolator to meet Canadian reg's). Since the AFP pump is regulated to 35 psi, there is very little change to indicated fuel pressure when boost pump is switched on or off with engine running. This makes sense to me, since the internal spring in the new fuel pump is holding pressure at 35 psi.
Engine performs normally, but on test flight I noticed that fuel flow gauge indicated 3-4 gph higher with boost pump on than with mechanical pump alone. Fearing a fuel leak, I returned to airport for trouble shooting. A helper looked for leaks with engine running and fuel pump on, no fuel coming from pump overflow line or cowl area, despite the fact that gauge indicated 2gph higher flow with boost pump, although engine did not change its idle tempo or EGT.
By the way, my Dynon fuel flow transducer is located in the cockpit, downstream of the Fuel tank selector, boost pump, and pressure regulator. No way for fuel to loop around it. Fuel pressure transducer is on firewall, "T'd" with flexible hose from the output fitting on mechanical fuel pump.
Removed cowls and found nothing wrong:
-No sign or smell of fuel leaked anywhere
-No fuel in the crankcase
-No fuel in the cockpit
-No leaks visible with boost pump on and mixture in idle cut-off position
-It took system 30 minutes to leak down from 35 to 30 psi after boosting in the idle cut-off position
Buttoned up cowls and test flew again, taking careful note of performance and readings. Take-off and climb performance is normal (great!), with CHT's and EGT's where I expect them. Application of boost pump increases indicated fuel flow 4-5 gph at cruise and climb, 2-3 gph at idle. No change to engine output or EGT in response to boost pump, however the engine does not run as well LOP as it used to. (Engine roughens up just LOP, resulting in having to run too close to peak, and correspondingly higher CHT's. LOP cruise yeilded 340 to 360, when I am used to seeing 320 to 330. Number 4 jug increased the most, which is where I pulled fuel pump blast air from. I don't think the blast air affected CHT, since I also installed my winter oil cooler shutter, which blocks off a big chunk of oil cooler air also coming from number 4 cylinder.
Writing this, I think I will look at the servo finger screen for debris, then run a GAMI test. Maybe I introduced debris downstream of fuel pump while fixing stuff which is contributing to LOP changes.
Any ideas what is causing indicated fuel flow to increase with boost pump application? I considered some electrical interference with the fuel flow transducer, but it always worked well before, and I don't like believing in coincidence. Prior to this work, fuel pressure would increase 3-5 psi, and fuel flow would temporarily increase with application of fuel pressure, but flow would settle down to previous value in a couple seconds
Thanks for any ideas
Jay
The new mechanical pump puts out 35 psi, about 3 psi more than the old one, but well within the specified 20-50 psi required for the servo. My Airflow Performance electric pump continues to work well.(stock Van's setup, except filter is replaced with firewall gascolator to meet Canadian reg's). Since the AFP pump is regulated to 35 psi, there is very little change to indicated fuel pressure when boost pump is switched on or off with engine running. This makes sense to me, since the internal spring in the new fuel pump is holding pressure at 35 psi.
Engine performs normally, but on test flight I noticed that fuel flow gauge indicated 3-4 gph higher with boost pump on than with mechanical pump alone. Fearing a fuel leak, I returned to airport for trouble shooting. A helper looked for leaks with engine running and fuel pump on, no fuel coming from pump overflow line or cowl area, despite the fact that gauge indicated 2gph higher flow with boost pump, although engine did not change its idle tempo or EGT.
By the way, my Dynon fuel flow transducer is located in the cockpit, downstream of the Fuel tank selector, boost pump, and pressure regulator. No way for fuel to loop around it. Fuel pressure transducer is on firewall, "T'd" with flexible hose from the output fitting on mechanical fuel pump.
Removed cowls and found nothing wrong:
-No sign or smell of fuel leaked anywhere
-No fuel in the crankcase
-No fuel in the cockpit
-No leaks visible with boost pump on and mixture in idle cut-off position
-It took system 30 minutes to leak down from 35 to 30 psi after boosting in the idle cut-off position
Buttoned up cowls and test flew again, taking careful note of performance and readings. Take-off and climb performance is normal (great!), with CHT's and EGT's where I expect them. Application of boost pump increases indicated fuel flow 4-5 gph at cruise and climb, 2-3 gph at idle. No change to engine output or EGT in response to boost pump, however the engine does not run as well LOP as it used to. (Engine roughens up just LOP, resulting in having to run too close to peak, and correspondingly higher CHT's. LOP cruise yeilded 340 to 360, when I am used to seeing 320 to 330. Number 4 jug increased the most, which is where I pulled fuel pump blast air from. I don't think the blast air affected CHT, since I also installed my winter oil cooler shutter, which blocks off a big chunk of oil cooler air also coming from number 4 cylinder.
Writing this, I think I will look at the servo finger screen for debris, then run a GAMI test. Maybe I introduced debris downstream of fuel pump while fixing stuff which is contributing to LOP changes.
Any ideas what is causing indicated fuel flow to increase with boost pump application? I considered some electrical interference with the fuel flow transducer, but it always worked well before, and I don't like believing in coincidence. Prior to this work, fuel pressure would increase 3-5 psi, and fuel flow would temporarily increase with application of fuel pressure, but flow would settle down to previous value in a couple seconds
Thanks for any ideas
Jay