[acts]

Hello,

I've searched a bunch for this topic but haven't quite found my scenario, witnessed on my last short 200 sm each-way, cross country.

Normal readings on mechanical fuel pressure gauge 'T'd off the outflow side of the high pressure mechanical fuel pump to the firewall with a connection at the firewall, then to a 40 hour since new mechanical gauge in the cockpit = ~25 psi steady with mechanical pump only; ~28 psi with boost pump on- from start-up to shut down.

Last week, warm day(80F) climbing from sea level, started to notice mechanical fuel pressure gauge reading drop at/above ~6000' MSL to ~18 psi by level-off cruise at 9500' one way and the same findings returning at 8500' MSL the other direction; ~22 psi with boost pump on (and back down to ~18 psi when turned back off). OAT up in that altitude range was between 55-60F. CHTs were all ~315F. I lost ~7" of MP in the climb which took about 10+/- minutes. Upon descending at/below ~6000' MSL both directions, the mechanical fuel pressure gauge reading increased back to the normal 25 psi for mechanical pump only; 28 psi with boost pump on. Despite all the reading changes, the engine ran perfectly normal and it didn't matter if I changed tanks or used the fuel pump when doing so or not if that matters.

I've read the Lycoming manual and the mechanical fuel pump range for my injected engine is 12-45 psi. First guesses could be atmospheric pressure change affecting simple/mechanical instrumentation or fuel pump starting to fail.

Can someone help explain this phenomenon?

Thanks in advance,
James

 

[Don at Airflow]

Fuel pressure drop

The high-pressure mechanical pump used on an IO Lycoming (P/N LW-15473) is rated at 22-26 PSI. Since the pump uses a diaphragm and a spring to generate the fuel pressure, and the diaphragm is referenced to ambient pressure, the ambient pressure is added to the spring pressure. So you will see some drop off in pressure as the ambient pressure decreases.

The Bendix, or Airflow Performance fuel injection systems are not effected by fuel pressure changes as long a the inlet fuel pressure is above the minimum pressure to satisfy all the pressure drops in the fuel metering system at that particular fuel flow. That is to say for a given power setting if fuel pressure is above the minimum required pressure to make everything work, say it?s 20-PSI. Then you turn on an auxiliary pump and the inlet fuel pressure goes up to 50 PSI. The fuel flow delivered to the nozzles would stay the same.


Don

 

[acts]

Thanks Don!

I appreciate you taking the time to write. This is a logical explanation of how ambient pressure changes would affect the workings within the pump mechanically. You've also helped separate the functionality of the Bendix fuel servo from the fuel pump.

I do have a blast tube running to the top of the fuel pump- for those who might have wondered about vapor lock.

Thanks again,
James