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Fuel Pressure Indication, correct or not

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VF84Sluggo

Well Known Member
Folks,

I ran my engine yesterday for the first time after installing my rehabbed fuel servo (Bendix RSA-5A) and divider. I also replaced the UMA mechanical fuel pressure gauge while the servo was being worked on (old one sprang an internal leak.) The servo came back with a new "Experimental" placard on it instead of the original Bendix RSA-5AD1 placard (thought this was odd)

When I turned on the boost pump I noticed I got fuel pressure with the mixture at idle cutoff. Moving the mixture to full rich made no change in the fuel pressure reading (20-25 psi.) Is this normal?

Can't remember if this was the case before all of this work, but I seem to recall moving the mixture to full rich for priming caused fuel pressure to rise/indicate. That rise and counting three or four seconds was my priming routine. That's what got my attention: fuel pressure was already showing, got no change when I went to prime for start, and no drop when going back to idle-cutoff.

Also, now after pulling the mixture to idle-cutoff for shutdown, fuel pressure remains indicated (trapped fuel pressure?). I think the fuel pressure went to zero before when the mixture was pulled to idle-cutoff. Fuel pressure will drop if I go to full rich after shutdown (engine off/boost pump off). Also will bleed down over time (like all night).

Anyway, just wondering if maybe after the overhaul, now when the mixture is at idle-cutoff in the cockpit, fuel might not be fully cutoff in the servo. Or maybe all this is normal, and I just didn't pay this close attention before :D

Thoughts?
 
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Where do you pick up the fuel pressure reading? If it’s before the servo, then that explains why you have pressure regardless of where the mixture is.
 
True. I thought I had pics that would show the fuel line connections, but I don't. I'll get some.
 
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Folks,

I ran my engine yesterday for the first time after installing my rehabbed fuel servo (Bendix RSA-5A) and divider. I also replaced the UMA mechanical fuel pressure gauge while the servo was being worked on (old one sprang an internal leak.) The servo came back with a new "Experimental" placard on it instead of the original Bendix RSA-5AD1 placard (thought this was odd)

When I turned on the boost pump I noticed I got fuel pressure with the mixture at idle cutoff. Moving the mixture to full rich made no change in the fuel pressure reading (20-25 psi.) Is this normal?

Can't remember if this was the case before all of this work, but I seem to recall moving the mixture to full rich for priming caused fuel pressure to rise/indicate. That rise and counting three or four seconds was my priming routine. That's what got my attention: fuel pressure was already showing, got no change when I went to prime for start, and no drop when going back to idle-cutoff.

Also, now after pulling the mixture to idle-cutoff for shutdown, fuel pressure remains indicated (trapped fuel pressure?). I think the fuel pressure went to zero before when the mixture was pulled to idle-cutoff. Fuel pressure will drop if I go to full rich after shutdown (engine off/boost pump off). Also will bleed down over time (like all night).

Anyway, just wondering if maybe after the overhaul, now when the mixture is at idle-cutoff in the cockpit, fuel might not be fully cutoff in the servo. Or maybe all this is normal, and I just didn't pay this close attention before :D

Thoughts?

Sounds right. Full rich is not full flow. The servo uses airflow as the primary means to meter fuel. With the boost pump on, I would NOT expect pressures below 25 when full rich without the engine running.
 
In fact, that's how I prime my IO-360 for a cold start. Throttle open, fuel pump on, watch for full pressure, then mixture rich until ~3 gph, then fuel pump off/throttle open 1/4" and ICO, and start. Fires up every time.
 
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