Bravo Fuel Pressure

[FlyDave]

My 1989 Bravo fuel pressure typically runs around low 20's PSI (almost in the yellow) on the mechanical pump. But when I turn on the boost pump it jumps to around 50 PSI (almost in the red) and FF drops by 2/10 gph. Any other Bravo owners experience this?

My mechanic says we need to look at the mechanical pump as there may be a leaky seal or diaphragm torn inside.

Any ideas?

Thanks,

Dave

[carusoam]

Check if the FP has a drain at the bottom.  Pumps usually get a drain to show fuel where to exit in the event of it's diaphragm going south.  The blue goo of 100LL will probably collect on the nipple.

Best regards,

-a-

[DVA]

FlyDave, the yellow arc top is at 24psi, the green “normal” goes from 25-55 psi. Are saying that you run in the yellow arc, that being below 25psi with the engine driven pump?

Also, early Bravo’s (27-0001 - 27-0052) had a low fuel pressure lamp set to go off below 24psi, are you between those serial numbers?

DVA

Edited June 26, 2016 by DVA

[FlyDave]

DVA,

Yes - see the accompanying picture. It runs in the yellow so around 23-24 PSI.

My plane is SN 9 and I do have that low fuel pressure lamp but it only comes on when the engine isn't running.

I might think this is failing instrumentation but the decrease in FF when the boost pump is turned on tells me something other than instrumentation is amiss.

Dave

[DVA]

What is the fuel pressure at idle? And what is the fuel flow at full power on the takeoff roll?

 

Edited June 27, 2016 by DVA

[FlyDave]

Idle is around 20. Takeoff the boost pump is on so it's around 55.

[Awful_Charlie]

Normal flight and taxi gives me about 30-35PSI with the engine pump only, the electric pump takes it right to the top of the green - I've not noticed any difference in fuel flow with the electric pump off or on.. I do get the odd dip in pressure, particularly after a long climb to the higher levels (which is noted in the POH) and sometimes in the cruise, but the engine doesn't miss a beat and there's nothing detectable in the JPI data, so I'm not worried about it at this stage. S/n 239 and with 1950 hours on the engine

[DVA]

FlyDave,

I’d have two things looked at: 1) On your plane, you have the low pressure light (only on early models), it should be going off if the fuel pressure is indeed below 24. Since it is not, you may have a faulty reading on the fuel pressure gauge at lower pressures. The FP gauge and the FP idiot light are not necessarily coincident, so you may actually have a higher pressure than you think (hence the reason you aren’t seeing the light). OR, the low pressure sensor AND the gauge are off - seems unlikely; this is easily verified by any A&P.  2) There is a mechanical fuel pump AD 2003-14-03 that is required at every 50 hours on certain pump models. While this AD is mainly looking for leaks, I have a seen few Lycoming owners who were unaware of this, and a few Annuals that didn’t check it, a small leak could cause fuel pressure issues (amongst other bad things). You may be all over this already, but I felt compelled to point it out in the interest of safety. 

Awful_Charlie describes s typical Bravo fuel pressure scenario; mine is exactly like his.

Let us know how you make out!

DVA

[FlyDave]

And just to confirm, you don't get a drop in FF with the boost pump on - correct?

[DVA]

46 minutes ago, FlyDave said:

And just to confirm, you don't get a drop in FF with the boost pump on - correct?

No, no drop.

That’s a little weird to me, as the pressure transducer is in the unmetered portion of the fuel flow circuit, i.e. prior to the fuel flow splitter on the top of the engine. That side of the circuit should be higher than the metered side toward the injectors if I recall.  I’ll go check the maintenance manual when I get home tonight.

[FlyDave]

Follow up:

My mechanic put a calibrated pressure gauge in the fuel line system and found that my fuel pressure gauge is fairly accurate to within 2-3 PSI. After he took the fuel system apart and T'ed his gauge into the system he was getting ~30 PSI on the mechanical pump and 55 PSI on the electric pump. He adjusted the pump output up a little and I'm now at 40 PSI in cruise on the mechanical pump and there is no drop in fuel flow when I turn the electric pump on.

There must have been something in the fuel line or stuck in the pump vanes (it has vanes not not a diaphragm) because he didn't do anything but disconnect the fuel line and let the fuel drain out (He may have blown out or purged some of the lines).

Anyway, issue resolved.

[Tony Armour]

Another data point:

Cruise: 38psi

Cruise with electric on: 55psi

[FoxMike]

Another data point:

Cruise pressure 34PSI about 1200hrs on engine.  Had to put change crank ( AD).  Overhauled fuel pump Criuse pressure 40PSI.