Boost pump on Bravo temporarily causing engine roughness

[rocketman]

The other day right after a proper preflight check, I turned the boost pump on in preparation for takeoff in my 1994 Bravo and noticed engine roughness and sputtering at 1000 RPM. Turning off the boost pump eliminated this problem. I taxied back to the apron and tried this several other times which the engine continued to run rough only with the boost pump on. I attribute this to flooding the engine with excess fuel. I then shut the engine off and restarted it 10 minutes  later and the problem went away. I reattempted duplicating the problem with the boost pump at various RPM's, even going through the same conditions on engine preflight checks to try to duplicate this condition but could not. I then flew the plane and everything was prefect. My rocker switch never experienced so much of my fingerprints for a 30 minute flight so now this intermittent problem cannot be duplicated.

 

Any thoughts on why this condition existed and what do I do now to check where the problem might be. 

[FlyDave]

I'm don't know what caused this problem but, in the Bravo, when I go full power, the boost pump comes on automatically and shuts off when I pull the MP back to ~33". So I don't flip the  boost pump switch on for takeoff - but I do for landing.

[Danb]

Same as Dave..but it seems like it was flooding the power at first try?? Has it been flown much lately and how about the outside temp?

[gjkirsch]

I only use the boost pump to prime the engine before starting and I verify it comes on during my take off run.  I do not use it before landing.  The engine is running good and I am at a low power setting, in my opinion, it does not make sense to pour more fuel into an engine that is leaned and at low power setting.  Additionally, I lean aggressively on the ground and only go full rich to do the run up.

[larryb]

I don't know the specifics of the Bravo fuel system. But a couple thoughts come to mind.

 

Did you happen to notice the fuel flow with and without the boost pump? Or do you have a JPI that has the data in memory? That might be a clue. When things are running normally, is there a difference in flow with/without the boost pump? Perhaps the fuel pressure regulator was stuck or had a bit of dirt in it and wasn't regulating properly for a time.

 

Larry

[Piloto]

It is not unsual to have some roughness at idle with the boost pump on. I have seen this a couple of times. Like others said just check for pump light when going full throttle. You may also notice that to keep the fuel pressure up above FL200 you need to turn the boost pump on.

 

José  

[rocketman]

Jose' - If its not unusual to have some engine roughness with the boost pump on low RPM, is there anything I should be worried about using the boost pump during landing when the RPM's are 1000 or less? After all, the POH does call for the electric fuel pump to be turned on during al landings.

[Piloto]

Jose' - If its not unusual to have some engine roughness with the boost pump on low RPM, is there anything I should be worried about using the boost pump during landing when the RPM's are 1000 or less? After all, the POH does call for the electric fuel pump to be turned on during al landings.

Like others have said I do not turn it on during landing. I did it one time on short final and scared me enough to turn it off. If you turn it on final it may force you to increase the RPM on the flare to keep the engine smooth, thus more floating on the flare. Depending on your approach speed the engine is windmilling and the roughness is not as noticeable as on the ground. Looks to me like the fuel servo is either poorly calibrated or poorly design. Fuel pressure changes should not have this effects at idle. But I have seen this in other Bravos and does not appear to be something wrong with your plane.

 

José

[Guest]

Jose' - If its not unusual to have some engine roughness with the boost pump on low RPM, is there anything I should be worried about using the boost pump during landing when the RPM's are 1000 or less? After all, the POH does call for the electric fuel pump to be turned on during al landings.

If turning on your fuel,boost pump causes engine roughness in a Lycoming powered airplane with a Bendix/ Precision fuel injection system there is something wrong. These servos are able to accept a wide range of fuel pressures.

On a TCM powered airplane it's a different scenario, the fuel system should be set up per TCM SID 97-3 E and the applicable airframe manual. In most cases the airframe fuel pressure is adjustable to prevent changes in engine operation with the pump on or off.

Clarence