David, don't know whether you have considered this - as you are aware the two pumps are able to work in parallel because the pumps have check valves.  So if one pump is off and the other on, the on pump does not pump fuel backwards through the off pump.  Since each pump produces the 42 psi pressure by itself, one would assume the pumping elements in both pumps are working properly.  Also assuming the pressure change is NOT due to a malfunctioning pressure sensor.

Here are some things that you might want to consider:

1.  That somehow when both pumps are on - one of the pumps check valves is not closing properly.  Not a  pump expert so not certain this could even happen - but the thought occurred to me.

2.  The effect of Increased fuel flow rate due to both pumps may be causing a pressure drop through an upstream orifice (like perhaps your "T").

3.  Turning on both pumps (assuming on the same circuit) might cause sufficient voltage drop in the line sufficient to decrease the pumping performance of both pumps - pump flow rates are sensitive to voltage variations.

4.  The pressure return regulator might be malfunctioning and opening due to the higher flow rate - easy to check, just temporarily block off the return.  If the symptom disappears then it’s the regulator.

Theoretically, if the fuel flow rate through the T's and pumps  does not change with both pumps on, then the pressure should remain the same  - which is what you are expecting, but not seeing. 

So since the pressure does change (drops) - what could cause this (assuming the pressure drop is real and not the result some strange malfunction of your gauge)?

  One thing that could cause it, is if the fuel flow rate increases through an upstream orifice in your fuel line (say at the "T" which feeds your two pumps)  then the pressure on the other (downstream) side will drop.  Turn one of the pumps off, the fuel flow rate drops, therefore there is less pressure drop across this orifice  and your fuel pressure on the downstream side of the orifice will again go up.

Its similar to a resistor in a circuit.  The resistor value will remain static - the same, but if you pull more current thought it, the voltage drop across it will increase resulting in less voltage on the downstream side.  Reduce the current flow back to previous rate and the voltage drop across the resistor decreases and therefore voltage on the downstream side increase.

Another possible cause might be if both of your pumps are fed off the same electrical line and there is sufficient resistance that when both pumps are on the voltage drop causes less performance by both pumps.

Just some WAGs that you might consider, if the suggestions make by others do not result in identifying the cause.

Good Luck

Ed

Edward L. Anderson
Anderson Electronic Enterprises LLC
305 Reefton Road
Weddington, NC 28104
http://www.andersonee.com
http://www.eicommander.com

Fuel is gravity routed from tanks to ganged fuel selector out to T-fitting with each pump on the sides of the T,  to another T,  into main line out to eng., pressure sender is mounted directly after the T  where the two pumps come together.   All this is in front of the wing spar near the floor in an RV6A. This would make the pumps in parallel & the pressure is the same from either pump, but both together pressure drops.   David 

From: "Mark Steitle" msteitle@gmail.com
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
Sent: Wednesday, June 13, 2012 8:36:19 AM
Subject: [FlyRotary] Re: Fuel presure question.

David,

How are your pumps plumbed... in parallel or series?  What kind of
pressure do you get when running the backup pump only?

Mark

On 6/13/12, Tracy <rwstracy@gmail.com> wrote:
> Hmmmm...  Mine goes up about a pound when I switch on the second pump.  What
> happens when you turn both off?
>
> Tracy
>
> Sent from my iPad
>
> On Jun 12, 2012, at 7:11 PM, hoursaway1@comcast.net wrote:
>
>> Question about fuel pressure indications, at fast idle 2200 RPM, fuel
>> pressure is 42 PSI on EM2, if I switch on the back-up pump with the
>> primary pump the pressure drops to 35 PSI, can here both pumps running, if
>> I switch off either pump pressure goes back to 42 PSI.  My system is 13B,
>> stock red injectors all four, pressure regulator is stock Mazda on the end
>> of fuel rail with vacume connection from dynamic chamber.  Thanks,  David
>> R. Cook  RV6A Rotary.
>

--
Homepage:  http://www.flyrotary.com/
Archive and UnSub:   http://mail.lancaironline.net:81/lists/flyrotary/List.html