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Low fuel pressure indication

 
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HCRV6(at)comcast.net
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PostPosted: Thu Jun 27, 2013 10:07 am    Post subject: Low fuel pressure indication Reply with quote
I have a puzzling problem and wondering if anyone can shed some light on it. Under continuous high power settings, like in climb to altitude after takeoff, my Grand Rapids EFIS fuel pressure indication drops to zero and even shows a low negative value after a couple of minutes, but the engine continues to run at full power. This with a carbureted O-360 and running on the engine driven pump only. If I switch the aux pump back on the pressure indication comes back to the normal 2 to 4 psi range. I had this problem a few years ago and installing a new VDO pressure sender from Grand Rapids cured it, but this time a new sender did not help. The pressure take off is at the carburetor and the sender is mounted on the firewall, connected with a -2 hose.

The thing that has me scratching my head is that the engine keeps running fine at high fuel flow even though the fuel pressure indicates zero or less, so I would suspect a bum sender again, but the fact that the pressure indication comes back to normal when I turn the aux pump back on has me baffled.

Harry Crosby
RV-6 N16CX, 1080 hours

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mrobert569(at)hotmail.com
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PostPosted: Thu Jun 27, 2013 10:25 am    Post subject: Low fuel pressure indication Reply with quote
Harry,
 
Keep in mind that when we measure pressure we are really talking pressure differential.  If the throttle is full open then the fuel pressure differential may actually be zero.  For carb equipped airplanes I would not get too carried away with a zero reading at full throttle as long as the engine is running good and you still see some pressure when you turn on the boost pump.
 
Mike Robertson
 
Date: Thu, 27 Jun 2013 18:07:25 +0000
From: HCRV6(at)comcast.net
To: RV-list(at)matronics.com
Subject: Low fuel pressure indication

I have a puzzling problem and wondering if anyone can shed some light on it.  Under continuous high power settings, like in climb to altitude after takeoff, my Grand Rapids EFIS fuel pressure indication drops to zero and even shows a low negative value after  a couple of minutes, but the engine continues to run at full power.  This with a carbureted O-360 and running on the engine driven pump only.  If I switch the aux pump back on the pressure indication comes back to the normal 2 to 4 psi range.  I had this problem a few years ago and installing a new VDO pressure sender from Grand Rapids cured it, but this time a new sender did not help.  The pressure take off is at the carburetor and the sender is mounted on the firewall, connected with a -2 hose.

The thing that has me scratching my head is that the engine keeps running fine at high fuel flow even though the fuel pressure indicates zero or less, so I would suspect a bum sender again, but the fact that the pressure indication comes back to normal when I turn the aux pump back on has me baffled.

Harry Crosby
RV-6 N16CX, 1080 hours

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flying-nut(at)cfl.rr.com
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PostPosted: Thu Jun 27, 2013 12:35 pm    Post subject: Low fuel pressure indication Reply with quote
BTDT!!!
The sender 'wiper' sits in one place at where ever the fuel pressure puts it.  The normal engine vibration wears a hole in the carbon path ..... and the reading goes to zero.  Turning on the electric pump moves the wiper off it's 'normal' spot and onto a good spot.
Replaced the sender and my problem went away.
My sender had about 2500 hours on it but it's a crapshoot when it will fail.
A prop balance is a good investment.
Linn


On 6/27/2013 2:07 PM, HCRV6(at)comcast.net (HCRV6(at)comcast.net) wrote:

[quote] p { margin: 0; } I have a puzzling problem and wondering if anyone can shed some light on it.  Under continuous high power settings, like in climb to altitude after takeoff, my Grand Rapids EFIS fuel pressure indication drops to zero and even shows a low negative value after  a couple of minutes, but the engine continues to run at full power.  This with a carbureted O-360 and running on the engine driven pump only.  If I switch the aux pump back on the pressure indication comes back to the normal 2 to 4 psi range.  I had this problem a few years ago and installing a new VDO pressure sender from Grand Rapids cured it, but this time a new sender did not help.  The pressure take off is at the carburetor and the sender is mounted on the firewall, connected with a -2 hose.

The thing that has me scratching my head is that the engine keeps running fine at high fuel flow even though the fuel pressure indicates zero or less, so I would suspect a bum sender again, but the fact that the pressure indication comes back to normal when I turn the aux pump back on has me baffled.

Harry Crosby
RV-6 N16CX, 1080 hours



Quote:


No virus found in this message.
Checked by AVG - www.avg.com
06/26/13 [b]

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vshimsl(at)live.com
Guest
PostPosted: Thu Jun 27, 2013 2:57 pm    Post subject: Low fuel pressure indication Reply with quote
Only way a working gauge pressure transducer would register negative is if its sensing a vacuum. I am keying in on fact you have takeoff at carburetor rather than the fuel pump outlet and that your boost pump readings are, compared to mine, low. I would also suspect you have the fuel pressure transducer mounted fairly high up on the firewall.

Is it possible to move the takeoff back to the fuel pump out port near the rear of the engine? Moving the ‘T’ to the fuel pump output decreases the height differential and also the length of hose.

I am thinking your problem will disappear, and your current transducer and quite possibly your old transducer, will work correctly reading more like 4 to 6 PSI with boost pump on.

I may be way off, but moving the Takeoff is something ‘relatively’ quick and inexpensive to try.

Regards,
VinceH
RV8432 flying



From: owner-rv-list-server(at)matronics.com [mailto:owner-rv-list-server(at)matronics.com] On Behalf Of Linn Walters
Sent: Thursday, June 27, 2013 1:34 PM
To: rv-list(at)matronics.com
Subject: Re: Low fuel pressure indication

BTDT!!!
The sender 'wiper' sits in one place at where ever the fuel pressure puts it. The normal engine vibration wears a hole in the carbon path ..... and the reading goes to zero. Turning on the electric pump moves the wiper off it's 'normal' spot and onto a good spot.
Replaced the sender and my problem went away.
My sender had about 2500 hours on it but it's a crapshoot when it will fail.
A prop balance is a good investment.
Linn
On 6/27/2013 2:07 PM, HCRV6(at)comcast.net (HCRV6(at)comcast.net) wrote:
Quote:

I have a puzzling problem and wondering if anyone can shed some light on it. Under continuous high power settings, like in climb to altitude after takeoff, my Grand Rapids EFIS fuel pressure indication drops to zero and even shows a low negative value after a couple of minutes, but the engine continues to run at full power. This with a carbureted O-360 and running on the engine driven pump only. If I switch the aux pump back on the pressure indication comes back to the normal 2 to 4 psi range. I had this problem a few years ago and installing a new VDO pressure sender from Grand Rapids cured it, but this time a new sender did not help. The pressure take off is at the carburetor and the sender is mounted on the firewall, connected with a -2 hose.

The thing that has me scratching my head is that the engine keeps running fine at high fuel flow even though the fuel pressure indicates zero or less, so I would suspect a bum sender again, but the fact that the pressure indication comes back to normal when I turn the aux pump back on has me baffled.
Harry Crosby
RV-6 N16CX, 1080 hours


Quote:

No virus found in this message.
Checked by AVG - www.avg.com
06/26/13

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[quote][b]

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HCRV6(at)comcast.net
Guest
PostPosted: Fri Jun 28, 2013 9:10 am    Post subject: Low fuel pressure indication Reply with quote
Thanks Mike, excellent point that was escaping me.

Harry
From: "Mike Robertson" <mrobert569(at)hotmail.com>
To: "rv list" <rv-list(at)matronics.com>
Sent: Thursday, June 27, 2013 11:25:15 AM
Subject: RE: Low fuel pressure indication

Harry,

Keep in mind that when we measure pressure we are really talking pressure differential. If the throttle is full open then the fuel pressure differential may actually be zero. For carb equipped airplanes I would not get too carried away with a zero reading at full throttle as long as the engine is running good and you still see some pressure when you turn on the boost pump.

Mike Robertson

Date: Thu, 27 Jun 2013 18:07:25 +0000
From: HCRV6(at)comcast.net
To: RV-list(at)matronics.com
Subject: Low fuel pressure indication

I have a puzzling problem and wondering if anyone can shed some light on it. Under continuous high power settings, like in climb to altitude after takeoff, my Grand Rapids EFIS fuel pressure indication drops to zero and even shows a low negative value after a couple of minutes, but the engine continues to run at full power. This with a carbureted O-360 and running on the engine driven pump only. If I switch the aux pump back on the pressure indication comes back to the normal 2 to 4 psi range. I had this problem a few years ago and installing a new VDO pressure sender from Grand Rapids cured it, but this time a new sender did not help. The pressure take off is at the carburetor and the sender is mounted on the firewall, connected with a -2 hose.

The thing that has me scratching my head is that the engine keeps running fine at high fuel flow even though the fuel pressure indicates zero or less, so I would suspect a bum sender again, but the fact that the pressure indication comes back to normal when I turn the aux pump back on has me baffled.

Harry Crosby
RV-6 N16CX, 1080 hours

[quote]

get=_blank>http://www.matronics.com/Navigator?RV-List
ttp://forums.matronics.com
=_blank>http://www.matronics.com/contribution



t="_blank">http://www.matronics.com/Navigator?RV-List
tp://forums.matronics.com
_blank">http://www.matronics.com/contribution

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ceengland7(at)gmail.com
Guest
PostPosted: Sat Jun 29, 2013 1:51 pm    Post subject: Low fuel pressure indication Reply with quote
There have been numerous threads on various lists & forums about low fuel pressure indications with carb'd engines. Most indicate a sensor design flaw, rather than an actual pressure issue. Here's a pretty good example from VAF:
http://www.vansairforce.com/community/showthread.php?t=90682&highlight=low+fuel+pressure

Mike,

I'm not following how manifold pressure would affect a fuel pressure reading, since with a carb'd engine the fuel pressure sensor is measuring pressure between the engine pump & the inlet to the carb bowl & isn't typically in the intake path. What am I missing?

Charlie

On 06/28/2013 12:10 PM, HCRV6(at)comcast.net (HCRV6(at)comcast.net) wrote:

[quote] p { margin: 0; } Thanks Mike, excellent point that was escaping me. 

Harry


From: "Mike Robertson" <mrobert569(at)hotmail.com> (mrobert569(at)hotmail.com)
To: "rv list" <rv-list(at)matronics.com> (rv-list(at)matronics.com)
Sent: Thursday, June 27, 2013 11:25:15 AM
Subject: RE: Low fuel pressure indication

Harry,
 
Keep in mind that when we measure pressure we are really talking pressure differential.  If the throttle is full open then the fuel pressure differential may actually be zero.  For carb equipped airplanes I would not get too carried away with a zero reading at full throttle as long as the engine is running good and you still see some pressure when you turn on the boost pump.
 
Mike Robertson
 
Date: Thu, 27 Jun 2013 18:07:25 +0000
From: HCRV6(at)comcast.net (HCRV6(at)comcast.net)
To: RV-list(at)matronics.com (RV-list(at)matronics.com)
Subject: Low fuel pressure indication

I have a puzzling problem and wondering if anyone can shed some light on it.  Under continuous high power settings, like in climb to altitude after takeoff, my Grand Rapids EFIS fuel pressure indication drops to zero and even shows a low negative value after  a couple of minutes, but the engine continues to run at full power.  This with a carbureted O-360 and running on the engine driven pump only.  If I switch the aux pump back on the pressure indication comes back to the normal 2 to 4 psi range.  I had this problem a few years ago and installing a new VDO pressure sender from Grand Rapids cured it, but this time a new sender did not help.  The pressure take off is at the carburetor and the sender is mounted on the firewall, connected with a -2 hose.

The thing that has me scratching my head is that the engine keeps running fine at high fuel flow even though the fuel pressure indicates zero or less, so I would suspect a bum sender again, but the fact that the pressure indication comes back to normal when I turn the aux pump back on has me baffled.

Harry Crosby
RV-6 N16CX, 1080 hours






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