Bad Fuel Pressure Sensor? Or bad main fuel pump?

[Saville]

So I was flying around the other day and I flipped to the screen, on my Trutrak EFIS GP FM, which shows Fuel pressure (among other things) and I saw this:




well I immediately flipped on the Aux fuel pump - and saw this:



and I immediately landed.

At no time did the engine falter in the slightest. I was at 5500 feet and the engine had about 149 hours TTSNEW. On the way back I shut off the aux pump and watched the indicator....it hung at 3.8 for just a second and then slowly wound down until it hit 1.9 whereupon it disappeared and displays what you see above. Aux pump back on...immediately comes back on and counts up to 3.8.

In talking with the TruTrak people, this is what happens when there is no data on the line.



Following weekend I pulled the cowl to check all the connections to the Fuel pressure sensor and they all seemed very sound.

So today I went up - within gliding distance of an airport - and took some data. "LEFT" and "RIGHT" mean left tank and right tank. MECH is engine driven pump; AUx is Aux fuel pump on

Startup Reading - just after startup - 1000rpm:

Pump LEFT RIGHT GPH
MECH 2.4 2.3 2.1
AUX 5.1 5.1 3.5


I idled it and got: 2.0 1.9 .5gph

Taxied to the runup area. By the time I got there the readings for MECH disappeared:

Runup Reading - 1800 rpm:

Pump LEFT RIGHT GPH
MECH --- --- 4.4
AUX 4.5 4.5 7.6


5500 feet 23 squared:

Pump LEFT RIGHT GPH
MECH --- --- ??/ can't read my writing
AUX 4.8 4.8 16.0

I may have been too lean here at 23 squared.

Ok so I head back and as I'm letting down I'm playing with the throttle: Prop at 2300.

I bring the MP back slowly to 15" stopping every couple of inches. At 9 inches the readings return - 1.9psi

Back up to 15" - readings disappear.....

Back down one inch at a time - readings appear at 9".

Back up 1 inch at a time to 15...reading disappears at around 12"

I repeat that two more times..same results.

Next time I do it, the readings do not disappear! Odd.

So I concentrate on landing, and take a reading at the hangar just before shutdown:

Hangar Reading:

Pump LEFT RIGHT GPH
MECH 5.2 5.1 1.7


Ok so my first inclination is to say that the Fuel pressure sensor may be bad. It was bought from TruTrak in 2010, 0 - 30 psi

SO# 27614

Looks like a plain old every day sensor:



But it is almost 6 years old.

I can't think of any other things to check. As I say the engine runs fine.

Any ideas welcome.

thanks!

[Jrskygod]

Good luck on it being the sender. My pump was bad and I had similar indications. I just couldn't believe that with a 0 fuel pump pressure reading that the fuel pump could be bad and engine still run fine. I installed a mechanical gauge for testing and sure enough under high power the pressure dropped way off. Replaced fuel pump and all is well. Pump had about 400 hours if I recall.

[Kyle Boatright]

Here's a thread from my similar experience earlier this year. Turned out to be the mechanical fuel pump.


http://www.vansairforce.com/communit...=132918&page=3

[Lenny Iszak]

That VDO pressure sensor has a reostat-like winding inside and it gets worn out at the spot that gets used most often. Hopefully it's that and not the pump.
Talk to Trutrak and see if you can use a voltage based Kavlico or Honeywell sensor. Those seem to be much more reliable.

Lenny

[Ed_Wischmeyer]

Had the same problem and replaced the fuel pump. Still have the problem.

Grrrr.

Ed

[Saville]

Originally Posted by Jrskygod

Good luck on it being the sender. My pump was bad and I had similar indications. I just couldn't believe that with a 0 fuel pump pressure reading that the fuel pump could be bad and engine still run fine. I installed a mechanical gauge for testing and sure enough under high power the pressure dropped way off. Replaced fuel pump and all is well. Pump had about 400 hours if I recall.

How did you run your tests with the mechanical gauge? On a ground run?

Do you happen to recall at what MP the reading went to zero?

thanks!

[TS Flightlines]

Guys---back in my old days of turning wrenches, we always ran a mechanical gauge test to verify the reading before condeming anything. The problem as I see it, (and I'm guilty too) is that we've become captive by all the electronic gizmos, and have moved away from what has worked for decades.
So when the pressure readings on the EFIS start to fluxuate, the most natural thing is to condem the pump.
I my old world, the natural thing was to condem the electronics----because it was the 'new thing'. And quite many times it was as simple a bad connection.
I'll go out on a limb and say thats probably still the case.

It would almost be prudent to place an extra bulkhead fitting in the firewall
(capped off of course) to allow a mechanical gauge to be temporarily installed in the cabin to check the pressures. The fuel pressuse hose could be re-routed to this fitting, and the gauge connected. Test flight and see if the symptoms repeat. If so, then you've eliminated the electronic devices, and can zero in of the pump. IF the symptoms go away, then zero in on the electronics.

We seen guys the spent countless hours and hundreds of dollars or more buying and swapping parts, only to find a mis-crimped terminal or a loose ground. Above all----be safe!
Tom

[Tom Martin]

I am in agreement with Tom Swearengen. Even though the sender is screwed in to what looks to be a perfect ground time and sealant may have compromised that ground. Take a bit of sandpaper and clean up a spot on the sensor. Now fasten a ground wire to the body of the sensor with a hose clamp. The other end of the wire goes to a good ground on the airframe.
This solved a similar issue that I had. A simple thing to check, then a mechanical gauge. The sensors are not that expensive, you will eventually have to replace it, mine lasted 400 hours and the replacement has 550 hours on it and I have a spare on the shelf....

[BillL]

Originally Posted by Lenny Iszak

That VDO pressure sensor has a reostat-like winding inside and it gets worn out at the spot that gets used most often. Hopefully it's that and not the pump.
Talk to Trutrak and see if you can use a voltage based Kavlico or Honeywell sensor. Those seem to be much more reliable.

Lenny

But do a diagnosis first. If you have an air pressure regulator, ohm meter, and manual pressure gage, you could pressurize and test the sensor. Since the above failure mode is sensitive to vibration, you might not learn anything, but you never know. This is what I would do, but I'm nuts.

The Kavlico is much more reliable and durable (non mechanical), if it works with your system. After replacement you can tear apart the old one and look for the spot on the wiper for confirmation.

[Jrskygod]

Saville I did as Tom S. Suggested. I used a known good mechanical gauge plumbed into the fuel system. I installed a bulkhead fitting in the firewall and connected the line to the electric sender to it. I made up a line for the cockpit side and temporarily installed the gauge. When flying the pressure dropped to almost zero under power. Flipped on boost pump and all was good. Apparently pump provided enough volume to supply engine at power as the engine always ran fine.