I've had the same issue with my AFS installation and asked about it on the AFS forums, but never got much of a response about it:
http://www.advanced-flight-systems.c...r-Failure-Mode

I just replaced my VDO oil pressure transducer for pretty much the same sort of problem. Oil pressure was starting to read high (as high as 95 psi, which would trigger an alarm), and was "jumpy" when I looked at the data. Changed it out for the same type of transducer and it's back where it was for 150+ hours, mid-80s and steady.

If either this one or the fuel pressure transducer fails, I'll change them both out for the newer Kavlico senders, which are supposed to be better (but requires more wires, and I don't have the patience at the moment to crawl under the panel and muck with wiring...at least not yet).

I suspect I'll have to, though, as occasionally I see fuel pressures in the 10-12 psi range *with the engine off, before first start of the day*, much as I saw on my oil pressure (15+ psi on occasion, on a cold engine, so clearly not correct). Sigh.

Sounds like you had an instrumentation problem, not an actual fuel pressure problem.

Basically, form what I learned, the VDOs are not very reliable.

Check the crank case vent.
 

Typically you would not expect the mechanical fuel pump to be able to produce increased pressure... But there is a failure mode where it will and that is operation under increased crank case pressure. The increased pressure on the large area of the diaphragm is additive with the mechanical spring and will increase the pumps ability to produce pressure.

With the pressures you are quoting, I would check for signs of forced oil leaks aroung the front main seal and accessories. If you don't see any evidence there, and you can blow air into the vent oil dip stick tube with little restriction, then that is not the issue.

Hope that helps.

Bill Rogers
RV-7

Thanks...
 

... ALOT for all the input guys!
You've made me come up with the following plan:

1) I'll check the grounding of the sensor first.

2) I'll also look for the little hole in the spider (I have never seen one?) and I'll also blow air into the "vent oil dip stick tube" (guess that is the oil dip stick tube?)

3) I'll X-change the fuel press sensors on my two -7's and then do a ground run with both planes. If the high fuel press follows the sensor over to my first 7, then the sensor is bad.
During the swap, I'll also check the fuel-pressure indication when the wire is disconnected. Might be interesting. (ref the post on the AFS forum)


Stand by for an update.

Update
 

Here's an update on the post above:

1) The grounding of the FF sensor (type VDO) was fine.

2) After checking of the grounding, (I did not do anything else) I did a ground run and THIS time the fuel press was ok. (28 PSI)
The last time the engine was run (the flight mentioned above) the fuel press showed 52 PSI at shutdown.
In other words: the fault was intermittent.

3) I swapped sensors with my first -7, and the fuel press was 30 PSI.

I contacted AFS and they said they have had some failures with the VDO sensors, so they are now using the Kavlico sensor instead.
The change required two additional wires and to cut one jumper inside the unit.

To avoid more work for now, and to be able to complete phase 1 asap, I ordered a VDO sensor.

The warranty had expired but since the service life of the sensor was only 19 hrs, AFS replaced it at no charge anyway. Great customer care!
Right now, the new VDO sensor is on it's was across the Atlantic.

Hopefully, that sensor will last alot longer than the previous one.
If not, guess I've to change to Kavlico sensor. We'll see...

I've just replaced my 3rd fuel pressure sender in 430 hrs. It's not a big job, but the VDO senders clearly aren't fit for purpose so I'm glad most EFIS manufacturers are going with solid state ones now. When this one inevitably fails sometime in 2015 I'll replace it with one of those..
Anyway I took a pic of the guts of this one.
As has been mentioned in other threads, the wiper makes a home for itself at the regular operating pressure and wears through the resistive coil. Resistance just prior to the break is around 55ohms and goes open circuit past that.
So the symptoms at failure are normal pressure indication until you turn on the fuel pump. This tips it past the break on the coil and pressure runs off the scale high.

Yeah, I just replaced my VDO 360-003 fuel pressure sensor (its 0-80psi unit without the ground terminal vs the 360-410 posted earlier). It took about 3 flights for it to go from first twitch to pretty much never reading right. (like 250psi :eek:).

Anyway. Too lazy to upgrade to the solid state sensor. :rolleyes: