High fuel pressure (52 PSI) on a Lyc IO-360. Normally 27 PSI. Any ideas to why?

Today was the 11th flight with my new -7, tach was around 19 hrs when it happened.

Until today, the fuel pressure has been steady at 27 PSI during flight.

About an hour into todays flight, (2:30 mins planned) I noticed that the fuel press was 32 PSI.
I kept an eye on it and about 10 mins later it was 35.
5 mins later it was 40.

The increase where slowly, so I assumed this probably wasn't an indicator malfunction. (No "jumpy" indications)

I decided to turn towards the homebase and during the turn it slowly climbed to 52 PSI.
I climbed to 8000' on course for home.

During the climb, the fuelpress decreased to 40 PSI.
After about 5 mins cruise at 8000' the fuel press decreased to 32 PSI

Here a pic:





A few minutes later, it climbed back up to 40:





When reducing power for landing, it climbed back up to 51 PSI.
I turned on the boost pump and it jumped to 55 PSI, so I quickly turned the boostpump off again.

When taxing in, the fuel press stayed at 51 PSI.

I pulled the cowls after shutdown and no leaks or blue stains anywhere.
Tomorrow I'll check the fuel filter.

I'll also check for good grounding at the fuel pressure sensor.

On my first -7 (Extreem IO-360 engine) the fuel press has been 30 PSI since the first flight.
On this -7, I've been satisfied with 27 since it has been steady.

The mechanical fuel pump was installed by Lycoming when I got the engine, so I haven't done anything with it.

Does anyone have an idea to what can be the cause of this?
To me, it looked like there really was a high fuel pressure and not an indicatotion problem, since the indications where so gradual.

Any ideas anyone?

Had a similar issue on my IO-375 with AFS instrumentation although the readings never got quite that high. Fixed with a new fuel pressure transducer.

I had an issue in Phase 1 with my Oil pressure spiking rapidly (which really got my attention) - turned out to be a bad ground on the transducer - might be something to check.

Have had the fluctuating fuel pressure twice with my Dynon transducer over the first 150 hours. Both times I fixed it by bleeding the line by pulling the transducer off the manifold and then pumping a bit a fuel through before screwing the transducer back on. On the second time I also added an extra ground write to make sure I kept a good ground. Hope this helps... I know it can be frustrating chasing these kinds of issues.

Alf, I had similar issues several years ago on my twin with IO470s.... After almost a month of searching, rebuilding the fuel pump, cleaning and/or replacing injectors, etc. I was ready to throw in the towel and give up.... I was putting the cowling back on when I found the needle in a hay stack! The body of the fuel injector (looks like a spider) there is a very small pin hole that allows air into the injector. There was a bug inside of this hole and was blocking the air from getting into the body of the injector. I am by no means a mechanic but thought I would share my experience .... We (my A&P and I ) cleaned it and it immediately fixed the issue. I spent over $1000 trying to fix it and it turned out to be a stupid bug building a mud nest (we call them dirt dauber) in the injector body. You may want to make sure the injectors and the body are all clean.
Good luck.

Fuel pressure sensor

I had this happen on my Dynon last year. When I called to order a replacement I found that they are supplying a different, higher-quality sensor now. It took a different excitation voltage so it was a bit of a pain to re-wire, but I did the oil pressure sensor at the same time, so they are both the newer, better style.

ao.frog said:

The increase where slowly, so I assumed this probably wasn't an indicator malfunction. (No "jumpy" indications)

Click to expand...

This is where you went wrong.

I don't think the mechanical pump is capable of doing this - the pressure is developed by the spring tension inside the pump on the return stroke of the pump lever. Rather, I believe it has to do with the transducer used by AFS for the fuel pressure sender. If it's like the ones on the 2 planes I have with AFS units, it is a VDO pressure-to-resistance unit and is sensitive to grounding issues and other mechanical factors. The one supplied by AFS is grounded through the pipe fitting which makes it possible to have an unreliable ground - VDO makes one that has a separate ground terminal with otherwise the same pressure and resistance ratings as the original, which makes it easier to connect a secure ground to it. I don't have the part number handy but will see if I can find it today.

The upside is that these VDO sensors aren't horribly expensive - that's a good thing because they have a limited life and eventually fail. I replaced one on the RV when it started to intermittently show either zero or off-the-scale high pressure. Really got my attention when it suddenly showed zero while I was over rather rugged terrain on a cross country flight...

I believe this is the VDO part for the fuel pressure transducer

http://www.vdo-instruments.com/sens...er-80-psi-floating-ground-1-8-27npt-32-1.html

80 PSI, 10-180 ohm with ground terminal.

We had 2 VDO sensors go bad. First one after 200 Hobbs. Second one immediately after install. Third one working fine with 20-30 Hobbs. I took the first one apart. The wiper had worn through most of the resistive coiled wire at one location. Probably from the wiper vibrating around the constant fuel pressure point.

The symptoms on the first one was the pressure would go max because the wiper would go past the worn through resistive wire which would then be an open circuit to the EFIS. The second one was just intermittent. It would be fine around 25-27 psi, then would creep up to 40. Then back down.

Apparently there is another type of sensor that is recommended. We have not gone that route yet.

Good luck.

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.com/Forum2/showthread.php?1005-Fuel-Pressure-Sender-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 ).

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