Flucuating fuel pressure indication on IO-360-A series engine

I've got a Lycoming IO-360-A1B6. The fuel pressure sender is between the engine driven fuel pump and the Bendix fuel injection servo. The fuel flow sender is between the electric boost pump and the engine driven fuel pump.

Historically, the fuel pressure has been in the high 20s. Two flights ago I got an alarm from my Grand Rapids EIS 4000 engine monitor as the fuel pressure indication exceeded 45 psi. It slowly drifted up to 50 psi. There was no change in the fuel flow or any other engine indications. I tried turning the boost pump on - the fuel pressure indication would decrease about five psi, then slowly increase back to 50 psi. Then after several boost pump OFF-ON cycles, the fuel pressure slowly decreased to the high 20s and stayed there for 30 minutes or so. Then, it slowly increased to 55 psi, and stayed there for about 10 minutes despite multiple boost pump OFF-ON cycles. I tried changing fuel flow with the mixture and rpm changes, but they had no effect on the fuel pressure. Then, for no obvious reason, the fuel pressure decreased back to the high 20s. On the next flight, the fuel pressure slowly varied between 23 and 36 psi.

Can anyone explain the mechanism that regulates the pressure output from the Lycoming fuel pump on a fuel injected engine? Are there failure modes that could cause the fuel pressure to vary above and below the normal range?

If the fuel pressure really is high, is there any risk of flying with it that way? Could the fuel injection servo be unable to function properly if the input fuel pressure is too high?

I suspect that this is more likely an indication issue than a real fuel pressure issue. I can't imagine that it is a bad ground, as that shouldn't lead to a fluctuating reading. But, perhaps the sender is failing. I'm tempted to replace the VDO 360 003 fuel pressure on spec.

Comments and advice are appreciated.

Kevin Horton said:

I suspect that this is more likely an indication issue than a real fuel pressure issue. I can't imagine that it is a bad ground, as that shouldn't lead to a fluctuating reading. But, perhaps the sender is failing. I'm tempted to replace the VDO 360 003 fuel pressure on spec.

Comments and advice are appreciated.

Click to expand...

I would definitely do this first.


Yes, the ground can do many strange things.

Kevin Horton said:

Can anyone explain the mechanism that regulates the pressure output from the Lycoming fuel pump on a fuel injected engine? Are there failure modes that could cause the fuel pressure to vary above and below the normal range?

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The spring in the mechanical fuel pump sets the pressure; since the spring can?t likely get stronger you probably have an indication error.

I have had a VDO fuel pressure sensor fail in my RV. It first indicated too high pressure then it went to completely off scale. I replaced the sender and it showed the correct pressure. I then cut the old one open and found that the sender works by sliding a wiper along a bar coiled with fine resistance wire. Because the wiper tends to sit in the same place most of the time and it fluctuates slightly, the wiper tends to wear through the wire over time. When this occurs continuity is broken. Then, the reading will be erratic and might be correct until the engine is started or the electric fuel pump is on - causing the reading to jump to a very high pressure.

This was after about 100 hours on the sender. I postulate that some device that could even the pressure pulses seen by the sender could greatly extend its life. A simple air reservoir - similar to those that are used to eliminate "water hammer" in housing plumbing - would probably work for this.

The earlier comment about how the mechanical fuel pump works is correct - the pressure is determined by the strength of the spring inside the pump. For this reason it's practically impossible that your fuel pressure is actually as high as the sender is indicating.

Lets think about the schematic for fuel pressure on the EIS.

You have 4.8v coming from a regulated supply through a 301 ohm resistor "tee"ed into the signal line that stretches from the EIS AUx port you selected to the VDO sensor. The gage case and threaded fitting are the connection to ground.

The EIS is "forward acting" on this setup, so increased voltage at the AUX port will cause higher indications.

There are a few ways for the voltage to go up at the aux port. All of them have the common fault of preventing electricity from passing to ground from the 301 ohm connection point.

1) Bad ground. Hose clamp a dedicated ground to the sensor case to check this. It is low on my list since you have been functional already.

2) A degraded harness connection somewhere between the VDO sensor and the "tee" junction of the 301 ohm resistor.

3) Bad sensor. They are 0-180 ohm if you care to check it by applying 0-5 bar pressure and testing with multimeter. The point in the previous post about the potentiometer wiper wearing a bad spot in the coil is definitely a valid failure mechanism.

I had a similar experience with my fuel pressure indicator a couple years ago. Mostly I was seeing intermittent extreme spikes in the pressure that were clearly not possible. I tried messing with the ground, concerned that there was perhaps not good continuity from the sender body to the airframe. I even soldered a ground wire onto the body. Seemed to work temporarily, but not too many hours later saw spikes again. Replaced the sender and all is normal the last 150 hours, typically ~21 psi for my IO-360.

Greg

Thanks for all the info guys. It is comforting to know that this has to be an indication issue. I'll check the list of things provided by Bill Pendergrass.

I've seen quite a few failure reports on VDO senders. Can anyone recommend another brand sender that is more reliable but physically and electrically compatible with the EIS 4000? I've got fuel injection, so I need to measure pressures up to 45 psi or so at full scale.

Kevin,

Unlike you to be stuck for answers

Funny you mention this, Jamie Lee had a faulty sensor for his O-360 recently. It also faulted high by about a factor of 2. In his case 3-4 PSI to 8+

new sensor installed yesterday and I test flew it after 100hrly......All was good again.

Funny thing is we have 600 hours on ours (IO540) and so far so good!

Cheers

David

Kevin,

I have 41 hrs on my EIS6000 and GRT supplied fp sender. A couple of hours back mine went from the normal 26-28 psi down to 18-20 just after a hot startup. I turned boost pump on for 15 sec and back off, then all was normal. Don't know if it was vapor issue or sender.

I do know that GRT sells a better quality fp sender.

Kevin Horton said:

Thanks for all the info guys. It is comforting to know that this has to be an indication issue. I'll check the list of things provided by Bill Pendergrass.

I've seen quite a few failure reports on VDO senders. Can anyone recommend another brand sender that is more reliable but physically and electrically compatible with the EIS 4000? I've got fuel injection, so I need to measure pressures up to 45 psi or so at full scale.

Click to expand...

Kevin:

GRT has a high dollar replacement for the VDO Fuel Pressure Sender. You will have to "Y" off one of the 5 volt lines to supply the voltage to the sender, in a manor similar to some of the other GRT sensors that need the 5 volt supply. Call Sandy or Carlos. About $130 IIRC a couple of years ago.

I did some detective work from GRT's part number and came up with a Digi-Key part number and saved a couple of dollars. Probably wasn't worth the effort tho.

RV10inOz said:

Kevin,

Unlike you to be stuck for answers

Click to expand...

One of the keys to success is recognizing that while there may be some subjects in which you know quite a bit, there are many other subjects where you know very little. You've got to be able to tell the difference between those two areas, and be sure to ask lots of questions when you are in your weak areas.

mannanj said:

Kevin:

GRT has a high dollar replacement for the VDO Fuel Pressure Sender. You will have to "Y" off one of the 5 volt lines to supply the voltage to the sender, in a manor similar to some of the other GRT sensors that need the 5 volt supply. Call Sandy or Carlos. About $130 IIRC a couple of years ago.

Click to expand...

Thanks for the info. I'll contact GRT and ask about the better quality sender once I rule out any electrical gremlins.

Kevin Horton said:

One of the keys to success is recognizing that while there may be some subjects in which you know quite a bit, there are many other subjects where you know very little. You've got to be able to tell the difference between those two areas, and be sure to ask lots of questions when you are in your weak areas.


Thanks for the info. I'll contact GRT and ask about the better quality sender once I rule out any electrical gremlins.

Click to expand...

Kevin:

I was wrong about the 5 Volt tap for the sensor. After checking the install info this afternoon, it is the regulated 12 Volt supply for the fuel flow sensor that you need to tap for the new higher quality sensor.

I had to replace 3 engine sensors on my AFS EFIS during my first year flying...the same erratic indications. Check sensors and grounds first.