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High Fuel Pressure Indication

jjconstant

Well Known Member
Hi all

I'm in the latter stages of Phase 1 and every now and then I get a high fuel pressure reading, using just the mechanical fuel pump. Sometimes on the ground right after starting, sometimes in the runup area, sometimes in flight.

I have the standard Van's vertical fuel injection and I set my high fuel pressure alarms at 36 psi. My normal indications are 27 to 31 psi. The first few times, I taxied back to the hangar, decowled, futzed around with the sender wires, opened the fitting to vent to atmosphere, etc.

My understanding is that the diaphragm in the mechanical fuel pump is only capable of either generating the appropriate fuel pressure or rupturing and generating LOW pressure, not high. Is that a reasonably accurate understanding?

What are some things that could generate actually HIGH fuel pressure with the mech pump only? Cloged or blocked lines/injectors upstream of the pump?

Or, is a high fuel pressure alarm actually a) useful and b) telling me anything other than my sender is unreliable?

Thanks for your time

Jeremy
RV7A Phase 1
 
Jeremy I also experience intermittent high fuel pressure readings. Beleive it or not, in my case they are caused by RF input to the system (EIS 4000). I improved my antenna efficiency and eliminated some feedback, but it still occurs like clockwork. Key the radio to transmitt and the fuel pressure starts a steady climb up. That antenna is on the bottom of the aircraft justahead of the spar.

Hope that might be it. Good luck
 
I just had my fuel flow transducer fail and before that it would show high readings intermittently. Failure mode showed 200 psi which a few on this forum said was not possible and they were right. This sensor shows high readings when it's ground fails, and the ground is the case itself and grounds to the manifold when it is screwed in., which in my opinion is real poor design. But that way they eliminate a wire and/or save money.

That said, I have an identical one coming from the good folks at GRT.


.
 
Bad Ground

If the case is not well grounded it will show a high pressure reading. Same with oil pressure. This is usually not a problem if the VDO sender is in the Vans's aluminum manifold, but I did not use them. I ended up soldering a ground wire to the case.
 
A bit more data

I've verified that it is not associated with transmitting on the radio. I've checked with a tee'd-in second indicator--within 1 psi. It is going into Van's aluminum manifold. I tried adding a band clamp with a redundant ground--no change.

I just checked with Mahlon Russel at Mattituck and he suggested checking that the breather tube was not blocked in any way because that would cause high fuel pressure. In the same email he mentioned that my "normal" readings of 28-31, while within spec, were already on the high side of what they consider normal, which is about 26psi at idle and between 20 and 24 at full power. He is concerned that at the redline of 45psi (remember I have transients up to 40 indicated) the engine may quit. He is recommending a new (mechanical) fuel pump. I verified that the breather was not blocked, disconnected it from the air/oil separator and ran it down toward the exhaust air exit area. No change.

I will be re-verifying the fuel pressure sender with another pressure indicator, but in the meantime what fuel pressures are people typically seeing with their fuel injected mechanical fuel pumps? If the fuel pressure reality check with another indicator verifys the readings are good, I'll go with Mahlon's recommendation of a new fuel pump:( I'm not interested in flirting with engine out possibilities if I can avoid it!


Jeremy
 
I have exactly the same problem - intermittent high pressure indications using mechanical pump, but flip on the electrical pump (AFP) and it stabilises at 27psi (using Dynon supplied fuel pressure sender and Dynon D180). I got a new sender - same problem. I took the manifold fitting off the firewall, cleaned up the back of it and the firewall itself, took the sender off, cleaned up around the screw fitting, reassembled and for good measure added a wire from one of the bolts holding the manifold in place to the forest of tabs I use for my earth connections - surely that must be adequately grounded - but the problem is still there. My plan now is to move the earth wire from the manifold to the sender itself and secure it using a jubilee clip - should ensure the body of the sender is properly grounded.

Have you checked the sender using some form of calibrated pressure system so you can see if the sender is sensing correctly? Have you used the same kit to ensure that eh pressure from the fuel pump at idle is what the fuel pressure readout is telling you?

It is my understanding that there is NO WAY for the EDP to generate such high pressures - and I certainly would't be rushing to change the pump without very clear indications that there is indeed a problem with the pump and to my mind you don't have that yet.

There are several similar threads on the site already.

Hope that helps? Good luck and let us know the answer

Chris
 
I am having the same issues. I do not use a manifold but clamp the sender to the engine mount using an adel clamp. I run a separate ground wire to the unit and use a hose clamp to hold the ground to the body of the sender.
This worked for 350 hours with the first sender and 8 hours with the replacement sender. When it first gave a high pressure reading turning on the boost pump would bring the pressure reading back in line. After an hour or so this would not work either. I just removed the ground wire clamp and sodered the wire directly to the case of the sender. A ground check shows that the unit still functions, however my high pressure readings only show up after 20 or thirty minutes into a flight. I will have to wait for a test flight to confirm my current set up. I would like to source another sender that has a built in ground tab. I am using the AFS 3500 system. Other then this annoying glitch it is a great unit.
 
troubleshooting

Chris: I don't know where I would find some kind of calibrated pressure system, but I will be checking again with an analogue pressure gauge...maybe two or three. If they all say the same, and it's within 1 or 2 psi of my electronic indicator, that's good enough for me. Thanks for letting me know that you're at around 27 psi. It's a helpful data point. Mine doesn't change with boost pump on or off. It only changes with time...maybe 5 to 10 seconds.

Tom: I'll try soldering on a separate ground wire instead of the band clamp and see if that does anything. BTW, I grew up close to where you are in Kitchener-Waterloo. Based on the weather down here in California this year you guys should still be under 6 feet of snow! A friend here wanted to drive to his cabin in Montana, but had to wait out a blizzard going accross Nevada...in May!

Back to the problem. I know that Grand Rapids now has a "better" quality sender, but I believe it has 3 wires instead of 1, so I have to do more investigating before I go that route. It may be a less expensive and time consuming next step before a new fuel pump!

Thanks for the ideas and data points...keep 'em coming.

Jeremy
 
High fuel pressure

It seems this is a common problem. I have intermittant high fuel pressure as well. I have an XP-360 fuel injected engine. I replaced the fuel pump to no avail. The problem goes away if the electric pump is turned on but then comes back. It does not coincide with any voice transmission. I think the only way to solve this is to temporaily install an analog fuel pressure gauge. When the pressure goes high, if the analog gauge goes high then there is a problem. If the analog gauge stays normal then it must be something electrical.
 
We have the same, but overall I am not concerned. Seems to me the majority of "high this / low that" problems are transducer / instrument problems. The Fuel Pressure on the Dynon "jumps" about, meaning about the correct ~25psi, but sometimes very high. As above, impossibly high, given the Mech Fuel Pump design.

Only "advice" I might offer is to be cautious when setting up Dynon warnings etc.? Just because you can set high and low alarm values for every reading under the sun, does not mean it is sensible. The test phase should be used to watch readings, see which are good, which curious, and set audio alarms for those which are truly urgent, visual alarms for the less urgent, and just colour changes for the trivial :)

I wonder if this Fuel Pressure issue is the RV layout of a long pipe, presumably largely air/vapour, leading up from the pump to the transducer? Do the pump pulses set up a harmonic in the pipe? Bearning in mind the mech pump + elec pump do manage to pulse sufficiently to make a fuel flow sensor between the 2 to overread considerably...

Andy
 
I've verified that it is not associated with transmitting on the radio. I've checked with a tee'd-in second indicator--within 1 psi. It is going into Van's aluminum manifold. I tried adding a band clamp with a redundant ground--no change.

I just checked with Mahlon Russel at Mattituck and he suggested checking that the breather tube was not blocked in any way because that would cause high fuel pressure. In the same email he mentioned that my "normal" readings of 28-31, while within spec, were already on the high side of what they consider normal, which is about 26psi at idle and between 20 and 24 at full power. He is concerned that at the redline of 45psi (remember I have transients up to 40 indicated) the engine may quit. He is recommending a new (mechanical) fuel pump. I verified that the breather was not blocked, disconnected it from the air/oil separator and ran it down toward the exhaust air exit area. No change.

I will be re-verifying the fuel pressure sender with another pressure indicator, but in the meantime what fuel pressures are people typically seeing with their fuel injected mechanical fuel pumps? If the fuel pressure reality check with another indicator verifys the readings are good, I'll go with Mahlon's recommendation of a new fuel pump:( I'm not interested in flirting with engine out possibilities if I can avoid it!


Jeremy

Hi Jeremy,

Just wondering if you resolved your high fuel pressure issue.
Did changing out the mechanical fuel pump resolve the problem ?
 
Confessions:

I have not changed out the mechanical fuel pump...I changed the alarm limits. I keep a wary eye on it, but nothing has stumbled or burped and it remains variable between 26 and 38 and has never gotten too close to the spec limit. I have come to the opinion that it is an indicator "feature" or an installation characteristic, though I don't know if that opinion is denial or simply gathering empirical data. I have the better fuel pressure sender which I'll install at annual. If that doesn't work, I'll start considering a new fuel pump.

Jeremy
 
High Fuel pressure

I see about 31 to 33 psi on my airplane. I have verified the sender is accurate by comparing it to a mechanical gauge that I tee'd in together and they are both within 1 psi of each others reading. When I add the boost pump, the pressure rises up to 36 psi.

My mechanical fuel pump was also weeping oil from the two upper gaskets so on the recommendation of Mahlon from Mattituck, he sent me a new mechanical fuel pump to swap out. I just installed it but have yet to fly it as it was raining hard and we even saw some hail as I was finishing the installation of the new pump. I will see tomorrow how the new pump works out for me and report back how it went.
 
I had similar issues with my pressures. Most of the time they would stay around 22-23 psi but intermittently spike to off-scale. Sometimes turning on the electric pump would stabilize them, but after a while that quit working. I tried the grounding business, and although it seemed to work initially, it never really fixed the problem completely. I finally decided to replace the sender unit and have had no pressure issues in the 25 hours since doing so.

greg
 
Don't know if it'll make you feel any better, but I've struggled with high FP problems as well. Dynon D-100 & 120. I'll get a high FP reading when I first start the engine, and while taxiing to the runway. By the time I'm in the air, it settles down to within normal limits (25-30 psi) and runs fine. I've learned to live with it as a minor annoyance. In flight, while it doesn't go high, it does fluctuate constantly, not unlike my MAP used to before I installed a filter to attenuate the vacuum pulses.
 
The sensor may be the problem

I get occasional high FP readings as well with my GRT EIS. I spoke to Greg Toman (who started GRT) at OSH and he said it is probably the sensor. The $35 standard sensors have a wiper arm moved by a diaphram which moves across a resistor. Vibration and time cause wear which causes high readings. He said those sensors only have a life of a couple hundred hours. :eek:
He said the higher priced ($135) sensors are all electronic and have a much longer life.
 
If you have fuel injected and running airflow performance, then check your fuel bypass valve. It is located on top part of the fuel pump/filter assembly and it is about 2" long. It has an o-ring that can get stock and not open up. At least this was the case for me, I would get high pressure with no specific pattern, meaning I could be on the ground, in the run up area or in the air. Once I replaced the o-ring, the issue went away. My FP with just the mechanical pump stays between 23 and 25 with 90% of the time at 24 PSI. Let me know if you need more detail or how to locate it.
 
Faulty Mechanical Fuel Pump

I had exactly the same problem with the fuel pressure on my IO-320. It was fine for the first 30 hours and then all of a sudden the fuel pressure started to creep up after flying for a little while. It would go up to over 60 psi if I let it, but if I flipped on the electric pump and it would settle right back down to below 30 psi. I read the other posts regarding grounding but just couldn't comprehend how a bad ground would get fixed by turning on the electric fuel pump. Therefore I replaced the mechanical fuel pump and have flown another 10-15 hours with no problem at all. After replacing the pump I did notice some weeping around the lower gasket on the faulty pump. My theory was that perhaps something was sticking internally and by turing on the electric pump it would somehow bias the diapham in the pump so the pressure wouldn't climb. Just a guess but it was replaced under warranty by Aero Sport so that's the good part. By the way, I have the AFS EFIS so I would get verbal warning the instant the pressure got to high. I could see that if you only had a mechanical gauge it might be hard to catch.
 
I see about 31 to 33 psi on my airplane. I have verified the sender is accurate by comparing it to a mechanical gauge that I tee'd in together and they are both within 1 psi of each others reading. When I add the boost pump, the pressure rises up to 36 psi.

My mechanical fuel pump was also weeping oil from the two upper gaskets so on the recommendation of Mahlon from Mattituck, he sent me a new mechanical fuel pump to swap out. I just installed it but have yet to fly it as it was raining hard and we even saw some hail as I was finishing the installation of the new pump. I will see tomorrow how the new pump works out for me and report back how it went.

How'd this work out for you Dave?

Any new theories on how a high fuel pressure indication when operating on the mech pump only is brought down into the green by turning on the high pressure boost pump?
 
Just to close the circle on the issue with my plane, at the annual after this issue surfaced I changed to the 3 wire $ender unit and my readings are stable now at 25 psi +/- 3 with and without boost pump on. In my case, I'm confident that it was a sender issue.
 
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