Bad sensor???

Today, during my lesson, my plane and engine flew perfectly, but after landing at one airport, on takeoff I glanced at my Skyview and it showed fuel pressure REALLY low, like 1.9, bouncing around there, 1.9 to 2.1, but running fine, and 2 minutes after taking off it was back up to 5.6-5.8.

I've checked, and usually have 5.8 with both pumps running, 5.6 with the electric fuse pulled, and the electric pump alone (plane off) gives about 3.6lbs by itself.

I'm thinking maybe a bad sensor or bad connection to the sensor. This is the second flight after it's annual, and it decides to act up in a couple different ways.

Funny how that is, as I doubt the two issues I had today are in any way due to the annual or anything the mechanic did, heck, I was there and helping for the engine part of the annual!

I'm going to wiggle all the connections (first thing to always do, that, or hit it with a hammer) and hope to never see those low pressures again!

Anyone else experience anything like this?

I worked on a -12 that had a low fuel pressure issue and I seem to remember he had the banjo fitting for the fuel return line with a restrictor fitting wrong (I don't remember the details) and that caused a low reading, even though the pressure wasn't actually low. Sorry I don't remember the details.

Chrysopelea said:

Today, during my lesson, my plane and engine flew perfectly, but after landing at one airport, on takeoff I glanced at my Skyview and it showed fuel pressure REALLY low, like 1.9, bouncing around there, 1.9 to 2.1, but running fine, and 2 minutes after taking off it was back up to 5.6-5.8.

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Any chance you shut the engine down for some period of time after landing? I'm thinking you may have experienced vapor lock. I learned this lesson on my 12 and now I leave the master on to keep the electric fuel pump running if I anticipate a hot engine restart after sitting for 10 minutes or so.

My buddy and I have both experienced this, each in his own plane. I think Jim may have it right in regard to vapor lock. We both had landed, shut-down, then restart within 10-15 minutes and both had acceptable fuel pressure on restart. However, soon after lift-off, the pressure warning came on and our gauges jumped around just as you describe. At that point in the take-off phase, it was safer to continue and fly the pattern. After a short period of time, fuel pressure indications returned to normal.

It was actually one of the cooler days we've had a lesson, and we landed, and taxied around and took back off, never stopping the engine.

It does sound like a possible vapor lock, as the engine never skipped a beat, and pressures returned after a few minutes.

I'm torn between running both pumps all the time while in flight, or using the electric only during takeoffs and landings. It was explained to me that pulling the electric pump fuse during flight will extend the electric fuel pump's life, and also will let me know when one, or the other goes bad. With both running, the electric or mechanical pump could go bad without my knowing, and then if the mechanical one went south, it'd be coitans for me I say, coitans.

My plan is to make a nifty little fuse holder for the pump fuse so it has a place once removed.

At my work we have a high powered laser, and I'm going to laser engrave up a plaque with my plane info, and home airport frequencies, as I seem to have trouble remembering them. It's going to be super cool, made of anodized titanium, probably blue like my plane. If we have any thin zirconium, I could make it black chrome looking, and paint in the engraving with white, that would be cool too.

Chrysopelea said:

It was explained to me that pulling the electric pump fuse during flight will extend the electric fuel pump's life, and also will let me know when one, or the other goes bad. With both running, the electric or mechanical pump could go bad without my knowing, and then if the mechanical one went south, it'd be coitans for me I say, coitans.

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The pump is rated for continuous duty operation.
They have been run in RV-12's that way for many years now.

I am not clear on how it is important to know that one of the pumps has failed.

If this logic is valid, then we should fly around with only one ign. system active.
That might extend the life of the second one and then you would know if/when the first one failed.

The likelihood of both pumps failing on any given flight is quite remote. You will likely know if the engine driven pumps fails because the electric pump is unable to keep the system pressure as high. You will know that the electric pump has failed when you go to do an engine start on the next flight.

BTW, I think you have an S-LSA?
If so it is not legal for you to modify the aircraft systems (add a switch to turn the aux pump on and off) without manufacturer approval, or operate the systems in a manner contrary to the POH.

If you remove the Fuel Pump fuse to disable the electric fuel pump, you also disable the two electric fans that provide cooling to the avionics compartment -- the fuel pump and fans are on the same circuit.

The "full-time" electric fuel pump serves several purposes:

1) Being ON full-time, the pilot is unlikely to forget to use it.
2) The immediate availability of a full-time electric pump may prove timely in a sudden mechanical pump failure at low altitude.
3) The full-time electric pump assists in reducing and clearing fuel vapor formation.

You can verify how each pump is functioning (on the ground and in the air) by momentarily disabling the electric pump and noting the resultant fuel pressure indications. Also, be sure to listen for the clicking of the electric fuel pump as part of your "pre-start" and "shut-down" checklists.

Many ROTAX operators have reported in this forum low fuel pressure indications similar to yours on take-off and climb-out.

You have an SLSA I recall. You will need factory authorization to install any kind of switch on your electric fuel pump. I suspect that such an approval will not be forthcoming.

Thanks for the advice. I will rethink this act.

Having the fuel pressure drop on takeoff and climbout is unnerving I must say.

I have checked pressures with and without the electric pump running. 5.6 without and 5.8 with both pumps running, so I lose .2lbs of pressure running on just the mechanical pump.

More than one person has suggested and told me they remove the fuse and use their electric pump only during takeoffs and landings, but after what I've read here I think I'll leave it in.

Also, I was talking about removing the fuse, not modifying the plane and adding a switch, but after hearing that doing so disables other fans and such, I think I'll just fly it with both pumps on all the time and enjoy!

Thanks!

The underlying issue of occasional low fuel pressure readings on takeoff/climb out and sometimes on decent as well is something that many of us continue to scratch our heads at. Anecdotally, I've spoken to several RV-12 owners who admit to seeing this.

On climb out, my pressure readings fairly often drop into the yellow range for a minute or two and then recover. Engine performance never seems to be an issue and because of that I may be more complacent about these readings than I should be. Rarely today the Skyview audio warning calls out which really gets your passenger's attention. Occasionally on a long decent, particularly from a higher altitude (ie >6,500ft) similar low pressure readings in the yellow range will occur. It is unfortunately inconsistent between flights and I?ve not identified specific differences between the events to better figure this out. The throttle is reduced for these decents, but of course its full on climb out.

Last year there were times I would more often see erratic pressure readings all the way down to the "red range" and the audio Skyview warning would call out. I went ahead and replaced the pressure sensor and while it may have improved I would still see this problem from time to time.

Despite the fire sleeve insulation, at one point I was suspicious that the fuel line that comes off the bango bolt over to the pressure sensor on the firewall was too close to the radiator hose (I had them zip tied together) possibly leading to a warmer temperature in the fuel line affecting the measurement. Long after engine shutdown those hoses are still warm. Yes the line is well insulated, but I was searching for ideas. After discussing this with an A&P friend, he suggested adding a small a standoff and even some aluminum reflector tape to the line to eliminate this as a possible cause. Since then, while I still see these low readings from time to time, they ?may? be less common and I rarely drop into the red range any longer. I don't believe this is necessarily resolved, but I'm not seeing the very low readings that I once did.



Again, engine appears to operate flawlessly during these low readings, but I would really like to understand the root cause.

I have been reading these RV-12 forums since the beginning. Based on the experiences of other that have posted here, the most common reason for low fuel pressure is (in order of likelihood) :
1. Faulty fuel pressure sensor or its electrical connections.
2. Failing engine driven fuel pump. (most noticeable during takeoff)
3. Vapor lock.
I can not recall anyone posting about a failed electric fuel pump, although a few have had the electric pump get stuck for an unknown reason when first turned on. They fixed it with a good rap or tap.
Although I have installed a fuel pump switch in my E-LSA, I never turn it off during flight. My pre-startup checklist includes making sure that every electrical switch is turned on. It is easy to ensure that both fuel pumps are working. Turn on the master switch and observe the fuel pressure. Start the engine and look for an increase in fuel pressure. It will not be much.
A fuse should never be replaced in flight or when the master switch is on. Doing so will distract the pilot from flying the plane and looking for traffic. And there is a good possibility that the fuse will blow from accidentally touching the instrument panel.
The best advice that I can give is to listen to Scott's advice.

Insulating the fuel line going to the pressure sensor will have absolutely no affect on the pressure. The pressure in that fuel line will be equal from end to end, regardless of whether the fuel line contains liquid fuel or fuel vapor or even air. If there were a pressure differential, then that would cause flow which would equalize the pressure.

Vapor Locks are more likely with Mogas than with Avgas, furthermore after discussing this with a "refiner", with Mogas vapor locks are more likely with lower Octane fuels. For example 91 Octane is more susceptible than 93 Octane.
After further investigations I believe that the fuel pump failure I thought I had a few months ago on takeoff was likely a vapor lock, result of several factors:
- I had switched from 93 Octane Ethanol Free to 91 as the 93 Ethanol free had disappeared from Service Stations in my area
- I later found small leaks in my fuel line at the switch off vane and at the red cube that might have resulted in small amount of air sucked in
- ... and aggravating factor (entirely my fault) I did not switch the electric fuel pump before take-off.

The electric fuel pump seems to play an important role in avoiding vapor locks. In my research I found that some old classic cars are subject to vapor locks when operating in high temperature weather. A standard solution to solve this problem is to add an electric pump on the fuel line between the tank and the engine... just like in the RV-12! This fuel pump while helping when "on" could make things worse when "off" as it is introducing a small drag (negative pressure) on the fuel that's pumped by the mechanical fuel pump.

I have not researched the influence of Ethanol on vapor lock but I would not be surprised if ethanol was also a negative factor if it is more volatile than ethanol free mogas.

I would recommend that anyone plagued by suspected vapor locks start by switching to Avgas or to a different source of mogas. Once vapor lock is confirmed you can concentrate on improving the cooling of fuel lines and checking for small leaks. Last I have to admit that VANs strategy of leaving the electric fuel pump on all time was the right thing to do from a safety stand point (if not for a convenient maintenance stand point).

There is (was) a recall by Dynon on faulty fuel pressure sensors. Go to the Dynon website and look for it. My sensor was one that was recalled and once I had it replaced, I no longer have an low fuel pressure issues. Also, it's important to drain and flush this line during the condition inspection per the instructions.

Were you using mogas or 100LL?

I was using 91 octane premium (Chevron) in my plane when I experienced this .

I THINK it's non ethanol, but in CA, often there's 10% in gas at pumps..

I was told to use Chevron from a busy station by a number of people.

Chrysopelea said:

...pulling the electric pump fuse during flight will extend the electric fuel pump's life, ...

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You don?t want to remove the fuse to open a circuit. I expect the ATC style fuse holder is rated for less than 50 mating cycles. Looking at the bigger picture, the $50 electric fuel pump will likely outlast the airframe and is less expensive than the Van?s fuse module.

Some data points;
Female Fast-On connectors are rated for 10 mating cycles.
PCB card edge connectors are rated for 50-100 mating cycles
Sub-D connectors are rated for 250-500 mating cycles

I forgot to mention in my post to check your supplier of ethanol free Mogas, I found that mine (Sunoco) has a disclaimer about using his Ethanol Free gas in airplanes! I suspect that it is a way to protect themselves as most of their customers for this type of gas are boat and vintage classic cars owners. I do not know if this is true for all suppliers. What this may also mean is that they do not have the same control on the quality of this type of gas as for 100LL Avgas.
As a result I am now back using 93 octane mogas with ethanol.

Chrysopelea said:

I was using 91 octane premium (Chevron) in my plane when I experienced this .

I THINK it's non ethanol, but in CA, often there's 10% in gas at pumps..

I was told to use Chevron from a busy station by a number of people.

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I don't want to start a gasoline war here, and I may get flamed for this suggestion, but why don't you try burning 100LL for a few tanks to see if the incident reoccurs?

MMiller said:

You don?t want to remove the fuse to open a circuit. I expect the ATC style fuse holder is rated for less than 50 mating cycles. Looking at the bigger picture, the $50 electric fuel pump will likely outlast the airframe and is less expensive than the Van?s fuse module.

Some data points;
Female Fast-On connectors are rated for 10 mating cycles.
PCB card edge connectors are rated for 50-100 mating cycles
Sub-D connectors are rated for 250-500 mating cycles

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That is really interesting information, and good to know. Who does the ratings?

Interesting, but I would think low insertion force would tell you there is a problem. At that time I'd just use some needle nose pliers to squeeze the socket connectors and tighten the fit.

BigJohn said:

I don't want to start a gasoline war here, and I may get flamed for this suggestion, but why don't you try burning 100LL for a few tanks to see if the incident reoccurs?

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This was the first time it has happened that I've noticed. Since I'm pretty sure it's a sensor issue, I'm going to fly it a couple times to see if it happens again before doing anything.

I KNOW both pumps work, so I'm not worried about fuel pressure.

Vapour lock

BigJohn said:

I don't want to start a gasoline war here, and I may get flamed for this suggestion, but why don't you try burning 100LL for a few tanks to see if the incident reoccurs?

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As per Big John's suggestion, try a tank of at least 30% min avgas I suspect from my own experience you will most likely be pleasantly surprised.