Fuel pressure, sensor

H. Paul Shuch is a Light Sport Repairman with Maintenance ratings for airplanes, gliders, weight shift control, and powered parachutes, as well as an independent Rotax Maintenance Technician at the Heavy Maintenance level. He holds a PhD in Air Transportation Engineering from the University of California, and serves as Director of Maintenance for AvSport of Lock Haven.

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BrianL99
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Fuel pressure, sensor

Postby BrianL99 » Sat Sep 12, 2015 8:01 pm

I'm having intermittent problems with my fuel pressure ...

My TruTrak EFIS had some anomalies that would result in my EMS #'s blanking out. TruTrak did a "tune up" on my EFIS and that seemed to cure the issues.

Yesterday and today, the fuel pressure reading on my EMS, blanked out a few times. The engine seemed to be running fine. I clicked on the auxiliary fuel pump and the numbers went back up ... I then shut the auxiliary pump off and all was well again.

I've heard there have been numerous problems with Rotax Fuel Pumps?

Can the engine run very long without fuel pressure? I'm fairly certainly it's an electronic glitch, but maybe not?

Can I add another, mechanical fuel pressure gauge?

Thoughts?

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drseti
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Re: Fuel pressure, sensor

Postby drseti » Sat Sep 12, 2015 9:01 pm

Rotax has had service bulletins against fuel pumps on several occasions. They have used three kinds: AC which was replaced by Pierburg which was replaced with Corona. Find out which kind you have, check it's serial number against the SBs, and see if you need to replace it. The current Corona is available for $106 from Lockwood, and is easy to change. If yours is listed in the most recent SB, you may still be able to get a refund from Rotax when you return your defective one.

No, the engine will not run more than a minute or so with zero file pressure. Once you've burned all the fuel in the carb float bowls, it quits. You can test this. Start the engine, and run it up to 4000 RPM (on the ground!) with the fuel selector valve turned off. Time how long the engine runs (and please report your result back here).

A mechanical fuel pressure gauge can tell you if your system is lying to you. But, if your plane is an SLSA, you need a Letter of Authorization from the manufacturer to install one.
The opinions posted are those of one CFI, and do not necessarily represent the FAA or its lawyers.
Prof. H. Paul Shuch, Ph.D., CFII, LSRM-A/GL/WS/PPC, iRMT
AvSport of Lock Haven
fly@AvSport.org
http://AvSport.org
http://facebook.com/SportFlying

BrianL99
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Re: Fuel pressure, sensor

Postby BrianL99 » Sun Sep 13, 2015 4:16 am

Thanks Paul.

The original owner told me that he hated the fuel pump SB and he went back to his original. When I bought it and SportAir did the annual, they went back to "current" one, which I'll check tomorrow.

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dstclair
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Re: Fuel pressure, sensor

Postby dstclair » Sun Sep 13, 2015 4:32 am

You may indeed have a bad pump or it may be operating normally for the Sting setup. What does "blanking out" mean? Exactly what are pressures you are seeing?

Sportair installed the 'new' pump on my plane a couple years ago and I would occasionally see the pressure drift lower -- usually during climb but not always. We investigated every possible cause and could not isolate. Turns this is accepted operation of the new fuel pump. Summary I posted on another forum:

Thought I'd update the thread and keep in mind I have a low wing. We put the new fuel pump on and it seemed fine for a few short flights. I also have cycled through the MoGas in the tank and have run 100ll for a bit over a tankful. I still occasional get a slow decrease in fuel pressure on climbs typically starting at 4.5 psi and dropping to as low as 2.4. If I turn on the Aux pump, I'm never below 3.5. Once in level flight the fuel pressure slowly decreased but always stayed in the green and eventually worked its way back to the mid 3s. I'm beginning to think this is just how the new pump works for my installation. From the SB:

"Low fuel pressure indications are possible and allowed. But the pressure must stabilize to the operating limit within 5 seconds. If not, the cause should be determined and rectified.
Due to the technical design and installation conditions (construction of the return line, etc.) pres- sure fluctuations, at the fuel pump part no. 893110 are possible. These pressure fluctuations within the specified operating limits are not considered to a problem."
dave

BrianL99
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Re: Fuel pressure, sensor

Postby BrianL99 » Sun Sep 13, 2015 5:41 am

It shows " ____ " fuel pressure".

Reading what you wrote David, I wouldn't be surprised if that's what's happening and my EFIS or sending unit isn't sensitive enough to show Fuel Pressure below 1.5/2 ? Is that possible?

roger lee
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Re: Fuel pressure, sensor

Postby roger lee » Sun Sep 13, 2015 8:44 am

Hi Brian,

This isn't the fuel pump. None of the 3 pumps that have been used by Rotax goes to zero and all the digital fuel read outs I know of will go down to at least 1 psi or lower. My Dynon will read in tenths. This isn't a symptom of a bad pump. You have a wiring issue. Could be a poor ground connection or just a loose signal wire.

I did a research project 3 years ago with a FD CT high wing plane on total loss of a fuel pump. By the way because of the design of all 3 of the fuel pumps if they truly failed they fail open because of the type of internal check valve which takes a pressure and flow change to close it and when there is no pressure and flow change it remains open.

Here is what I did for the project.

I removed both fuel hoses off the fuel pump and put a piece of tubing inline joining them together. This then was just head pressure from the wings which is approx. 2 ft. I ran the engine for 15 minutes at full power this way without a hickup.
I then removed the pump off the engine and hooked the hoses back up. I put a blind on the open fuel pump hole on the engine. Now the pump was inline, but would not work at all since it was dangling off the engine. So now the fuel had to flow through a dead pump. Now I ran the engine. I ran it at full power and after a few moments it started to stutter. I reduced throttle and was able to maintain up to 5000 rpm with a completely non-functioning pump. That's enough to fly where you would want to go. A low wing would be a different story, but most low wings have electric aux pumps.
Even though the minimum fuel pressure in the manual is 2.2 psi the engine will operate on less. I have seen 1.5 psi without any issues. The max pressure is 7.2 psi. The old max was 5.8 psi which should be changed on everyone's instruments that use the newer corona pump.

Fuel pumps don't usually quit working they usually start leaking and the owner or mechanic finds it. A leaking pump will run for many hours and you wouldn't know the difference in the cockpit.


You need to check your wiring from the fuel sender to the instrument. Tighten any grounds along the way. Wire connections are considered hand tight, but wrench loose until proven otherwise.



The Pierberg pumps were good pumps, but the company decided they didn't want their pumps on aircraft any longer. Then Rotax got the AC (stands for aircraft) pump, but had some issues with quality control over the years it was in service. That lead up to the use of the Italian Corona fuel pump. The first year they had some over pressure issues. From what I heard from Rotax was because diaphragms were too tight. Now they should be fine. They do tend to have higher pressures over the last two pumps. An electric pump when used should only raise the pressure .5 - 1 psi more.
Roger Lee
Tucson, Az.
LSRM-A, Rotax Instructor & Rotax IRC
(520) 574-1080 (Home) Try Home First.
(520) 349-7056 (Cell)

BrianL99
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Re: Fuel pressure, sensor

Postby BrianL99 » Sun Sep 13, 2015 12:11 pm

Thank you Roger, we're lucky to have you here!

roger lee
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Re: Fuel pressure, sensor

Postby roger lee » Sun Sep 13, 2015 12:27 pm

Reposted from the Rotax forum.

This is my own 2 cents and has nothing to do with Rotax.

Just food for thought.

I have found intermittent low pressure issues over the years with the fuel pressure senders. The senders that tend to hang down so the orifice is pointing upward and below the carbs seem to have these issues. If you use auto fuel the fuel sits in the hose for long periods, goes through many heating and cooling cycles. After about a year I find some of these have that small pressure sender orifice starting to plug from the old stale auto fuel. I had this issue the first year I owned my plane with intermittent low pressure alarms. I did the following and re-located my sender high up on the firewall. That was 7 years ago and I have never had an issue since and still have the same sender. No more killing vibration or stale fuel.

You have 3 choices. One replace the sender (last choice for me, but the sender could always be bad). Two, remove it and take a can of aerosol carb spray with the long plastic nozzle and hold it up against the sender orifice and give it 4-5 quick short blast of carb cleaner. Keep you face out of the way. :silly:
I have tried this and many engines and so far has worked 100% of the time.

The last thing you can do to help yourself after one of the previous two things is re-locate the fuel pressure sender so the orifice points down or at least horizonta to the engine and locate it high, equal to or higher than the carb balance tube.
This will allow the fuel to drain away from the sender orifice when the plane is sitting in the hangar overnight or for longer periods. When you start back up the fuel pressures right back up. All that I have done this mod to has never again had an issue and the sender's last for years.
Roger Lee
Tucson, Az.
LSRM-A, Rotax Instructor & Rotax IRC
(520) 574-1080 (Home) Try Home First.
(520) 349-7056 (Cell)

Wm.Ince
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Re: Fuel pressure, sensor

Postby Wm.Ince » Sun Sep 13, 2015 1:28 pm

Thanks Roger, for all that information. Excellent.
Bill Ince
CTSW
Retired Heavy Equipment Operator

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designrs
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Re: Fuel pressure, sensor

Postby designrs » Mon Sep 14, 2015 8:24 am

Condition: Low Fuel Pressure, usually when returning to full or cruise power after a lower throttle setting.

I had my mechanic replace the VDO fuel pressure sender with a new VDO of the same.
Taxi is 4.x psi. Normal cruise is 3.x psi.
Still have low fuel pressure warnings. 2.2.. 2.1... even 1.8 and 1.7

It has something to do with:
1) Cooler temperature (say hitting the coast after flying inland)
2) Backing off the throttle for descent or slower flying... then when cruse or full power is applied, the fuel pressure drops. (most common)

Then, if I back off to partial power 4,500 or 3,500 RPMs fuel pressure will climb back in the 3.x range.
Then edging the throttle forward causes fuel pressure to drop.
After a while it stabilizes and I'm cruising along as normal at 3.x psi.

I have a newer style fuel pump installed last December.
Plane runs fine with the electric fuel pump on or off... although I do switch the electric pump on after low fuel pressure occurs. The electric pump adds about 0.2 psi.

Originally, I thought it was an ethanol MOGAS issue, but I've had the same thing happen when running 100% 100LL.

Don't think it's wiring, as clearly the issue is predictably RPM related.

Suggestions?
Thanks!

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Re: Fuel pressure, sensor

Postby Nomore767 » Mon Sep 14, 2015 10:24 am

I've had something similar with my 912ULS.

The RV-12 has an electric fuel pump running all the time. Seems when fuel FLOW is high PRESSURE will drop down and as fuel FLOW decreases, say in climb, or evening in cruise, fuel PRESSURE will rise.

Occaisionally, in colder weather after take off I've seen the fuel PRESSURE drop into the low caution range and then go back to normal.
Folks on the VAF site had noted similar things and I have to say I've never had anything wrong with the way the engine runs.

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drseti
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Re: Fuel pressure, sensor

Postby drseti » Mon Sep 14, 2015 12:16 pm

Howard, what you describe is a perfect example of Daniel Bernouli's principle. :)
The opinions posted are those of one CFI, and do not necessarily represent the FAA or its lawyers.
Prof. H. Paul Shuch, Ph.D., CFII, LSRM-A/GL/WS/PPC, iRMT
AvSport of Lock Haven
fly@AvSport.org
http://AvSport.org
http://facebook.com/SportFlying

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designrs
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Re: Fuel pressure, sensor

Postby designrs » Mon Sep 14, 2015 1:23 pm

OK. The weird thing is that is never happens on initial climb out. But guaranteed, do a touch & go or low pass (always in hot weather in FL) and I'll get low fuel pressure on climb out. Possibly below 1.9. Not an easy thing to ignore! What should I do?
- Richard

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drseti
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Re: Fuel pressure, sensor

Postby drseti » Mon Sep 14, 2015 1:48 pm

designrs wrote:What should I do?


Climb out with your aux (electric) fuel boost pump on, at least until you get to what you consider a safe re-start altitude, should the engine actually quit.
The opinions posted are those of one CFI, and do not necessarily represent the FAA or its lawyers.
Prof. H. Paul Shuch, Ph.D., CFII, LSRM-A/GL/WS/PPC, iRMT
AvSport of Lock Haven
fly@AvSport.org
http://AvSport.org
http://facebook.com/SportFlying

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designrs
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Re: Fuel pressure, sensor

Postby designrs » Mon Sep 14, 2015 1:59 pm

Yeah, but if it's vapor restriction or a bad clamp / seal on a fuel line then I suppose it doesn't matter how many fuel pumps you have pumping. Pushing it at a few hundred feet AGL is not fun. Thinking I need a solution to troubleshoot and resolve the problem.


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