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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bstrachan
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Re: Fuel pressure, sensor

Postby bstrachan » Sun Jan 24, 2016 11:45 am

FYI: I haven't had a low fuel pressure condition in my StingSport since late last summer. I switched to Chevron premium back then. After a couple of months I went back to Shell premium with their new mystery additive. Nothing changed in the airplane. The weather is cooler and the fuel is different, but I've had low fuel pressure indications in cool weather previously. So is it the fuel that was causing my problems? I don't see any new posts to this thread. Comments?

roger lee
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Re: Fuel pressure, sensor

Postby roger lee » Mon Jan 25, 2016 9:13 am

Fuel is rarely the issue even when people want to point fingers. Some vapor formation in a hose can produce low pressures especially after you land and the plane sits a while then you go to re-start. Bad senders which are absolutely notorious for being wrong causes more issues than anything else. The second thing that causes erroneous readings are poor grounds that have become corroded and or loose. I see the grounding issue and bad senders 95% of the time. Some engines under certain types of tight cowling and especially with bare fuel hose versus fire sleeve hose can develop vapor in flight and this is a huge reason to make sure your plane has the recirculation line setup.
Roger Lee
Tucson, Az.
LSRM-A, Rotax Instructor & Rotax IRC
(520) 574-1080 (Home) Try Home First.
(520) 349-7056 (Cell)

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bstrachan
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Re: Fuel pressure, sensor

Postby bstrachan » Mon Jan 25, 2016 10:01 am

Roger, I believe you are 100% correct about fuel pressure senders, etc. But do these things fix themselves? I haven't touched anything in the airplane since before the problem went away. I did change out my FP sender but that was a long time ago.

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designrs
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Re: Fuel pressure, sensor

Postby designrs » Mon Jan 25, 2016 10:20 am

UPDATE:
OK. Here's the update on my fuel pressure issue.
I upgraded the fuel system to the new SportCruiser fuel system.

Major changes include:
1) check valve after the Gascolator, on the bypass around the electric fuel pump
2) elimination of a redundant (parallel) fuel line from the electric pump to the old spider fitting
3) elimination of the spider fitting
4) new fuel block (banjo assembly) from Rotax

Noted Improvements & Observations:
1) highly effective electric pump operation
2) no pressure ossilation when the electric pump is switched on
3) lower fuel pressures can occur in temperatures above 83 degrees F, however those temporary conditions are now quickly remedied by switching on the electric pump.

* In the previous fuel system setup, the electric pump did not remedy the problem, nor prevent low pressure from occurring on final or climb out. The new system does.

Side notes re: new Rotax fuel block (banjo assembly):

1) manufacturers may or may not follow the exact Rotax illustrations in the installation guide
2) restrictor for return line is within the top bolt of the banjo assembly (labeled FUEL). This is the restrictor option that I observed from SportCruiser.
OR
in the double hose banjo fitting (restrictor is either used for return line, or removed if the fitting is used for other connection purposes)

** I am not a certified mechanic or authority. Check and verify information with authorized experts.
- Richard

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designrs
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Re: Fuel pressure, sensor

Postby designrs » Mon Jan 25, 2016 10:24 am

bstrachan wrote:Roger, I believe you are 100% correct about fuel pressure senders, etc. But do these things fix themselves? I haven't touched anything in the airplane since before the problem went away. I did change out my FP sender but that was a long time ago.


Low fuel pressures seem to be an issue with Rotax, especially above 83 degrees F, MOGAS or 100LL. Management of the condition often depends upon the specific fuel system installation.

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designrs
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Re: Fuel pressure, sensor

Postby designrs » Mon Jan 25, 2016 12:16 pm

Those interested can find specific SportCruiser fuel system photos and diagrams here:
http://www.scflier.com/index.php?/topic ... rm/?p=1806

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bstrachan
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Re: Fuel pressure, sensor

Postby bstrachan » Mon Mar 28, 2016 11:24 am

I haven't seen any fuel pressure squawks here for a long time. Did the problem go away all over the country? A nationwide change in fuel formulation is the only thing I can think of that would explain that.

I haven't had a low fuel press warning light for months.

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designrs
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Re: Fuel pressure, sensor

Postby designrs » Mon Mar 28, 2016 8:53 pm

Glad your issue finally resolved itself. My issue was occurring with both MOGAS and 100LL. Issue resolved by updating the fuel system, with the primary components being a check valve, hose rerouting, and new banjo fuel block from Rotax. Results are more stable pressures and a highly effective application of the electric boost pump.
- Richard

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bstrachan
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Re: Fuel pressure, sensor

Postby bstrachan » Mon Mar 28, 2016 10:34 pm

designrs wrote:Glad your issue finally resolved itself. My issue was occurring with both MOGAS and 100LL. Issue resolved by updating the fuel system, with the primary components being a check valve, hose rerouting, and new banjo fuel block from Rotax. Results are more stable pressures and a highly effective application of the electric boost pump.


I'm still wondering what is going on here. There were many complaints on this forum from people who had this same low fuel pressure problem and now they have all gone away. You did something to fix yours, but I did nothing and if anybody else took any action they haven't bothered to let us know about it. Yet the issue seems to have disappeared. This makes no sense.

Whatever, it's good to hear you succeeded in fixing your airplane. Planning the trip out West anytime soon?

Barrie

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Re: Fuel pressure, sensor

Postby Kregster » Thu Mar 31, 2016 4:53 pm

I too had the problem. Put the new fuel system, banjo bolt and check valve on.. Runs like a kitten. FYI, I think the aux fuel pump seems now to provide a true boost in pressure readings where before it may have been negligible. Fly Safe.

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bstrachan
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Re: Fuel pressure, sensor

Postby bstrachan » Thu Jun 02, 2016 6:46 pm

My low fuel pressure issue in the StingSport came back in spades yesterday. I flew to Valle (40G) in northern AZ, spent a couple of hours at the museum there, then left for Cedar City in the middle of a hot afternoon. Taxied out and did normal runup.... when I opened the throttle and started the takeoff roll, the LOW FUEL PRESS light came on. I turned on the AUX pump and it immediately flooded the engine!!! Luckily I hadn't rotated... got the airplane stopped on the runway, ran the engine for awhile to clear it out, back taxied, took off and had uneventful 1 1/2 hour flight home (partway at 11,500 going thru the Tuckup corridor). I did get one LOW FUEL PRESS light on the way, but it went out as soon as I started the AUX pump, and didn't come back when I turned the pump off.

This certainly sounded like vapor locking to me. This morning I disconnected the fuel return line from the "spider" and turned the AUX pump on. Nothing!! The "restrictor" (a Mikuni carb jet) in the return line was plugged up solid. No way the anti-vapor lock system could work with that plugged. I cleaned out the orifice and put everything back together. Will test fly it on one of these first nice warm days and report results.

roger lee
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Re: Fuel pressure, sensor

Postby roger lee » Thu Jun 02, 2016 11:04 pm

Low fuel pressure signs often happen after landing and taking off again after .5 - 1 hour later. These usually do not cause engine issues, but cause the pilot to worry. before the digital age most did not have fuel pressure indicators so they never knew and the issues usually corrects itself in a few minutes as cooler fuel flows. This is one important reason to have and re-circulation line. People that don't have fuel hose in fire sleeve see this even more. I have seen this in many aircraft, but haven't heard of an engine shutdown.
Roger Lee
Tucson, Az.
LSRM-A, Rotax Instructor & Rotax IRC
(520) 574-1080 (Home) Try Home First.
(520) 349-7056 (Cell)

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bstrachan
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Re: Fuel pressure, sensor

Postby bstrachan » Fri Jun 03, 2016 9:20 am

Roger, thanks for pointing out the "restrictor" in the return line is actually a Mikuni carb jet. I had a job getting mine out. They put it in with LocTite and the tiny little screwdriver slot took it in the shorts. I was able to go to the local Polaris store and buy a better jet with a shoulder on it and a sturdier screwdriver slot. This made a much cleaner installation.

All my fuel lines are in aluminum firesleeve. Maybe this is why I had as little vapor locking as I did, even with the return line completely blocked. I still wonder what was in that orifice... it was hard as the hubs of hell... might have been some of the LocTite they slopped on there when they installed the carb jet Way Back When? I never see anything in the gascolator or in the sump drain.

As to my takeoff experience, I have never had anything like that happen before. After the 1 1/2 hour flight to Valle and sitting in the hot sun for 2 hours, the engine was heat soaked, no doubt about it. My guess is, the fuel lines were all full of vapor. I started up and taxied out on the fuel that was in the carb bowls (if you ride a motorcycle and have ever forgotten to turn the fuel valve on, you know how far you can go on the gas in the bowls). Runup was normal, but when I opened the throttle for takeoff, the LOW FUEL PRESS light came on. Don't ask me why it didn't come on before. Anyway I flipped the AUX pump on (book says NOT TO USE IT ON TAKEOFF) and that immediately flooded the engine. If that had happened 30 seconds later, we would have been in the sagebrush. I don't have a clue as to why this happened the way it did. If you have any ideas I would like to hear them.

I am hoping that fixing the return line will result in no more fuel pressure problems. I'll report as I learn more.

Barrie

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drseti
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Re: Fuel pressure, sensor

Postby drseti » Fri Jun 03, 2016 11:50 am

bstrachan wrote: Anyway I flipped the AUX pump on (book says NOT TO USE IT ON TAKEOFF) and that immediately flooded the engine.


If you ever wondered why the book says not to use aux pump on takeoff, now you know!
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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bstrachan
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Re: Fuel pressure, sensor

Postby bstrachan » Fri Jun 03, 2016 8:35 pm

Thanks for your input. Very helpful.


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