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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Joined: Sat Mar 27, 2010 4:03 pm
Location: Cedar City, Utah

Re: Fuel pressure, sensor

Postby bstrachan » Sat Sep 03, 2016 11:10 pm

Here's a constructive suggestion if you are having intermittent low fuel pressure indications on your Rotax powered airplane. Check the restrictor in the fuel return line that comes out of the "octopus" on top of the engine. It was easy to do, on mine anyway. Pop the quick release hose clamp open with a screwdriver, take the hose off with your Harbor Freight hose removal pliers, and see if you can run a piece of thin safety wire into the restrictor. Mine was plugged up solid. No idea how it happened or what the foreign matter was... I've never seen a bit of debris in the gascolator or the sump.

The fuel return line is there to prevent vapor locking, and it seems to do a good job when it is not plugged.

roger lee
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Location: Tucson, Az. Ryan Airfield (KRYN)

Re: Fuel pressure, sensor

Postby roger lee » Sun Sep 04, 2016 8:39 am

You can still have debris at the fuel junction block on the carb crossover tube and it would be more likely with rubber hose vs Rotax red Teflon hose. The reason you may have debris here that the gascolator or another filter didn't catch is this. First the fuel pump has a fine screen filter inside. So anything up to the fuel pump gets stopped, but after the pump up to the fuel block assembly and to the carbs has no more filters. So the affected hose is the one that comes off the bottom of the fuel pump. So this is where your debris most likely came from. It's that short 14" piece of hose off the pump. If you are using the Rotax supplied red Teflon hose debris would be extremely hard to happen after the pump.
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 » Sun Sep 04, 2016 9:49 am

What you say makes perfect sense. But whatever was blocking my restrictor was HARD, like dried up Loc-tite or ??? I couldn't poke it out with a needle, I had to take a tiny drill to it. I have no idea what it was. You ever see anything like this? In any case, cleaning out the restrictor seems to have solved my low fuel pressure problem (knock on wood).

Wm.Ince
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Re: Fuel pressure, sensor

Postby Wm.Ince » Sun Sep 04, 2016 10:33 am

roger lee wrote:You can still have debris at the fuel junction block on the carb crossover tube and it would be more likely with rubber hose vs Rotax red Teflon hose. The reason you may have debris here that the gascolator or another filter didn't catch is this. First the fuel pump has a fine screen filter inside. So anything up to the fuel pump gets stopped, but after the pump up to the fuel block assembly and to the carbs has no more filters. So the affected hose is the one that comes off the bottom of the fuel pump. So this is where your debris most likely came from. It's that short 14" piece of hose off the pump. If you are using the Rotax supplied red Teflon hose debris would be extremely hard to happen after the pump.
Thanks Roger.
When you perform a 5-year hose change, on a carbed 912 now, do you install the Teflon hoses as a standard replacement?
Bill Ince
CTSW
Retired Heavy Equipment Operator

roger lee
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Location: Tucson, Az. Ryan Airfield (KRYN)

Re: Fuel pressure, sensor

Postby roger lee » Sun Sep 04, 2016 1:04 pm

Hi Bill,

No I don't, but I take special care not to have debris issues. That said no one can ever guarantee anything 100%. We can just do the best we can. If the engine already has the Teflon hoses from the fuel distribution block to the carbs then they are replaced on condition so then it just boils down to that 15" hose off the fuel pump to that block. I've only had a problem twice and that was many years ago when we first started doing hose changes. Since then my more sterile techniques have served me well SO far.
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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Joined: Sat Mar 27, 2010 4:03 pm
Location: Cedar City, Utah

Re: Fuel pressure

Postby bstrachan » Mon Nov 13, 2017 11:12 am

Some of you may remember my pathetic bleating about low fuel pressure indications in my 2006 912ULS powered Sting Sport. After almost losing it on takeoff at Valle (as I reported) I decided I had to do something or die. Here's what I did.

1. As mentioned in my previous post, I fixed the vapor return line that comes out of the "spider" on top of the engine back to the main tank. No idea how mine got plugged up, but having this return working properly is MANDATORY.

2. I had occasionally noticed a big WHOOSH when I opened the main tank to refuel after flying. My buddy, the previous owner of the airplane, had observed the same thing. I checked the main tank vent several times, it was always open, but somehow the convoluted plumbing of that vent tube wasn't allowing air into the tank to replace fuel consumed and the fuel pump was sucking a vacuum on the tank. So I drilled a tiny hole (#55 drill) in the non-vented fuel cap. No more WHOOSH. And yes, I know the fuel cap is located in a low pressure area on top of the wing. Most - not all - fuel caps, vented or no, are on top of the wing. All I can tell you is that tiny hole breaks the vacuum in the tank, and no fuel syphons out.

Since making these two adjustments (it's been over a year) I have had ZERO low fuel pressure indications. It runs a pretty steady 3 PSI which I believe is what those Bing carbs want to see. I burn Shell 91 octane (highest octane they sell here at 5700 feet), I check every batch, and I have never found any water or ethanol in this fuel.

Just for what it's worth.

Barrie


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