R

RFSchaller

Well Known Member
Today I aborted a takeoff with a hi-fuel flow alarm. My fuel pressure was (and is) about 3.5 psi and I have about 11 GPH indicated fuel flow at WOT on runup where I used to have about 6 GPH. I checked and there are no fuel leaks (thank you, God!). My "red cube" detector K constant is just where it was before the problem, and the fuel flow indication follows throttle position (i.e. increases as throttle opens and decreases when closed). With the engine shutdown I have 1.1 - 2.3 GPH with the electric pump running. When I pulled the fuel pump fuse with the engine shutdown the indications (Pressure and Flow) went to zero and returned to 3.5# and 1.1 GPH when I put the pump fuse back in. It looks like the return line flow may have inexplicably increased, but I haven't confirmed this. Has anybody else had this problem or can you give me any insight on the return line: is it a regulator or just a restricting orifice? Does anyone know of any potential failure mode that could cause this (e.g. orifice blew out down the return line)? I plan to look at the return line flow with a borescope tomorrow with the electric pump running and the engine shutdown. BTW I have a Dynon 180 with the old AC style fuel pump and I have the old style pressure sensor (P/N 100411-00) and "red cube" flow sensor.
 
First suspect is the fuel flow sensor.
Second suspect is the engine driven fuel pump.
Third suspect is vapor lock.
Joe Gores
 
As discussed on here many times, the VDO fuel pressure sensor is all but worthless and its most likely your flow sensor failed (a new one cost $250). The engine will run fine on the electric pump only in the unlikely event your fuel pump fails. If you need it, buy a Kavlico. You don't need the flow sensor at all. Fill your tank up when you leave and fly by time. Eventually you can use that to determine your flow...if its a Rotax 912ULS it will be somewhere between 4.2 and 4.5. These are both Dynon supplied items and needless to say they are not worth the work to install them. Do yourself a favor and turn em off. Its a VFR airplane...and a great one at that...go fly and spend your time looking at the great view.
 
Thanks, guys. I don't see how the fuel pressure sensor is an issue since it seems to be responding. The fuel flow sensor is a pulse generator with a small turbine. I would expect it to freeze and fail to zero flow.

I did some research in the ROTAX parts manual and found there is a restrictor jet in the return line banjo connection. I'm going to pull the banjo tomorrow and see if the jet is still there. It is a removable part, so maybe it backed out.

I'll keep everyone posted on what I find.

Rich
 
See section 46, page 11!

If you built the airplane then in section 46-11 you verified operation of the banjo bolt assembly for proper fuel flow back to the tank;).
 
Rich,

The incorrect high fuel flow indication has been an ongoing problem for several of us. It usually occurs at altitudes above 5000 feet. I have been working with Dynon to try and isolate the cause. I just ran a test for them last week, and but my email bounced back and I have to resend it. We are trying to determine if this is a Dynon problem, or for some as yet unexplained reason the return flow suddenly increases.

There are previous threads on this if you search. I am interested to know what your pressure altitude was when you got the alarm.

While others do not necessarily agree, I consider this a serious issue. Sudden unexplained alarms are not a good thing.

Please post your altitude when you saw this behavior, and results of any testing you do.
 
John,

Have you given any thought to placing another, completely independent, fuel flow sensor in series with the existing. Maybe at the outlet of the gascolator?

Anyone know if a small sensor is available that uses a completely different technology compared to the installed? Maybe someone can comment.
 
Problem Troubleshooting Results

Here's what I found today:

The return flow restricting orifice in th VA-216 assembly was in place and secure. There is no evidence of a fuel leak downstream of the red cube sensor. A borescope showed the return flow barely bubbling out of the return line with the engine shut down and the electric fuel pump running (3.7 psi). A subsequent WOT run up had normal fuel flow indications peaking at an indication of 7 GPH.

All indications point to the red cube giving a false high reading. It doesn't make much sense to me given the design of the sensor, but I can't argue with the evidence. My solution was to raise the alarm point to 16GPH. I figure that will eliminate most spurious alarms while giving me some monitoring for a major leak.

BTW the PA was 1075 FT and the DA was 4250 FT.

I hope this helps others who may similar indications.
 
What was the outside temperature?
If the alarm set-point is too high, there will not be a warning when the engine driven fuel pump goes bad. Others have had a high fuel flow alarm (along with low pressure) when that pump fails.
Joe Gores
 
3.7 psi from the elec. pump seems high to me. I don't get nearly that much pressure from mine. Wonder what others are seeing from theirs?
 
That's the problem with the digital world. We used to say "about three and a half" . Now we lock in on the third significant digit on a sensor that's probaly good to 10% !

Rich
 
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