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8.5 psi Fuel Pressure!!

R

rdockery_3

Member
I noticed today when I landed my fuel pressure was really high, it was 8.5 psi. I turned the electric fuel pump off and it was the same and then I shut the engine down and turned the master off and it was still reading 8.5 psi. I did a total of 3 run ups at different times throughout my flying today and it was normal psi and when I checked it in flight it was normal. Anybody have any idea what is going on?
 
Normal for a carb engine would be 6-8. I did run into one once that failed high. If I remember right is was about 12 or 15psi. There's no adjustment. We cured that one by replacement, which seems a little drastic so far. The danger is that high pressure would overwhelm the float needle and flood. That wouldn't be good, but is probably not happening as of yet. If the pressure remains high after shutdown it just means there are no leaks and the needle and seat are holding the pressure. Monitor and see if it keeps climbing over time. Come back tomorrow and look at it before starting and see if it's still high and if not, what pressure the boost pump throws.

Another possibility is that the fuel pressure sensor is getting flakey. We put a reference gauge on my buddy's plane to confirm before he sprung for a new pump.

Seems like in the hot months, most people are worrying about low pressure. ;-)

Ed
 
Bicyclops has it wrong for Rotax 912ULS engine. There is a small orifice that bleeds fuel back into the fuel tank. The orifice is located in the block in the fuel rail that feeds the carbs. When you shut off engine the fuel pressure will bleed to zero as fuel stops flowing back to the tank the pressure because one atmosphere...
 
If the gauge is still showing 8.5 psi when the master is turned off then possibly there is something wrong with the gauge...?
 
Know nothing about rotax carbs so can’t speak to what is too high. However, i have experience with typical diaphragm style fuel pumps. While they typically fail with low or no pressure, they CAN fail with high pressure. I have seen this several times back in the 70’s 80’s in the auto world - same pump that lyc uses and speculate rotax is similar.

Checking sensors is a good first step, but please don’t assume that the pump can’t fail with over pressure. If it gets bad enough, it will overcome the needle pressure and cause an overly rich or even a flooded condition.
 
Piper J3 said:
. There is a small orifice that bleeds fuel back into the fuel tank. The orifice is located in the block in the fuel rail that feeds the carbs. When you shut off engine the fuel pressure will bleed to zero as fuel stops flowing back to the tank the pressure because one atmosphere...
Click to expand...
+1. This is the first place I would begin troubleshooting.
 
You should perform SD-00017 Fuel return line blockage test, My guess is your fuel pressure transducer is bad...
 
Piper J3 said:
You should perform SD-00017 Fuel return line blockage test, My guess is your fuel pressure transducer is bad...
Click to expand...
If memory serves, the return line to the tank is smaller diameter than the feed thus maintaining the pressure. The carb feed is upstream of the return line obviously.

Unlikely for the pressure to be maintained after shutdown. I would look to a failing sender.
 
flightdoc101 said:
If memory serves, the return line to the tank is smaller diameter than the feed thus maintaining the pressure. The carb feed is upstream of the return line obviously.

Unlikely for the pressure to be maintained after shutdown. I would look to a failing sender.
Click to expand...
No.... it's done with an orifice.
 
I'm pretty sure it is a sending unit problem. Yesterday after the plane had sat for 3 days the fuel pressure read 3.1 psi with the master switch off and engine not running. I started the plane up and it still read the same 3.1 psi.
 
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